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1994-05-10
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┌─────────────────┐
│ Texture Mapping │
└─────────────────┘
Written for the PC-GPE by Sean Barrett.
┌───────────────────────────────────────────┐
│ THIS FILE MAY NOT BE DISTRIBUTED │
│ SEPARATE TO THE ENTIRE PC-GPE COLLECTION. │
└───────────────────────────────────────────┘
-=-=-=-=- -=-=-=-=-=- -=-=-=-=-=- -=-=-=-=-=- -=-=-=-=-=- -=-=-=-=-=-
TEXTURE
TEXUE almost everything you need to know to texture map with the PC
TXR
X
-=-=-=-=- -=-=-=-=-=- -=-=-=-=-=- -=-=-=-=-=- -=-=-=-=-=- -=-=-=-=-=-
Copyright 1994 Sean Barrett.
Not for reproduction (electronic
or hardcopy) except for personal use.
Contents:
0. warnings
1. terminology and equations
2. the basics
Perfect texture mapping
DOOM essentials
Wraparound textures
Non-rectangular polygons
3. the hazards
going off the texture map
divide by 0
it'll never be fast enough
4. the complexities
handling arbitrarily-angled polygons
lighting
slow 16-bit VGA cards
mipmapping
-=-=-=-=-=-
| Note: I am not providing any references as I simply
| derived the math myself and worked out the various
| techniques for myself (the 32-bit ADC trick was
| pointed out to me in another context by TJC,
| author of the Mars demo) over the last two years
| (since Wolfenstein 3D and Underworld came out).
-=-=-=-=-=-
TEXTURE
TEXUE
TXR 0. Warnings
X
I assume a RIGHT-handed 3D coordinate system, with X positive
to the right, Y positive disappearing into the screen/distance,
and Z positive up. To adjust this to the typical left-handed
3D space, simply swap all the 3D Ys & Zs.
I assume the screen space is positive-X to the right,
positive-Y goes down. Adjust the signs as appropriate for
your system.
I will present code and pseudo-code in C. I also include
some relatively tight inner loops written in assembly, but I'm
omitting the details of the loop setup. The inner loops, while
usually from real, working code, should generally be taken
as examples showing how fast it ought to be possible to run
a given task, not as necessarily perfect examples. I often
use 32-bit instructions (sorry 286 programmers) because they
can double the performance. However, I write in real mode,
because 16-bit addressing is often convenient for texture maps,
and it's straightforward to use segment registers as pointers
to texture maps. The translation to protected mode should not
prove problematic, but again, these should more be taken as
examples rather than simply being used directly. I optimize
for the 486, but I skip some obvious optimizations. For example,
I write "loop", because it's simpler and more clear.
Production code for the 486 should explicitly decrement and
then branch. Similarly, I write "stosb", etc. etc.
TEXTURE
TEXUE
TXR 1. Terminology and Equations
X
You really probably don't want to read this section first,
but rather refer back to it whenever you feel the need. So
skip up to section 2 and refer back as appropriate. I could've
made this an appendix, but it seems too important to put last.
TEX
TE Terms
T
texture: A texture is a pixelmap of colors which is mapped
onto a polygon using "texture mapping". The size
of the polygon has nothing to do with the size (the
number of pixels) in the texture map.
run: A run is a row or column of pixels. Normal texture
mapping routines process one run in an "inner loop".
arbitrarily-angled polygon: a polygon which isn't a floor,
wall, or ceiling; technically, a polygon which isn't
parallel to the X or Z axes (or X or Y axes in a
Z-is-depth coordinate system).
texture space:
polygon space:
polygon coordinate space:
Since a texture is flat, or two-dimensional, the relation
of the texture to the 3D world can be described with a
special coordinate space known by one of these names.
Because it is only 2D, the space can be characterized with
the location of the texture space in 2D, and two 3D vectors
which represent the axes of the coordinate space. Sometimes
called "uv" space, because the name of the coordinates are
usually u & v.
TEX
TE Notation
T
Vectors appear in all caps.
Components of vectors are P = < Px, Py, Pz >.
Certain variables have consistent usage:
x,y,z are coordinates in three-space
i,j are screen coordinates
u,v are coordinates in texture space
a,b,c are "magic coordinates" such that
u = a/c, v = b/c
TEX
TE Equations
T
Don't let this scare you off! Go read section 2, and
come back to this when you're ready.
Let P,M, and N be vectors defining the texture space: P is the
origin, and M and N are the vectors for the u&v axes.
Assume these vectors are in _view space_, where view space is
defined as being the space in which the transformation from
3D to 2D is:
(x,y,z) -> (i,j)
i = x / y
j = z / y
In other words, you have to adjust P, M, and N to be relative
to the view, and if you have multiplications in your perspective
computation, you have to multiply the appropriate components of
P, M, and N to compute them. Note that since typically in 3D
up is positive, and in 2D down is positive, there may be a
multiplication by -1 that needs to go into Py, My, Ny. Note that
this also assumes that (0,0) in screen space is at the center
of the screen. Since it's generally not, simply translate
your screen coordinates (i,j) as if they were before applying
the texture mapping math (or if you're funky you can modify
your viewspace to pre-skew them).
For example, if your transforms are:
i = Hscale * x / y + Hcenter
j = -Vscale * z / y + Vcenter
Then you should simply multiply Px, Mx, and Nx by Hscale,
and multiply Py, My, and Mz by -Vscale. Then just remember
to subtract Hcenter and Vcenter from the i,j values right
before plugging them into the texture mapping equations.
We begin by computing 9 numbers which are constant
for texture mapping the entire polygon (O stands for
Origin, H for Horizontal, and V for vertical; why I use
these names should become clear eventually):
Oa = Nx*Pz - Nz*Px
Ha = Nz*Py - Ny*Pz
Va = Ny*Px - Nx*Py
Ob = Mx*Pz - Mz*Px
Hb = Mz*Py - My*Pz
Vb = My*Px - Mx*Py
Oc = Mz*Nx - Mx*Nz
Hc = My*Nz - Mz*Ny
Vc = Mx*Ny - My*Nx
Ok. Then, for a given screen location (i,j), the formula
for the texture space (u,v) coordinates corresponding to
it is:
a = Oa + i*Ha + j*Va
b = Ob + i*Hb + j*Vb
c = Oc + i*Hc + j*Hc
u = a/c
v = b/c
TEXTURE
TEXUE
TXR 2. The Basics
X
So you've got your polygon 3D engine running, and
you'd like to start adding a bit of texture to your
flat- or Gouraud-shaded polygons. Well, it will make
it look a lot cooler. But let's point out the
disadvantages of texture mapping right away:
Slower
Sometimes hard to see polygon edges
Each of these has certain ramifications on the
overall approach you want to take with your code,
which we'll come back to later, in sections 3 and 4.
Practical advice: Don't try to get your riproaringly
fast texture mapper running first. Get a very simple,
slow, "perfect" texture mapper working first, as described
in the first subsection below. This will allow you to make
sure you've gotten the equations right. Realize that I
can't present the equations appropriate to every scenario,
since there are simply too many spaces people can work
in. I've chosen to present the math from an extremely
simple coordinate space which keeps the texture mapping
relatively simple. You'll have to work out the correct
transformations to make it work right, and a slow but
correct texture mapping routine may help you tweak the
code as necessary to achieve this. Use very simple
polygons to start your testing; centered and facing the
viewer should be your very first one (if done correctly,
this will simply scale the texture).
TEX
TE Perfect Texture Mapping
T
To start with, we'll do slow but exact "perfect" texture
mapping of a square tile with a simple texture map mapped onto
it. The polygon is defined in three-space using four points,
and the texture map is 256x256 pixels. Note that this is all
talking about using floating point, so those of you working in
C or Pascal are fine. Those in assembly should realize that
you have to do a bit of extra work to use fixed point, or you
can beat out the floating point by hand if you want.
First we have to "map" the texture onto the polygon.
We have to define how the square texture map corresponds
to the square polygon. This is relatively simple. Let
one corner of the polygon be the origin (location <0,0>)
of the texture map. Let each of the other corners
correspond to corners just off the edge of the texture
map (locations <256, 0>, <256, 256>, and <0, 256>).
We'd like to use the equations in section 1, which
require vectors P, M, and N, where P is the origin,
and M & N are the axes for u&v (which are _roughly_
the coordinates in the texture map, but see below).
In other words, P, M and N tells us where the texture
lies relative to the polygon. P is the coordinate in
three-space where the origin of the texture is. M
tells us which way the "horizontal" dimension of the
texture lies in three-space, and N the "vertical".
Suppose the polygon has four vertices V[0], V[1],
V[2], and V[3] (all four of these are vectors). Then,
P is simply V[0]. M is a vector going from the origin
to the corner <256, 0>, so M is a vector from V[0] to
V[1], so M = V[1] - V[0]. N is a vector from the origin
to the corner <0, 256>, so N is V[3] - v[0].
P = V[0]
M = V[1] - V[0] { note these are vector subtractions }
N = V[3] - V[0]
Again, remember that we need P, M, and N in the viewspace
discussed with the equation, so make sure you've transformed
the Vs appropriately, or you can compute P, M, and N in world
or object space and transform them into viewspace.
Compute the 9 magic numbers (vectors O, H, and V) as described
in section 1. Now, take your 3D polygon and process it as normal.
Scan convert it so that you have a collection of rows of pixels
to process.
Now, iterate across each row. For each pixel in the polygon
whose screen coordinates are <i,j>, apply the rest of the math
described in section 1; that is, compute a, b, and c, and from
them compute <u,v>.
I said before that <u,v> are basically the texture map
coordinates. What they are in truth are the coordinates in texture
map space. Because of the way we defined texture map space,
we'll actually find that u and v both run from 0..1, not 0..256.
This is an advantage for descriptive purposes because u and v
are always 0 to 1, regardless of the size of the texture map.
So, to convert u&v to pixelmap coordinates, multiply them
both by 256. Now, use them as indices into the texture map,
output the value found there, and voila, you've texture mapped!
The loop should look something like this:
[ loop #1 ]
for every j which is a row in the polygon
screen = 0xA0000000 + 320*j
for i = start_x to end_x for this row
a = Oa + (Ha * i) + (Va * j)
b = Ob + (Hb * i) + (Vb * j)
c = Oc + (Hc * i) + (Vc * j)
u = 256 * a / c
v = 256 * b / c
screen[i] = texture_map[v][u]
endfor
endfor
Once you've got that working, congratulations! You're
done dealing with the annoying messy part, which is getting
those 9 magic numbers computed right. The rest of this is
just hard grunt work and trickery trying to make the code
faster.
From here on in, I'm only going to look at the inner
loop, that is, a single run (row or column), and let the
rest of the runs be understood.
TEX
TE Prepare to meet thy DOOM
T
This subsection is concerned with vastly speeding
up texture mapping by restricting the mapper to walls,
floors and ceilings, or what is commonly called
DOOM-style texture mapping, although it of course predates
DOOM (e.g. Ultima Underworld [*], Legends of Valour).
[* Yes, Underworld allowed you to tilt the view,
but it distorted badly. Underworld essentially used
DOOM-style tmapping, and tried to just use that on
arbitrarily-angled polygons. I can't even begin to
guess what Underworld II was doing for the same thing.]
To begin with, let's take loop #1 and get as much
stuff out of the inner loop as we can, so we can see
what's going on. Note that I'm not going to do low-level
optimizations, just mathematical optimizations; I assume
you understand that array walks can be turned into
pointer walks, etc.
[ loop #2 ]
a = Oa + Va*j
b = Ob + Vb*j
c = Oc + Vc*j
a += start_x * Ha
b += start_x * Hb
c += start_x * Hc
for i = start_x to end_x
u = 256 * a / c
v = 256 * b / c
screen[i] = texture_map[v][u]
a += Ha
b += Hb
c += Hc
endfor
With fixed point math, the multiplies by 256
are really just shifts. Furthermore, they can be
"premultiplied" into a and b (and Ha and Hb) outside
the loop.
Ok, so what do we have left? Three integer adds,
a texture map lookup, and two extremely expensive fixed-point
divides. How can we get rid of the divides?
This is the big question in texture mapping, and most
answers to it are _approximate_. They give results that
are not quite the same as the above loop, but are difficult
for the eye to tell the difference.
However, before we delve into these, there's a very
special case in which we can get rid of the divides.
We can move the divide by c out of the loop without
changing the results IFF c is constant for the duration
of the loop. This is true if Hc is 0. It turns out that
Hc is 0 if all the points on the run of pixels are the same
depth from the viewer, that is, they lie on a line of
so-called "constant Z" (I would call it "constant Y" in
my coordinate system).
The requirement that a horizontal line of pixels be the
same depth turns out to be met by ceilings and floors.
For ceilings and floors, Hc is 0, and so the loop can be
adjusted to:
[ loop #3 ]
__setup from loop #2__
u = 256 * a / c
v = 256 * b / c
du = 256 * Ha / c
dv = 256 * Hb / c
for i = start_x to end_x
screen[i] = texture_map[v][u]
u += du
v += dv
endfor
Now _that_ can be a very fast loop, although adjusting
the u&v values so they can be used as indices has been
glossed over. I'll give some sample assembly in the next
section and make it all explicit.
First, though, let's look at walls. Walls are almost
identical to floors and ceilings. However, with walls,
Vc is 0, instead of Hc. This means that to write a loop
in which c is constant, we have to walk down columns instead
of across rows. This affects scan-conversion, of course.
The other thing about walls is that with floors, since
you can rotate about the vertical axis (Z axis for me, Y axis
for most of you), the horizontal runs on the floors cut
across the texture at arbitrary angles. Since you're
bound to not tilt your head up or down, and since the
polygons themselves aren't tilted, you generally find
that for walls, Va is 0 as well. In other words, as you
walk down a column of a wall texture, both a & c are constant,
so u is constant; you generally only change one coordinate,
v, in the texture map. This means the inner loop only needs
to update one variable, and can be made to run _very_ fast.
The only thing missing from this discussion for creating
a DOOM clone is how to do transparent walls, how to do
lighting things, and how to make it fast enough. These will
be discussed in section 4, although the some of the speed
issue is addressed by the inner loops in the next subsection,
and the rest of the speed issue is discussed in general terms
in section 3.
TEX
TE ...wrapped around your finger...
T
So far, we've only looked at texture mapping a single
polygon. Of course, it's obvious how to texture map
a lot of polygons--just lather, rinse, repeat. But it
may seem sort of wasteful to go through all the 3D math
and all over and over again if we just want to have one
long wall with the same texture repeating over and over
again--like linoleum tiles or wallpaper.
Well, we don't have to. Let's think about this
idea of a "texture map space" some more. We defined
it as being a coordinate system "superimposed" on
the polygon that told us where the texture goes.
However, when we implemented it, we simply used the
polygon itself (in essence) as the coordinate space.
To see this, make a polygon which is a rectangle,
perhaps four times as long as it is tall. When it
is drawn, you will see the texture is distorted,
stretched out to four times its length in one dimension.
Suppose we wanted it to repeat four times instead?
The first step is to look at what the definition
of the texture map space means. The texture map space
shows how the physical pixelmap itself goes onto the
polygon. To get a repeating texture map, our first
step is to just get one of the copies right. If
we set up our P,M, & N so that the M only goes one
quarter of the way along the long edge of the
rectangle, we'll map the texture onto just that
quarter of the rectangle.
Here's a picture to explain it:
Polygon A-B-C-D Texture map
A E u=0 u=1
o--------o-___________ B v=0 11112222
|111112222 ---------o 11112222
|111112222 | 33334444
|111112222 | 33334444
|333334444 | v=1
|333334444 |
|333334444 ___________---------o
o--------o- C
D F
So, we used to map (u,v)=(0,0) to A, (u,v)=(1,0) to B, and
(u,v) = (0,1) to D. This stretched the texture map out to
fill the entire polygon map.
Now, instead, we map (u,v)=(1,0) to E. In other words,
let P = A, M = E-A, and N = D-A. In this new coordinate
space, we will map the texture onto the first quarter of
the polygon.
What about the rest of the polygon? Well, it simply
turns out that for the first quarter 0 <= u <= 1. For
the rest, 1 <= u <= 4.
To make the texture wrap around, all we have to do is
ignore the integer part of u, and look at the fractional part.
Thus, as u goes from 1 to 2, we lookup in the texture map using
the fractional part of u, or (u-1).
This is all very simple, and the upshot is that, once
you define P, M, and N correctly, you simply have to mask
your fixed-point u&v values; this is why we generally use
texture maps whose sides are powers of two, so that we can
mask to stay within the texture map. (Also because they
fit conveniently into segments this way, and also so that
the multiply to convert u&v values from 0..1 to indices
is just a shift.)
I'm assuming that's a sufficient explanation of the
idea for you to get it all setup. So here's the assembly
inner loops I promised. I'm not going to bother giving
the ultra-fast vertical wall-drawing case, just the
horizontal floor/ceiling-drawing case.
Note that a mask of 255 (i.e. for a 256x256 texture)
can be gotten for free; however, no program that I'm
aware of uses texture maps that large, since they
simply require too much storage, and they can cache very
poorly in the internal cache.
First, here's your basic floor/ceiling texture mapper,
in C, with wraparound, and explicitly using fixed point
math--but no setup.
[ loop #4 ]
mask = 127, 63, 31, 15, whatever.
for (i=0; i < len; ++i) {
temp = table[(v >> 16) & mask][(u >> 16) & mask];
u += du;
v += dv;
}
Now, here's an assembly one. This one avoids the
shifts and does both masks at the same time, and uses
16 bits of "fractional" precision and however many bits
are needed for the coordinates. Note that I assume
that the texture, even if it is 64x64, still has each
row starting 256 bytes apart. This just requires some
creative storage approaches, and is crucial for a fast
inner loop, since no shifting&masking is required to
assemble the index.
mov al,mask
mov ah,mask
mov mask2,ax ; setup mask to do both at the same time
loop5 and bx,mask2 ; mask both coordinates
mov al,[bx] ; fetch from the texture map
stosb
add dx,ha_low ; update fraction part of u
adc bl,ha_hi ; update index part of u
add si,hb_low ; these are constant for the loop
adc bh,hb_hi ; they should be on the stack
loop loop5 ; so that ds is free for the texture map
This code is decent, but nowhere near as fast as it can be.
The main trick to improving performance is to use 32 bit adds
instead of two adds. The problem with this is that extra operations
are required to setup the indexing into the texture map. Through
the use of the ADC trick, these can be minimized. In the following
code, bl and bh are unchanged. However, the top half of EBX now
contains what used to be in si, and the other values have been
moved into registers. ESI contains hb_low in the top half, and
ha_hi in the low 8 bits. This means that ADC EBX,ESI achieves
the result of two of the additions above. Also, we start using
BP, so we move our variables into the data segment and the texture
map into FS.
loop6 and bx,mask2
mov al,fs:[bx]
stosb
add dx,bp ; update fractional part of u
adc ebx,esi ; update u (BL) and frac. part of v (EBX)
adc bh,ch ; update index part of v
dec cl
jnz loop6
This is a bit faster, although it has one bug. It's
possible for the addition into BL to overflow into BH. It might
not seem to be, since BL is masked every iteration back down to
stay in 0..127, 0..63, or whatever. However, if the step is
negative, then BL will be decremented each iteration, and may
"underflow" and subtract one from BH. To handle this, you need
a seperate version of the loop for those cases.
If you're not doing wraparound textures, you can speed the
loop up a bit more by removing the and. You can run entirely
from registers except for the texture map lookup. Additionally,
unrolling the loop once cuts down on loop overhead, and is crucial
if you're writing straight to the VGA, since it doubles your
throughput to a 16-bit VGA card.
Here's a very fast no-wraparound texture mapper. It uses
the ADC trick twice. Note that the carry flag is maintained
around the loop every iteration; unfortunately the 'and' required
for wraparound textures clears the carry flag (uselessly). EBX
and EDX contain u & v in their bottom 8 bits, and contain the
fractional parts of v & u in their top bits (note they keep the
_other_ coordinate's fractional parts). You have to have prepped
the carry flag first; if you can't figure this technique out, don't
sweat it, or look to see if someone else has a more clear discussion
of how to do fast fixed-point walks using 32-bit registers.
This loop is longer because it does two pixels at a time.
loop7 mov al,[bx] ; get first sample
adc edx,esi ; update v-high and u-low
adc ebx,ebp ; update u-high and v-low
mov bh,dl ; move v-high into tmap lookup register
mov ah,[bx] ; get second sample
adc edx,esi
adc ebx,ebp
mov bh,dl
mov es:[di],ax ; output both pixels
inc di ; add 2 to di without disturbing carry
inc di
dec cx
jnz loop7
I went ahead and 486-optimized the stosw/loop at the end to make
cycle-counting easier. All of these instructions are single cycle
instructions, except the branch, and the segment-override. So you're
looking at roughly 15 cycles for every two pixels. Your caching
behavior on the reads and writes will determine the actual speed.
It can be unrolled another time to further reduce the loop overhead;
the core operations are 9 instructions (10 cycles) for every two
pixels. Note the "inc di/inc di", which protects the carry flag.
If you unroll it again, four "inc di"s will be required. Unroll it
another time, and you're better off saving the carry flag, adding,
and restoring, for example "adc ax,ax/add di,8/shr ax,1", rather
than 8 "inc di"s.
TEX
TE Lost My Shape (trying to act casual)
T
Non-rectangular polygons are trivial under this system.
Some approaches require you to specify the (u,v) coordinates
for each of the vertices of the polygon. With this technique,
you instead specify the 3D coordinates for three of the
"vertices" of the texture map. So the easiest way of handling
a texture of a complex polygon is simply to use a square
texture which is larger than the polygon. For example:
P1 P2
x B _______ C x
/ \
/ \
A / \
\ / D
\ /
\ _______ /
x F E
P3
Now, we simply define the texture map such that P is P1,
M is P2-P1, and N is P3-P1. Then, if our texture looks like
this:
u=0 u=1
------------
v=0 |..XXoooooo..
|.XXXXoooooo.
|XXXXXXoooooo
|.XXXXXXoooo.
v=1 |..XXXXXXoo..
Then the regions marked by '.' in the texture map will
simply never be displayed anywhere on the polygon.
Wraparound textures can still be used as per normal,
and concave polygons require no special handling either.
Also, you can get special effects by having M and N not
be perpendicular to each other.
TEXTURE
TEXUE
TXR 3. The Hazards
X
This sections discusses some of the pitfalls and
things-aren't-quite-as-simple-as-they-sounded issues
that come up while texture mapping. All of the
information is, to some extent, important, whether
you've encountered this problem or not.
TEX
TE Cl-cl-cl-close to the Edge
T
At some time when you're texture mapping, you'll
discover (perhaps from the screen, perhaps from a
debugger) that your U & V values aren't within the
0..1 range; they'll be just outside it.
This is one of these "argh" problems. It is
possible through very very careful definition of
scan-conversion operations to avoid it, but you're
likely to encounter it.
If you use wraparound textures, you may not ever
notice it, however, since when it happens, the
texture will simply wraparound and display an
appropriate pixel.
If not, you may get a black pixel, or just
garbage. It'll only happen at the edges of your
polygon.
The reason this happens is because your scan-conversion
algorithm may generate pixels "in the polygon" whose
pixel-centers (or corners, depending on how you've
defined it) are just outside the texture--that is,
they're outside the polygon itself.
The right solution to this is to fix your scan-converter.
If your texture mapper computes u&v coordinates based on
the top-left corner of the pixel (as the one I've defined
so far has), make sure your scan-converter only generates
pixels whose top-left corner is really within the polygon.
If you do this, you may need to make a minor change to my
definition of M & N, but I'm not going to discuss this
further, since you probably won't do this.
A second option is to define P, M, and N such that the
texture map space is slightly bigger than the polygon; that
is, so that if you go just off the edge of the polygon,
you'll still be within the texture map.
This is a pain since you end up having to transform extra
3D points to do it.
The third, and probably most common solution, is to always
use wraparound textures, which hide the problem, but prevent
you from using textures that have one edge that highly contrasts
with another.
The fourth, and probably second most common solution,
and the one that turns out to be a real pain, is to "clamp"
the u&v values to be within the texture all the time.
Naively, you just put this in your inner loop:
if (u < 0) u = 0; else if (u > 1) u = 1;
if (v < 0) v = 0; else if (v > 1) v = 1;
Of course, you don't really do this, since it'd slow
you down far too much. You can do this outside the loop,
clamping your starting location for each run. However,
you can't, under this system, clamp the ending value
easily.
Remember that in the loop we update u and v with
(essentially) Ha/c and Hb/c. These are constant
across the entire run, but not constant across the
entire polygon, because c has different values for
different runs.
We can compute du and dv in a different way to
allow for clamping. What we do is we explicitly compute
(a,b,c) at (start_x, j) as we did before, but we also
compute (a,b,c) at (end_x, j). From these we compute
(u,v) at start_x & at end_x. Next we clamp both sets
of u & v. Then we compute du and dv with
du = (u2 - u1) / (end_x - start_x - 1)
dv = (v2 - v1) / (end_x - start_x - 1)
This is slightly more expensive than the old way,
because we have to compute u2 and v2, which requires
extra divides. However, for methods that explicitly
calculate u&v sets and then compute deltas (and we'll
see some in section 4), this is the way to go.
One final thing you can do is interpolate the (a,b,c)
triple from the vertices as you scan convert. This will
guarantee that all (a,b,c) triples computed will lie be
within the polygon, and no clamping will be necessary
(but deltas must still be computed as above). However,
you have to make sure the (a,b,c) values you compute at
the vertices are clamped themselves, which is not too hard
by a bit more complicated than clamping (u,v) values.
TEX
TE Out of This Domain -- Zero's Paradox
T
Divides by zero are ugly. We programmers don't
like them. If this were an ideal world (a quick
nod to mathematicians and some physicists), the
texture mapping equations would be divide-by-zero-free.
Unfortunately, it's a repercussion of the exact
same problem as above that you can bump into them.
Remember above, I noted that it's possible to
get (u,v) pairs with a value just outside of the
0..1 range, because a pixel we're texture mapping
isn't even in the polygon?
Well, even worse, it's possible for this pixel,
which isn't in the polygon, to be along the horizon
line (vanishing point) for the polygon. If this
happens, your Y value (sorry, Z for most of you)
would be infinite if you tried to compute the 3D
coordinates from the screen coordinates; and in
the (u,v) computation, you end up with a 0 value
for c. Since u = a/c, blammo, divide by 0.
Well, the solution is simple. Test if c is 0,
and if it is, don't divide. But what _should_ you do?
Well, let's look at an "even worse" case. Suppose
the pixel is so far off the polygon it's across the
horizon line. In this case, we'll end up with c having
the "wrong" sign, and while our divide won't fault on
us, our u&v values will be bogus.
What do we do then?
We can't clamp our a&b&c values very easily.
Fortunately, it turns out we don't have to. If
this happens, it means the edge of the polygon
must be very close to the horizon, or the viewer
must be very, very flat to the polygon (if you know
what I mean). If so, the viewer can't really tell
what should be "right" for the polygon, so if we
screw up the u&v values, it really doesn't matter.
So the answer is, don't worry if c gets the
wrong sign, and if c comes out to be 0, use any
value for u&v that you like--(0,0) makes an obvious
choice.
I've never had a serious problem with this, but
it is possible that this could actually give you some
pretty ugly results, if, say, two corners of a polygon
both "blew up", and you treated them both as being
(0,0). It can also cause problems with wraparound
polygons not repeating the right amount.
TEX
TE Do the Dog
T
Most polygon 3D graphics engines probably use
the painter's algorithm for hidden surface removal.
You somehow figure out what order to paint the polygons
in (depth sort, BSP trees, whatever), and then paint
them back-to-front. The nearer polygons obscure the
farther ones, and voila!, you're done.
This works great, especially in a space combat
simulator, where it's rare that you paint lots of pixels.
You can texture map this way, too. For example,
Wing Commander II doesn't texture map, but it does
real time rotation, which involves essentially the
same inner loop. Wing Commander II is fast--until
a lot of ships are on the screen close to you, at which
point it bogs down a bit.
If you care about not slowing down too much in the above
case, or you want to do an "indoor" renderer with lots of
hidden surfaces, you'll find that with texture mapping,
you can ill-afford to use the painter's algorithm.
You pay a noticeable cost for every pixel you texture
map. If you end up hiding 80% of your surfaces (i.e. there
are five "layers" everywhere on the screen), you end up
"wasting" 80% of the time you spend on texture mapping.
To prevent this, you have to use more complex methods
of hidden surface removal. These will probably slow you down
somewhat, but you should make up for it with the gain in texture
mapping.
The essential idea is to only texture map each screen pixel once.
To do this, you do some sort of "front-to-back" painting, where
you draw the nearest surface first. Any pixel touched by this
surface should never be considered for drawing again.
There are many ways to do this. You can process a single
scanline or column at a time and use ray-casting or just
"scanline processing", then resolve the overlap between the
runs with whatever method is appropriate. You can stay
polygonal and maintain "2D clipping" information (a data
structure which tracks which pixels have been drawn so far).
Beyond getting a fast inner loop for texture mapping,
getting a fast hidden-surface-removal technique (and a fast
depth-sorting technique if appropriate) is probably the
next most crucial thing for your frame rate.
But the details are beyond the scope of this article.
Note that if you attempt to use a Z-buffer, you will
still end up paying all of the costs of texture mapping for
every forward-facing polygon (or at least 50% of them if
you get really tricky; if you get really, really tricky,
the sky's the limit.) I strongly doubt that any PC game
now out, or that will come out in the next year, will
render full-screen texture mapping through a Z-buffer.
(Superimposing a rendered image on a Z-buffered background
is a different issue and is no doubt done all the time.)
TEXTURE
TEXUE
TXR 4. The Complexities
X
In this section we will discuss lots of miscellaneous
topics. We'll look at some more optimizations, such as
considerations for dealing with slow VGA cards, and how to
texture map arbitrarily-angled polygons without doing two
divides per pixel. We'll talk about a technique that lets
you use textures with high-frequency components, and one way
to integrate lighting into texture-mapping.
TEX
TE Arbitrarily-Angled Polygons
T
First suggestion: Don't. Set up your world to
have all (or mostly) walls and floors. Supporting
arbitrarily-angled polygons is going to slow you
down, no matter what.
The original texture mapping loop, which supported
arbitrarily-angled polygons, required two divides per
pixel. We don't have to go that slow, but we'll never
go as fast as DOOM-style rendering can go. (However,
as you start to use more sophisticated lighting algorithms
in your inner loop, the cost of handling arbitrarily-
angled polygons may start to become less important.)
There is one way to texture map such polygons
"perfectly" without two divides per pixel, and a
host of ways to do it "imperfectly". I'll discuss
several of these ways in varying amounts of detail.
Your best bet is to implement them all and see
which ones you can get to run the fastest but still
look good. You might find that one is faster for
some cases but not for others. You could actually
have an engine which uses all the methods, depending
on the polygon it's considering and perhaps a "detail"
setting which controls how accurate the approximations
are.
The "perfect" texture mapping algorithm is
described in another article, "Free-direction texture
mapping". I'll summarize the basic idea and the
main flaw. The basic idea is this. For ceilings
and walls, we were able to walk along a line on
the screen for which the step in the "c" parameter
was 0; this was a line of "constant Z" on the
polygon.
It turns out that every polygon has lines of
"constant Z"--however, they can be at any angle,
not necessarily vertical or horizontal.
What this means, though, is that if you walk
along those lines instead of walking along a horizontal
or vertical, you do not need a divide to compute your
texture map coordinates, just deltas.
The details can be found in the other article.
The slope of the line to walk on the screen is something
like Hc/Vc.
Note, however, that the "DOOM" approach was _just_
an optimization for a special case. The wall & ceiling
renderers produce exactly the same results as a
perfect texture mapper, for the polygons that they
handle (ignoring rounding errors and fixed-point precision
effects). This is not true for the "free-direction"
texture mapper. While there is a line across the
screen for which the polygon has constant Z,
you cannot walk exactly along that line, since you
must step by pixels. The end result is that while
in the texture map space, you move by even steps,
in the screen space, you move with ragged jumps.
With perfect texture mapping, you always sample
from the texture map from the position corresponding
to the top-left/center of each pixel. With the
free-direction mapper, you sample from a "random"
location within the pixel, depending on how you're
stepping across the screen. This "random" displacement
is extremely systematic, and leads to a systematic
distortion of the texture. I find it visually
unacceptable with high-contrast textures, compared to
perfect texture mapping, but you should try it and decide
for yourself. The technically inclined should note that
this is simply the normal "floor" renderer with an extra
2D skew, and that while 2D skews are trivial, they are
non-exact and suffer from the flaw described above.
The only other alternative for arbitrarily-angled
polygons is to use some kind of approximation. We
can characterize u and v as functions of i (the horizontal
screen position; or use 'j' if you wish to draw columns);
for instance, u = a / c, where a = q + i*Ha, c = p + i*Hc.
So we can say u(i) = (q + i*Ha) / (r + i*Hc).
Now, instead of computing u(i) exactly for each i,
as we've done until now, we can instead compute some
function u'(i) which is approximately equal to u(i) and
which can be computed much faster.
There are two straightforward functions which we
can compute very fast. One is the simple linear
function we used for DOOM-style mapping, u'(x) = r + x*s.
Since the function we're approximating is curved (a
hyperbola), a curved function is another possibility,
such as u'(x) = r + x*s + x^2*t. (SGI's Reality Engine
apparently uses a cubic polynomial.)
If you try both of these approximations on a very
large polygon at a sharp angle, you will find that
they're not very good, and still cause visible
curvature. They are, of course, only approximations.
The approximations can be improved with a simple
speed/quality trade-off through subdivision. The
idea of subdivision is that the approximation is
always of high quality for a small enough region,
so you can simply subdivide each region until the
subregions are small enough to have the desired
quality.
There are two ways to subdivide. One simple way
is to subdivide the entire polygon into smaller
polygons. This should be done on the fly, not
ahead of time, because only polygons that are at
"bad" angles need a lot of subdivision. After
dividing a polygon into multiple smaller ones,
render each one seperately. Use the original
P, M, and N values for all of the new polygons
to make the texture remain where it should be
after subdivision.
The (probably) better way to subdivide is to
subdivide runs instead of polygons, and so I'll
discuss this in more detail. The essential thing
is that to do an approximation, you evaluate the
original function at two or more locations and
then fit your approximate function to the computed
values. One advantage of run subdivision is that
you can share points that you evaluated for one
subrun with those of the next.
First lets turn back to the two approximations
under consideration. The first is what is called
"bilinear texture mapping", because the function
is linear and we're tracking two ("bi") values.
To use this method, we compute the function at
both endpoints: u1 = u(start_x), u2 = u(end_x).
Then we compute our start and step values. To
keep things simple, I'm going to assume the approximation
function u'(x) is defined from 0..end_x-start_x, not
from start_x..end_x.
So, the linear function u'(x) = r + s*x, where
u'(0) = u1 and u'(end_x - start_x) = u2 is met by
letting r = u1, s = (u2 - u1) / (end_x - startx).
Now, suppose our run goes from x = 10 to x = 70.
If we evaluate u(10), u(20), u(30), u(40),... u(70),
then we can have six seperate sections of bilinear
texture mapping.
For a quadratic, there are several ways to compute
it. One way is to compute an additional sample in the
middle; u3 = u((start_x + end_x)/2). Then we can
fit u1,u2, and u3 to u'(x) = r + s*x + t*x^2 with:
len = (end_x - start_x)
k = u1 + u2 - u3*2
r = u1
s = (u2 - u1 - 2*k)/len
t = 2*k / len^2
Note that to use this in code, you cannot simply
use a loop like this:
r += s;
s += t;
because the r,s, and t values aren't correct
for discrete advancement. To make them correct,
do this during the setup code:
R = r
S = s + t
T = 2*t
Then the loop of (...use R..., R += S, S += T) will work correctly.
The biquadratic loop will be slower than the linear
loop, but will look better with fewer subdivisions. You
can share one of the endpoints from one biquadratic
section to the next. Note, though, that you require twice
as many calculations of u&v values for the same number of
subdivisions with a biquadratic vs. a bilinear.
Another thing to do is to choose how to subdivide the run
more carefully. If you simply divide it in half or into quarters,
you'll discover that some of the subruns come out looking better
than others. So there are some things you can do to improve the
subdivision system. Another thing you can do is to try to make
most of your subruns have lengths which are powers of two. This
will let you use shifts instead of divides when computing r,s, and
t, which cuts down on your overhead, which lets you use more
subdivisions and get the same speed.
Note something very important. Subdivision increases the
overhead per run; biquadratic and other things increase the
cost of the inner loop. Before you go crazy trying to optimize
your arbitrarily-angled polygon renderer, make sure you're
rendering some "typical" scenes. The "right" answer is going
to depend on whether you have lots of very shorts runs or
fewer, longer runs. If you optimize based on a simple test
case, you may end up suboptimal on the final code.
You probably still want to have both a column-based and a
row-based renderer, and use whichever one the polygon is
"closer to" (e.g. if Hc is closer to 0 than Vc, use the row-based).
Note that the free-direction renderer looks its worst (to me)
for very small rotations, i.e. when Hc or Vc are very close to 0.
Since in these cases not much subdivision is needed, even if you
choose to use a free-direction mapper as your primary renderer,
you might still want to have "almost wall" and "almost floor"
renderers as well.
Finally, there is one more approximation method you can use,
which is faster than any of the ones discussed so far, but is
simply totally and utterly wrong. This is the approach used
by Michael Abrash in his graphics column in Dr. Dobbs. While
it's quite wrong, it works on polygons which are entirely
constant Y (sorry, Z), and can be a noticeable speedup.
What you do is 2D (instead of 3D) interpolation. You mark
each vertex with its coordinates in the texture map. Then when
you scan convert, you interpolate these values between vertices
on the edges of your runs. Thus, scan conversion will generate
runs with (u,v) values for the left and right end. Now simply
compute (du,dv) by subtracting and dividing by the length (no
clamping will be necessary), and use your fast bilinear inner
loop. When combined with 3D polygon subdivision, this approach
can actually be useful.
A cheat:
When the player is moving, set your internal quality
settings a little lower. When the player stops, switch
back to the normal quality; if the player pauses the game,
render one frame in normal quality.
If done right, you can get a small boost to your fps
without anyone being able to tell that you did it. You
may have to use normal quality if the player is only
moving very slowly, as well.
Note that while this may sound like an utterly cheap
trick just to improve the on-paper fps number, it's actually
quite related to the "progressive refinement" approach used
by some real VR systems (which, when the viewer isn't moving,
reuse information from the previous frame to allow them to
draw successive frames with more detail).
There are a number of ways of improving this cheat
intelligently. If the player is moving parallel to a polygon,
that polygon will tend to be "stably" texture mapped (similar
mapping from frame to frame). If there is any distortion from
your approximation, this will be visible to the player. So
this means a rule of thumb is to only cheat (draw with
above-average distortion) on polygons that are not facing
parallel to the direction of motion of the player.
TEX
TE Light My Fire
T
If you're texture mapping, it's generally a good idea
to light your polygons. If you don't light them, then it
may be difficult to see the edge between two walls which
have the same texture (for instance, check out the "warehouse"
section of registered DOOM, which is sometimes confusing
when a near crate looks the same color as a far crate).
Lighting is actually pretty straightforward, although
you take a speed hit in your inner loop. I'm not going
to worry about actual lighting models and such; see other
articles for discussion on how to do light-sourced polygons.
Instead I'm going to assume you've computed the lighting
already. We'll start with "flat-run" shading, wherein an
entire run has the same intensity of light falling on it.
DOOM uses flat-run shading. A given polygon has a certain
amount of light hitting it, which is the same for the entire
polygon. In addition, each run of the polygon is sort-of
lit by the player. Since runs are always at a constant
depth, you can use constant lighting across the run and
still change the brightness with distance from the player
(DOOM uses something that resembles black fog, technically).
So the only real issue is _how_ you actually get the
lighting to affect the texture. Several approaches are
possible, but the only one that I think anyone actually uses
is with a lighting table.
The lighting table is a 2D array. You use the light
intensity as one index, and the pixelmap color as the
other index. You lookup in the table, and this gives
you your final output color to display. (With two
tables, you can do simple dithering.) So the only thing
you have to do is precompute this table.
Basically, your inner loop would look something like this:
...compute light...
for (i=start_x; i <= end_x; ++i) {
color = texture[v >> 16][u >> 16];
output = light_table[light][color];
screen[i] = output;
u += du;
v += dv;
}
The next thing to consider is to Gouraud shade your
texture map. To do this, you need to compute the light
intensity at the left and right edge of the run; look
elsewhere for more details on Gouraud shading.
Once you've got that, you just do something like this:
z = light1 << 16;
dz = ((light2 - light1) << 16) / (end_x - start_x);
for (i=start_x; i <= end_x; ++i) {
color = texture[v >> 16][u >> 16];
output = light_table[z >> 16][color];
screen[i] = color;
u += du;
v += dv;
z += dz;
}
Note that you shouldn't really do this as I've written the
code. light1 and light2 should be calculated with 16 bits of extra
precision in the first place, rather than having to be shifted
left when computing z. I just did it that way so the code
would be self-contained.
I'm going to attempt to give a reasonably fast assembly
version of this. However, there's a big problem with doing
it fast. The 80x86 only has one register that you can
address the individual bytes in, and also use for indexing--BX.
This means that it's a real pain to make our inner loop alternate
texture map lookup and lighting fetch--whereas it's almost
trivial on a 680x0. I avoid this somewhat by processing two
pixels at a time; first doing two texture map lookups, then
doing two lighting lookups.
Here's a flat-shading inner loop. I'm doing this code off the
top of my head, so it may have bugs, but it's trying to show
at least one way you might try to do this. Since I use BP,
I put variables in the FS segment, which means DS points
to the texture, GS to the lighting table.
mov ch,fs:light
adc ax,ax
loop8 shr ax,1 ; restore carry
mov cl,[bx] ; get first sample, setting up cx for color lookup
adc edx,esi ; update v-high and u-low
adc ebx,ebp ; update u-high and v-low
mov bh,dl ; move v-high into tmap lookup register
mov ah,[bx] ; get second sample, save it in ah
adc edx,esi
adc ebx,ebp
mov dh,bl ; save value of bl
mov bx,cx ; use bx to address color map
mov al,gs:[bx] ; lookup color for pixel 1
mov bl,ah ; switch to pixel 2
mov ah,gs:[bx] ; lookup color for pixel 2
mov es:[di],ax ; output both pixels
mov bl,dh ; restore bl from dh
mov bh,dl
adc ax,ax ; save carry so we can do CMP
add di,2
cmp di,fs:last_di ; rather than having to decrement cx
jne loop8
For a Gouraud shading inner loop, we can now have three
different numbers u, v, and z, which we're all adding at every
step. To do this, we use THREE adc, and we have to shuffle
around which high-bits correspond to which low-bits in a
complex way. I'll leave you to figure this out for yourself,
but here's an attempt at the inner loop.
loop9 shr ax,1 ; restore carry
mov al,fs:[bx] ; get first sample
mov ah,cl ; save away current z-high into AH
; this makes AX a value we want to lookup
adc edx,esi ; update v-high and u-low
adc ebx,ebp ; update u-high and z-low
adc ecx,z_v_inc ; update z-high and v-low
mov bh,dl ; move v-high into tmap lookup register
mov ch,fs:[bx] ; get second sample, save it in ch
mov dh,bl ; save value of bl
mov bx,ax
mov al,gs:[bx] ; lookup first color value
mov bl,ch
mov bh,cl
mov ah,gs:[bx] ; lookup second color value
mov es:[di],ax ; output both pixels
mov bl,dh ; restore bl from dh
adc edx,esi
adc ebx,ebp
adc ecx,z_v_inc
mov bh,dl
adc ax,ax ; save carry
add di,2
cmp di,last_di ; rather than having to decrement cx
jne loop9
Notice that both of these loops are significantly slower
than the original loop. I'm not personally aware of any
generally faster way to do this sort of thing (but the code
can be tweaked to be faster). The one exception is
that for flat-run shading, you could precompute the entire
texture with the right lighting. This would require a lot
of storage, of course, but if you view it as a cache, it
would let you get some reuse of information from frame to
frame, since polygons tend to be lit the same from frame to
frame.
Finally, here's a brief discussion of transparency.
There are two ways to get transparency effects. The first
one is slower, but more flexible. You use _another_ lookup
table. You have to paint the texture that is transparent
after you've drawn things behind it. Then, in the inner
loop, you fetch the texture value (and light it) to draw.
Then you fetch the pixel that's currently in that location.
Lookup in a "transparency" table with those two values as
indices, and write out the result. The idea is that you
do this: table[new][old]. If new is a normal, opaque,
color, then table[new][old] == new, for every value of old.
If new is a special "color" which is supposed to be transparent,
then table[new][old] == old, for every value of old. This causes
old to show through. In addition, you can have translucency
effects, where table[new][old] gives a mixture of the colors
of old and new. This will let you do effects like the
translucent ghosts in the Ultima Underworlds.
However, the above approach is extremely slow, since you
have to load the value from the pixel map and do the extra
table lookup. But it works for arbitrary polygons. DOOM
only allows transparency on walls, not on ceilings and floors.
Remember we noticed that the special thing about walls is
that u is constant as you draw a column from a wall; you
are walking down a column in the texture map at the same
time you are drawing a column on screen. What this means
is that you can use a data structure which encodes where
the transparency in each column of the texture map is, and
use that _outside_ the inner loop to handle transparency.
For example, your data structure tells you that you have
a run of 8 opaque pixels, then 3 transparent pixels, then
5 more opaque ones. You scale 8, 3, and 5 by the rate at
which you're walking over the textures, and simply treat
this as two seperate opaque runs.
The details of this method depend on exactly how you're
doing your hidden surface removal, and since it doesn't
generalize to floors&ceilings, much less to arbitrarily
angled polygons, I don't think going into further detail
will be very useful (I've never bothered writing such a
thing, but I'm pretty sure that's all there is to it).
TEX
TE The Postman Always Rings Twice
T
If you're going to write to a slow 16-bit VGA card, you
should try your darndest to always write 2 pixels at a time.
For texture mapping, your best bet is to build your screen
in a buffer in RAM, and then copy it to the VGA all at once.
You can do this in Mode 13h or in Mode X or Y, as your heart
desires. You should definitely do this if you're painting
pixels more than once while drawing.
If, however, you wish to get a speedup by not paying
for the extra copy, you might like to write directly to the
VGA card from your inner loop.
You might not think this is very interesting. If the
write to the screen buffer in regular RAM is fast, how much
can you gain by doing both steps at once, instead of splitting
them in two?
The reason it is interesting is because the VGA, while
slow to accept multiple writes, will let you continue doing
processing after a single write. What this means is that
if you overlap your texture mapping computation with your
write to the VGA, you can as much as double your speed on
a slow VGA card. For example, the fastest I can blast my
slow VGA card is 45 fps. I can texture map floor-style directly
to it at 30 fps. If I texture map to a memory buffer,
this is still somewhat slow, more than just the difference
between the 30 and 45 fps figures. Thus, my total rate if
I write to an offscreen buffer drops as low as 20 fps, depending
on exactly what I do in the texture map inner loop.
Ok, so, now suppose you've decided it might be a speedup
to write directly to the VGA. There are two problems. First
of all, if you're in mode X or Y, it's very difficult to
write two bytes at a time, which is necessary for this
approach to be a win. Second of all, even in mode 13h, it's
difficult to write two bytes at a time when you're drawing
a column of pixels.
I have no answer here. I expect people to stick to
offscreen buffers, or to simply process columns at a time
and write (at excruciatingly slow rates on some cards) to
the VGA only one byte at a time.
One option is to set up a page flipping mode 13h (which
is possible on some VGA cards), and to paint two independent
but adjacent columns at the same time, so that you can write
a word at a time. I have a very simple demo that does the
latter, but it's not for the faint of heart, and I don't
think it's a win once you have a lot of small polygons.
Another answer is to have a DOOM-style "low-detail"
mode which computes one pixel, duplicates it, and always
writes both pixels at the same time.
A final answer is just to ignore the market of people
with slow VGA cards. I wouldn't be surprised if this
approach was commonplace in a year or two. But if you do
so with commercial software, please put a notice of this
requirement on the box.
TEX
TE Mipmapping (or is it Mip-Mapping?)
T
Mipmapping is a very straightforward technique that
can be used to significantly improve the quality of your
textures, so much so that textures that you could not
otherwise use because they look ugly become usable.
The problem that mipmapping addresses is as follows.
When a texture is far in the distance, such that its
on-screen size in pixels is significantly smaller than
its actual size as a texture, only a small number of
pixels will actually be visible. If the texture contains
areas with lots of rapidly varying high contrast data,
the texture may look ugly, and, most importantly,
moire artifacts will occur. (To see this in DOOM, try
shrinking the screen to the smallest setting and going
outside in shareware DOOM. Many of the buildings will
show moire patterns. In registered DOOM, there is
a black-and-blue ceiling pattern which has very bad
artifacts if it is brightly lit. Go to the mission
with the gigantic round acid pool near the beginning.
Cheat to get light amplification goggles (or maybe
invulnerability), and you'll see it.)
Mipmapping reduces these artifacts by precomputing
some "anti-aliased" textures and using them when the
textures are in the distance.
The basic idea is to substitute a texture map half as
big when the polygon is so small that only every other
pixel is being drawn anyway. This texture map contains
one pixel for every 2x2 square in the original, and is
the color average of those pixels.
For a 64x64 texture map, you'd have the original
map, a 32x32 map, a 16x16 map, an 8x8 map, etc.
The mipmaps will smear out colors and lose details.
You can best test them by forcing them to be displayed
while they're still close to you; once they appear to
be working, set them up as described above.
Mipmapping causes a somewhat ugly effect when you
see the textures switch from one mipmap to the next.
However, especially for some textures, it is far less
ugly than the effect you would get without them.
For example, a fine white-and-black checkerboard
pattern (perhaps with some overlaid text) would look
very ugly without mipmapping, as you would see random
collections of white and black pixels (which isn't too
bad), and you would see curving moire patterns (which
is). With mipmapping, at a certain distance the whole
polygon would turn grey.
I do not believe any existing games for the PC
use mipmapping. However, examining the data file
for the Amiga demo version of Legends of Valour showed
smaller copies of textures, which made it look like
mipmapping was being used.
Mipmapping requires 33% extra storage for the
extra texture maps (25% for the first, 25% of 25%
for the second, etc.).
This may also be a good idea for 2D bitmaps which
are scaled (e.g. critters in Underworld & DOOM, or
ships in Wing Commander II--although none of those
appeared to use it.)
SGI's Reality Engine does mipmapping. Actually,
it does a texturemap lookup on two of the mipmaps,
the "closer" one and the "farther" one, and uses
a weighted average between them depending on which
size is closer to correct. (The RE also does
anti-aliasing, which helps even more.)
TEXTURE
TEXUE
TXR Where Do We Go From Here?
X
The above discussion mostly covers what is basically
the state of the art of texture mapping on the PC. Hopefully
in the future every game will be at least as fast as the
inner loops in this article allow.
As long as people want full-screen images, it'll
be a while before we have enough computational power
to do more than that. But if we did have more power,
what else could we do with it?
o Better lighting
o Colored lighting (requires complex lookup tables)
o Phong shading (interpolation of normals--one sqrt() per pixel!)
o Higher resolution (640x400, or 640x400 and anti-alias to 320x200)
o A lot more polygons
o Bump mapping (can be done today with huge amounts of precomputation)
o Curved surfaces
