Encyclopedia of Graphics File Formats Companion/GFF_CD.ISO/formats/radiance/spec/tutorial.txt

Text File | 1994-06-01 | 29KB | 768 lines

                        RADIANCE Tutorial

                            Greg Ward
                   Lawrence Berkeley Laboratory


1. Introduction

RADIANCE is a lighting simulation program that synthesizes images
from 3-dimensional geometric models of physical spaces. The input
model describes each surface's shape, size, location and
composition. A model may contain many thousands of surfaces, and
is often produced by a separate CAD program. In addition to
arbitrary (planar) polygons, RADIANCE directly models spheres and
cones. Generator programs are provided for the creation of more
complex shapes from these basic surface primitives. Examples
include boxes, prisms and surfaces of revolution. A
transformation utility permits the simple duplication of objects
and hierarchical construction of a scene.

This tutorial assumes a certain familiarity with the UNIX
operating system and its text editing facilities. Ask your system
administrator for a basic introduction.

The RADIANCE reference manual will be required to understand the
following examples of scene creation and program interaction.
Text in italics is user input.


2. Input of a Simple Room

In this example, we will use a text editor to create the input
for a simple room containing a box, a ball, and a light source.
In most applications, a CAD system would be used to describe a
scene's geometry, which would then be combined with surface
materials, light fixtures, and (optionally) furniture. To get a
more intimate understanding of the input to RADIANCE, we will
start without the advantages of a CAD program or an object
library.

The scene we will be working towards is shown in Figure 1. It is
usually helpful to start with a simple drawing showing the
coordinate axis and the relative locations of major surfaces.

The minimum input required to get an image is a source of
illumination and an object to reflect light to the "camera".* We
will begin with two spheres, one emissive and one reflective.
First we define the materials, then the spheres themselves.
Actually, the order is only important insofar as each modifier
definition (i.e. material) must appear before its first
reference. (Consult the RADIANCE manual for an explanation of the
primitive types and their parameters.) Start your favorite text
editor ("vi" in this example) to create the following file called
"room.rad":


*In fact, a RADIANCE renderer can be thought of an invisible
camera in a simulated world.

     % vi room.rad

     #
     # My first scene.
     #

     # this is the material for my light source:

     void light bright
     0
     0
     3 100 100 100

     # this is the material for my test ball:

     void plastic red_plastic
     0
     0
     5 .7 .05 .05 .05 .05

     # here is the light source:

     bright sphere fixture
     0
     0
     4 2 1 1.5 .125

     # here is the ball:

     red_plastic sphere ball
     0
     0
     4 .7 1.125 .625 .125

Now that we have a simple scene description, we can look at it
with the interactive viewing program, rview. First, however, we
must create the "octree" file that will be used to accelerate the
rendering process. To accomplish this, type the following
command:

     % oconv room.rad > test.oct

Note that the suffixes ".rad" and ".oct" are not enforced by the
program, but are merely a convenience to aid the user in
identifying files later. The command getinfo can be used to get
information on the origin of binary (unviewable) files created by
RADIANCE utilities. Try entering the command:

     % getinfo test.oct

The usefulness of such a function will be apparent when you find
yourself with a dozen files named "test.pic".

To make an image of our scene, we must select a suitable set of
view parameters telling RADIANCE where to point its camera. To
simplify our example, we will use the same starting position for
all our renderings, and change views only once rview is started:

     % rview -vp 2.25 .375 1 -vd -.25 .125 -.125 -av .5.5.5
          test.oct

The "-vp" option gives the view point, and the "-vd" option gives
the view direction vector. The "-av" option specifies the amount
of light globally present in the scene, permitting portions of
the scene that are not illuminated directly to be visible. Rview
has many more options, and their default values may be discovered
using:

     % rview -defaults

You should start to see an image of a red ball forming on your
scene. Take this opportunity to try each of rview's commands, as
described in the manual. If you make a mistake in a view
specification, use the last command to get back to where you
were. It is probably a good idea to save your favorite view using
the rview command:

     : view default.vp

You can create any number of "viewfiles" with this command, and
retrieve them with:

     :last viewfile

If you look around enough, you may even be able to see the light
source itself. Unlike many rendering programs, the light sources
in RADIANCE are visible objects. This illustrates the philosophy
behind the program, which is the simulation of physical spaces.
Since it is not possible to create an invisible light source in
reality, there is no reason to do it in simulation.

Still, there is no guarantee that the user will create physically
meaningful descriptions. For example, we have just floated a red
ball next to a light source somewhere in intergalactic space. In
the interest of making this scene more realistic, let's enclose
the light and ball in a room by adding the following text to
"room.rad":

     % vi room.rad

     # the wall material:

     void plastic gray_paint
     0
     0
     5 .5 .5 .5 0 0

# a box shaped room:

!genbox gray_paint room 3 2 1.75 -i

The generator program genbox is just a command that produces a
RADIANCE description, and it is executed when the file is read.
It is more convenient than specifying the coordinates of four
vertices for each of six polygons, and can be changed later quite
easily. (See the manual genbox page for further details.)

You can now look at the modified scene, but remember first to
regenerate the octree:

          % oconv room.rad > test.oct
          % rview -vf default.vp-av .5 .5 .5 test.oct

This is better, but our ball and light source are still floating,
which is an unrealistic condition for most rooms. Let's put a box
under the table, and a rod to suspend the light from the ceiling:

     # a shiny blue box:

     void plastic blue_plastic
     0
     0
     5 .1 .1 .6 .05 .1

     !genbox blue_plastic box .5 .5 .5 \xform -rz15 -t .5 .75 0

     # a chrome rod to suspend the light from the ceiling:

     void metal chrome
     0
     0
     5 .8 .8 .8 .9 0

     chrome cylinder fixture_support
     0
     0
     7
          2    1    1.5
          2    1    1.75
          .05

Note that this time the output of genbox was "piped" into another
program, xform. Xform is used to move, scale and rotate RADIANCE
descriptions. Genbox always creates a box in the positive octant
of 3-space with one corner at the origin. This was what we wanted
for the room, but we wanted the box moved away from the wall and
rotated slightly. First we rotated the box 15 degrees about the
z-axis (pivoting on the origin), then translated the corner from
the origin to (.5,.75,0). By no small coincidence, this position
is directly under our original ball.

After viewing this new arrangement, you can try changing some of
the materials -- here are a few examples:

     # solid crystal:

     void dielectric crystal
     0
     0
     5 .5 .5 .5 1.5 0

     # dark brown:

     void plastic brown
     0
     0
     5 .2 .1 .1 0 0

     # light gray:

     void plastic white
     0
     0
     5 .7 .7 .7 0 0

To change the ball from red_plastic to the crystal defined above,
simply replace red plastic sphere ball with crystal sphere ball.
Note once again that the definition of the new materials must
precede any references to them. Changing the materials for the
floor and ceiling of the room is a little more difficult. Since
genbox creates six rectangles, all using the same material, it is
necessary to replace the command with its output before we can
make the required changes. To do this, enter the command
directly:

     % genbox gray_paint room 3 2 1.75 -i>> room.rad

The double arrow ">>" causes the output to be "appended" to the
end of the file, rather than overwriting its contents. Now edit
the file and change the ceiling material to "white", and the
floor material to "brown". (Hint: the ceiling is the polygon
whose z coordinates are all high. And don't forget to remove the
original genbox command from the file!)

Once you have chosen a nice view, you can generate a
high-resolution image in batch mode using the rpict command:

     % rpict -vf myview -av .5 .5 .5 test.oct > test.pic & [PID]

The ampersand "&" causes the program to run in the background, so
you can log out and go home while the computer continues to work
on your picture. The bracketed number [PID] printed by the
C-shell command interpreter is the process id that can be used
later to check the progress or kill the program. This number can
also be determined by the ps command:

     % ps

The number preceding the rpict command is the process id. If you
want to kill the process later, use the command:

     % kill PID

If you only want a progress report, use the form:

     % kill -ALRM PID

This sends an "alarm" signal to rpict, which then prints out the
percentage completion. Note that this is a special feature of
rpict, and will not work with most programs. Also note that this
works only for the current login session. If you log on later on
a different terminal (or window), rpict will not send the report
to the correct place. It is usually a good idea, therefore, to
give rpict an error file argument if it is running a long job:

     % rpict -e errfile ...

Now sending an alarm signal will result in rpict reporting to the
end of the specified error file. Alternatively, you can use the
-t option to generate reports automatically at regular intervals.
You can check the reports at any time by printing the file:

     % cat errfile

This file will also contain a header, and any errors that
occurred.


3. Addition of a Window

Adding a window to the room requires two basic steps. The first
step is to cut a hole in the wall an put in a piece of glass. The
second step is to put something outside to make the view worth
having. Since there are no explicit holes allowed in RADIANCE
polygons, we use the trick of coincident edges (making a seam) to
give the appearance of a hole. The new polygon for the window
wall is shown in Figure 2.

To create the window wall, change the appropriate polygon in the
scene file (modified part in italics). If you haven't done so
already, follow the instructions in the previous section to
change the genbox command in the file to its corresponding
polygons.

% vi room.rad

gray_paint polygon room.5137
0
0
30

     3     2     1.75
     3     2     0
     3     0     0
     3     0     1.75
     3     .625  1.75
     3     .625  .625
     3     1.375 .625
     3     1.375 1.375
     3     .625  1.375
     3     .625  1.75

Next, create a separate file for the window. (The use of separate
files is desirable for parts of the scene that will be
manipulated independently, as we will see in a moment.)

     % vi window.rad

     # an 88% transmittance glass window has a transmission of
        96%:

     void glass window_glass
     0
     0

     3 .96 .96 .96

     window_glass polygon window
     0
     0
     12
          3     .625      1.375
          3     1.375     1.375
          3     1.375     .625
          3     .625      .625

The vertex order is very important, especially for polygons with
holes. Normally, vertices are listed in counterclockwise order as
seen from the front. However, the hole of a polygon has its
vertices listed in the opposite order. This insures that the seam
does not cross itself.

The next step is the description of the scene outside the window.
A special purpose generator, gensky, will create a description of
the sun and sky which will be stored in a separate file. The
arguments to gensky are the month, day and hour (local standard
time). The following command produces a description for 10:00
solar time on March 20th at latitude of 40 degrees:

     % gensky 3 20 10 -o 0 -m 0 -a 40 > sky.rad


The file "sky.rad" contains only a description of the sun and the
sky distribution. The actual sky and ground are still undefined,
so we will create another short file containing a generic
background:

     % vi outside.rad

     #
     # A standard sky and ground to follow a gensky sun and sky
       distribution.
     #

     skyfunc glow sky_glow
     0
     0
     4 .9 .9 1.15 0

     sky_glow source sky
     0
     0
     4 0 0 1 180

     skyfunc glow ground_glow
     0
     0
     4 1.4 .9 .6 0
     ground_glow source ground
     0
     0
     4 0 0-1 180

We can now put these elements together in one octree file using
     oconv:

     % oconv outside.rad sky.rad window.rad room.rad > test.oct

Note that the above command results in the following error
     message:

     oconv: fatal - (outside.rad): undefined modifier "skyfunc"

The modifier is undefined because we put "outside.rad", which
uses skyfunc before "sky.rad" where skyfunc is defined. It is
therefore necessary to change the order of the files so that
skyfunc is defined before it is used:

     % oconv sky.rad outside.rad window.rad room.rad > test.oct

Now let's look at our modified scene, using the same rview
      command as before:

     % rview -vf default.vp-av .5 .5 .5 test.oct

As you look around the scene, you will need to adjust the
exposure repeatedly to be able to see detail over the wide
dynamic range now present. To do this, wait a few seconds after
choosing each new view and enter the command:

     : exposure 1

or simply:

     : e 1

All commands in rview can be abbreviated by one or two letters.
Additional control over the exposure is possible by changing the
multiplier factor to a value greater than one to lighten or less
than one to darken. It is also possible to use absolute settings
and spot normalization. See the rview manual entry for details.

You may notice that other than a patch of sun on the floor, the
window does not seem to illuminate the room. In RADIANCE, certain
surfaces act as light sources and others do not. Whether or not a
surface is a light source is determined by its material type.
Surfaces made from the material types light, illum, spotlight and
(sometimes) glow will act as light sources, whereas surfaces made
from plastic, metal, glass and other material types will not. In
order for the window to directly illuminate the room, it is
therefore necessary to change its material type. We will use the
type illum because it is specially designed for "secondary" light
sources such as windows and other bright objects that are not
merely emitters but have other important visual properties. An
illum will act as a light source for parts of the calculation,
but when viewed directly will appear as if made from a different
material (or disappear altogether).

Rather than modify the contents of "window.rad" which is a
perfectly valid description of a nonsource window, let's create a
new file, which we can substitute during octree creation, called
"srcwindow.rad":

     % vi srcwindow.rad

     #
     # An emissive window
     #

     # visible glass window_glass
     0
     0
     3 .96 .96 .96

     # window distribution function, including angular
          transmittance

     skyfunc brightfunc window_dist
     2 winxmit winxmit.cal
     0
     0

     # illum for window, using 88% transmittance at normal
          incidence:

     window_dist illum window_illum
     1 window_glass
     0
     3 .88 .88 .88

     # the source polygon:

     window_illum polygon window
     0
     0
     12
          3     .625  1.375
          3     1375  1.375
          3     1.375 .625
          3     .625  .625

There are a couple of things you should notice in this file. The
first definition is the normal glass type, window_glass, which is
used for the alternate material for the illum window_illum. Next
is the window distribution function, which is the sky
distribution modified by angular transmittance of glass defined
in winxmit.cal. Finally comes the illum itself, which is the
secondary source material for the window.

To look at the scene, simply substitute "srcwindow.rad" for
"window.rad" in the previous oconv command, thus:

     % oconv sky.rad outside.rad srcwindow.rad room.rad >
          test.oct

You can look at the room at different times by changing the
gensky command used to create "sky.rad" and regenerating the
octree. (Although the octree does not strictly need to be
recreated for every change to the input files, it is good to get
in the habit until the exceptions are well understood.)


4. Outside Geometry

If the exterior of a space is not approximated well by an
infinitely distant sky and ground, we can add a better
description to calculate a more accurate window output
distribution as well as a better view outside the window. Let's
add a ground plane and a nearby building to the "outside.rad"
file we created earlier.

     # Terra Firma:

     void plastic ground_mat
     0
     0
     5 .28 .18 .12 0 0

     ground_mat ring groundplane
     0
     0
     8

          0    0    -.01
          0    0    1
          0    30

     # A big, ugly, mirrored glass building:

     void mirror reflect20
     0
     0
     3.15 .2 .2

     !genbox reflect20 building 10 10 2 | xform -t 10 5 0

Note that the groundplane was given a slightly negative z value.
This is very important so that the ground does not peek through
the floor we have defined. The material type mirror, used to
define the neighboring structure, is special in RADIANCE.
Surfaces of this type as well as the types prism1 and prism2
participate in something called the "virtual light source"
calculation. In short, this means that the surfaces of the
building we have created will reflect sunlight and any other
light source present in our scene. The virtual light source
material types should be used with care since they can result in
substantial growth in the calculation. It would be a good idea in
the example above to remove the bottom surface of the building
(which cannot be seen from the outside anyway) and change the
roof type to metal or some non-reflecting material. This can be
done using the same manual process described earlier for changing
the room surface materials.

Now that we have a better description of the outside, what do we
do with it? If we simply substitute it in our scene without
changing the description of the window illum, the distribution of
light from the window will be slightly wrong because the
"skybright" function only describes light from the sky and the
ground, not from other structures. Using this approximation might
be acceptable in some cases, but let's assume that accuracy is
one of our main goals in life. There are two ways to an accurate
calculation of light from a window. The first is to treat the
window as an ordinary window and rely on the default
interreflection calculation of RADIANCE, and the second is to use
the program mkillum to calculate the window distribution
separately so we can still treat it as an illum light source.
Let's try them both.


Using the default interreflection calculation is probably easier,
but as we shall see it takes a little longer to get a good result
in this case. To use the interreflection calculation, we first
create an octree using the original glass window and our new
description of the outside:

     % oconv sky.rad outside.rad window.rad room.rad > test.oct

Now we just add a few new options to our rview command like so:

     % rview -vf default.vp -dr 1 -av .5 .5 .5 -ab 1 -ad 200 -as
          80 -ar 80 test.oct

The -dr parameter tells rview how many reflections to allow in
the virtual light source calculation. Because of the geometry of
our scene, there will never be more than one reflection from our
exterior building. The -ab parameter tells rview how many diffuse
interreflections to follow. When this value is 0, there is no
interreflection calculation. By setting this parameter to 1, we
compute the first diffuse interreflection (eg. from the sun patch
to the wall) as well as the contribution from the sky and
exterior objects through the window. The -ad option tells rview
how many rays to use in its initial sampling of the hemisphere
for the diffuse interreflection calculation. We have set this
value high enough so the program can find the relatively small
window and its sun patch. The -as option tells the program how
many rays to send to particularly bright areas of the hemisphere.
Since we know that we will have such bright areas, we set this
value to something which is around half the initial sample. The
-ar option tells rview at what resolution it can relax the
accuracy of the interreflection calculation. This number is
relative to the overall size of our scene, which can be
determined using getinfo on the octree like so:

     % getinfo -d test.oct

This command reveals that our scene has an overall dimension of
60 (due to the large ring we have used for our ground plane). In
order to have good resolution in our interior space, we set the
value so that 60 divided by this parameter is somewhat less than
one quarter the dimension of our interior space. Unfortunately,
many of the calculation parameters used in Radiance are difficult
to set correctly the first time, and require some experience as
well as knowledge about the calculation.

Probably the first thing you notice after starting rview is that
nothing happens. It takes the calculation a while to get going,
as it must trace may rays at the outset to determine the
contribution at each point from the window area. Once rview has
stored up some values, the progress rate improves, but it never
really reaches blistering speed.

A more efficient alternative in this case is to use the program
mkillum to create a modified window file that uses calculated
data values to define its light output distribution. Applying
mkillum is relatively straightforward in this case. You simply
give it an octree defined with the normal window and the file
describing this window and it creates a new window description
using calculated data values:

     % oconv sky.rad outside.rad window.rad room.rad > test.oct
     % mkillum -av 18 18 18 -ab 0 test.oct < window.rad >
          mkiwindow.rad

The -av value is appropriate for the outside, which is much
brighter, as suggested by the output of the gensky command stored
in "sky.rad". The -ab option is set to 0, because outside the
building we do not expect interreflections to play as important a
role as in the interior (plus we are trying to save some time).
The mkillum command may take a few minutes, so be patient. If we
look at the output file from mkillum, we see that it uses a
pattern type called brightdata, which refers to a data file
called "Illum.dat". This file contains the results of an rtrace
calculation, which traced many rays in each direction from the
window in order to determine its brightness distribution. Now we
can create an octree using this new file and render it with
rview, this time without the interreflection calculation:

     % oconv sky.rad outside.rad mkiwindow.rad room.rad >
          test.oct
     % rview -vf default.vp -av .5 .5 .5 test.oct

You will notice that the calculation proceeds much more quickly,
and even produces a smoother looking result. Aside from waiting
for mkillum to finish, there is an additional price for this
speed advantage, however. The contribution from the sun patch on
the floor is no longer being considered, since we are not
performing an interreflection calculation inside our space. The
light from the window is being taken care of by the mkillum
output, but the solar patch is not. In most cases, we endeavor to
prevent direct sun from entering the space, and in the morning
hours this is true for our model, but otherwise it is necessary
to use the diffuse interreflection calculation to correctly
account for all contributions.


5. Discomfort Glare

Especially in scenes containing daylight, situations arise where
the occupant is uncomfortable due to large differences in the
brightness of the visual field. For example, trying to look at
someone's face when they are backlit by a bright window is
difficult and painful. We would therefore like some way to
predict the effects of such bright sources of illumination when
designing a space. RADIANCE provides just such a calculation.

Let's start by generating a fisheye picture of our design space:


     % rpict -vta -vp 1.5 .8 1 -vd 0 1 0 -vh 240 -vv 180 -av
          .5 .5 .5 test.oct > fish.pic

This picture will be used to identify sources of glare from a
particular view point (1.5, 1, 1) about a particular direction
(0, 1, 0). Since RADIANCE pictures contain true floating point
radiance values, they can be used to analyze a visual environment
for problems such as discomfort glare. Once rpict has finished
(and this may take the better part of an hour), you may display
the image using ximage or whatever display program you have
available on your system. A fisheye perspective is a type of
distortion that allows a wider field of view than a standard
perspective image.

This larger field is used by the program findglare to locate any
and all bright spots that might affect visual comfort. Findglare
can also use direct calculation with rtrace, but this takes a
long time without providing the nice visual feedback one gets
from an image.

To facilitate the use of findglare and the associated programs
glarendx and xglaresrc, a script has been written called glare.
This script asks you some questions and to make it a little
easier to get a nice result. To start the script, simply type
glare on the command line. When it asks you a name for the glare
file you want to work with, enter something innocuous like
"test.glr". It will tell you that the file does not exist, so we
will have to create it. It then prompts you for the name of the
picture and the name of the octree, which are "fish.pic" and
"test.oct", respectively. When it asks for the view, just hit
return because we want to use the default view from the picture
file. Glare then asks what parameters it should use for the
rtrace calculation. (Findglare will still use rtrace to calculate
any points that it needs which are off the picture we have given
it.) For this, you should give the -av setting from before (.5 .5
.5). Then glare will ask you if the sources of glare are small,
which they are not, and if you would like to set the glare
threshold manually, which you would not. Finally, you will be
asked what maximum angle you want to calculate away from the view
center. Let's use 60 degrees (to the right and left). Glare then
proceeds to call the program findglare to locate the actual glare
sources. Once findglare has finished, glare will call the program
xglaresrc (if you are running X) to circle the glare sources
found on the image.

You then have a choice of a few different glare indices you may
calculated from the computed sources. The Guth visual comfort
probability (VCP) is perhaps the easiest to understand for those
who are not familiar with glare metrics, because it gives a
result in terms of a percentage of people who would be satisfied.
Calculating the Guth VCP in this case, we see that looking
straight ahead (ie. 0 degrees), we see that less than 15% of the
people would find this visual environment comfortable. Although
the visual comfort probability increases as we look to the left
(positive angles), it never gets over 20% for the range of angles
we've given. That is primarily because the window is unshielded
and very near the horizontal line of sight. Skylights and ceiling
fixtures are a little nicer from a glare standpoint for that
reason.