APDL Public Domain 1/APDL_PD1A.iso/art_draw/3d_tutor/3DText

Text File | 1991-12-09 | 7.7 KB | 168 lines

Since the 1980's, several films have been screened that feature
3D vision. The effect can be quite fantastic - depending on the
technique used. Probably the most well known method is using the
red/green glasses, but this does have the disadvantage that
colour cannot be used. Another technique is using polarised
light. This requires two projectors which polarise the beams of
light differently (90° 'apart'). The glasses then block out one
of the beams by having polarised lenses. This is debatably the
most effective method, allowing full colour.
Another form of three dimensional vision is to have a single
screen that flickers between the display for the left and right
eyes. The glasses are then synchronised to do the same. The Sega
system uses this form, but it suffers from a very flickery image.
Because the average user does not have access to polarised light
projectors or 'flickery' glasses, the first method is a good
choice to demonstrate the 3D effect.

The first thing we must understand before we venture into the
world of 3D is how the eye perceives 'depth'. Our eyes,
surprisingly enough, move independently. They move so as to focus
in on an object.
When an object is close to 'infinity' (as far away as possible)
our eyes are near parallel, that is point directly ahead and each
eyes' line of sight converges a long distance away. When our eyes
point near parallel, our brain assumes the object to be a long
way away.
When an object is near, the eyes swing towards into other,
converging at a closer point. This can be seen when somebody
tries to look at their nose. If the eyes point inwards, our brain
believes the object to be close.

Now, how do we convert that theory onto the screen? Well, under
normal circumstances, each eyes' line of sight converges at the
screen. If an object is to be seen behind the monitor (further
away), we must instruct each eye to swing outwards, towards the
parallel. This can be expressed as : the left eye's image must be
further to the left, and the right eye's image must be further to
the right. From this, we should realise that the image passed to
the left eye should be on the left, and the image for the right
eye, on the right. Therefore, we can expect forward projections
(in front of the monitor) to have the right eye's image at the
left, and the left image at the right. This will make the lines
of sight converge before the screen.

Note however, that the distance between the left and right images
should not exceed the distance between each eye, otherwise the 3D
effect is lost. Indeed, due to the inability to exactly match the
colour of the glasses, a separation of about 32 OS units is all
we can expect.

Now is where the glasses come in - separating the left eye's
image and the right eye's. The lenses in each eye are a different
colour - normally red and green, or red and blue. The glasses I
used were red in the left eye and blue in the right. The same
glasses could possibly be obtained from your local cinema, as
they were used for the film, "Freddy's Dead". Anyway, one lens
will totally block out the colour of the other. Therefore, by
making the left image the colour of the left lens, and the right
image the colour of the right lens, it is possible to separate
the picture on the screen.

Probably the hardest part of creating a 3D image is finding the
exact colour of each lens. The simplest way of doing this is to
run listing one (found on the free subscription disc). It will
initially draw a red circle in the centre of the screen. Place a
lens over it and alter the colour (which is displayed as three
figures at the top of the screen) by using the keys as described.
When the circle appears as dark as possible in one lens, but is
still bright in the other, note down the colour numbers (and
which lens it was visible in) and repeat with the other lens. You
may find all this easier in a dark room.

An example of the sort of effect that can be achieved with the
information described here can be seen with listing two - a three
dimensional star field. Close all the curtains and doors - the 3D
effect can best be seen in total darkness - and run the program.

If you own a television, you will notice a poor effect, if any.
The only way to avoid this is to use a good quality PAL encoder,
or use a monitor. The better the quality of the monitor, the more
amazing the effect. If you can see 'ghosting', that is seeing
both red and blue (or whatever colours the lenses are) lines, try
moving away from the screen more - on a good multisync, it is
possible to sit normally at the computer and still see a clear
picture.

If you find stars a little boring, take a look at listing three.
This is the source code to create the module '3DPlotMod'. This
module provides one SWI that can be accessed from BASIC, or most
other languages. This function allows 3D vector graphics to be
produced, by passing the x,y and z coordinates. The appropriate
red and blue shapes will be drawn. To use it from BASIC, for
example, you should enter :

SYS "3D_Plot",plot action,x,y,z

x,y and z are the coordinates of the line, arc or whatever you
wish to draw. The plot action parameter is the same as for VDU25
(Plot). For those of you without PRM, here's a summary of the
codes :

All codes can be expressed as "<base> + <action>". the Base
number tells the computer what shap you wish to draw. Action
tells the computer how you want to draw it. The base values are
as follows :

0 - draw a line, or move to the point
64 - plot a point
80 - draw a filled triangle
96 - draw a filled rectangle
112 - draw a filled parallelogram
144 - draw a circle outline
152 - draw a filled circle
160 - draw an arc
168 - draw a segment
176 - draw a sector
192 - draw an ellipse outline
200 - draw a filled ellipse

The action values that should be added to the above bases are as
follows :

0 - move relative to the last position visited (MOVE BY)
1 - draw relative to the last position visited
4 - move to the given position
5 - draw at the given position

Codes 0 and 1 can be illustrated : If the last point you moved
(or drew) to was 400,600 and you issued a move by (value 0)
100,-200 the new position would be 500,400.

As an example, let us draw a line from 300,676,375 to 274,607,374
What you must do is :
Move to one end ...
SYS "3D_Plot",4,300,676,375
And then draw a line to the other ...
SYS "3D_Plot",5,274,607,374

To draw a more complex shape, such as a triangle, move to the two
corners and issue a 85 (ie. 80 + 5) for the final corner.

Listing four is a final summary of the ideas described here. It
contains the start-field from listing two, and adds a spinning
pyramid bouncing backwards and forwards.
The pyramid is drawn in machine code for speed, and uses the
3DPlotMod for the vector graphics. Note that to run this program,
you must have the Module 3DLines loaded - this can be achieved by
double-clicking on the 3DPlotMod icon in the desktop.

The application !3DSetup is a simple program that defines the
colours used for the left and right lenses. This can be used to
adjust the colours to suit you - everyones eyes' see light and
colour differently. Run the application as normal by double-
clicking on its icon. Then, to change the left lens colour, press
'L', or 'R' to change the right one.
Then, follow the instructions on the screen. When you exit from
the program (not by pressing Escape), the settings you have
changed will be saved to disc. Note that this will not work if
you are running !3DSetUp from an archive, ie. follow the
instructions in the magazine on how to unpack the archive.
From now on, all the programs will use these settings for their
colours.

  Many thanks to Vincent Sanders and the local cinema for their
  tremendous support for this project and their help with the
  research into 3D techniques.