RE: [Algorithms] Lightmap Terrain

[Akbar A. <sye...@ea...>]

what algo did q3 use to light it's textures?

peace,
akbar A.

-----Original Message-----
From: gda...@li...
[mailto:gda...@li...]On Behalf Of Klaus
Hartmann
Sent: Thursday, August 24, 2000 5:30 PM
To: gda...@li...
Subject: Re: [Algorithms] Lightmap Terrain


Sylvain, :)

> Hi!
>
> I trying to find how to generate a lightmap/illumination/shadow on a
> terrain.
> Do you know some good tutorial/code/links talking about that?

Unfortunately, no, I don't know of any such papers and good code is also
very rare (I wasn't able to find any good code, yet). The only useful
related paper I know of is "Grid Tracing: Fast Ray Tracing for Height
Fields", by famous Mr. F. Kenton Musgrave. The paper does, however, not
explain how to light a surface, it only describes a fast algorithm to trace
a ray and intersect it with a height field. It is useful for many things.
Shadow feelers are a nice example. If you want to know, if a point on the
height field surface lies in shadow, then you can cast a ray from that point
to the light source, and then use the algorithm to trace the ray, and test
wheter it intersects with the height field (or any other object) before it
reaches the light source. If there is an intersection then the point lies in
shadow, and you decrease the intensity at that point.

> I want something to generate a lightmap on a (1025x1025 terrain). A good
> generation
> if possible like compute the the light from a point.

A simple, but not perfect, way to calculate the light at a point on the
height field is to use the dot product:
[1] Calculate the unit vertex normal, N, at the point, P, you want to
illuminate
[2] Calculate the unit direction vector, L, that points from point P towards
the point light source (or pre-calculate the single unit direction vector
for a directional light source).
[3] If the dot product I = L _dot_ N is smaller or equal to 0, then angle
between N and L is >= 90 degree, and the light source has no effect on point
P. If I is greater than 0, then the angle between L and N is smaller than 90
degree, and you can use I to illuminate the point:
point.r  = material.r * I * lightSource.r;
point.g = material.g * I * lightSource.g;
point.b = material.b * I * lightSource.b;

Note, that I actually is the cosine of the angle between the two *unit*
vectors, L and N. In general:
U _dot_ V = |U| * |V| * cos(angle(U, V))

where, U and V are two vectors, and angle(U, V) is the angle between the two
vectors U, and V. If U and V are both unit vectors, then:
U _dot_ V = 1 * 1 * cos(angle(U, V))
<=>
U _dot_ V = cos(angle(U, V))

So, if in the above example, N and L are not unit vectors, then you need to
compute I as follows:
I = (L _dot_ N) / ( |L| * |N|)

Here's a small performance tip w.r.t. point light sources (and spot lights).
If you use a point light source, then you need to re-compute L all the time,
because L depends on the point P. However, normalizing L all the time is
really expensive, and it is a waste of performance to normalize L, if the
light has no effect at the point P (the angle between N and L is greater
than 90°). You can speed this up as follows:

L = compute NON-normalized vector L
I = L _dot_ N;
if (I <= 0) then return; /light has no effect*/
I /= |L|;
point.r  = material.r * I * lightSource.r;
point.g = material.g * I * lightSource.g;
point.b = material.b * I * lightSource.b;

> I want generate the lightmap
> just before the use of the terrain and i dont want something that i need
to
> pre-calculate and put the result on a file or something like.

Not sure what you mean here...

HTH,
Niki


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