Shadow buffer

OpenGL
1

This site has some example
http://wiki.vrmedia.it/index.php?title=Shadow_Mapping

The C++ code


glMatrixMode(GL_TEXTURE);
glLoadIdentity();  // clear things up
glTranslate(0.5, 0.5, 0.5); // we have to clamp values in the [0.0, 1.0] range, not [-1.0, 1.0]
glScale(0.5, 0.5, 0.5);
glMultMatrix(light_mat_proj); // now multiply by the matrices we have retrieved before
glMultMatrix(light_mat_modelview);
 
// finally, multiply by the *inverse* of the *current* modelview matrix
var s_mat_m = glGet(GL_MODELVIEW_MATRIX);
if (! InvertMatrix(&s_mat_m))
   Quit("Singular view matrix!"); // this should not happen
 
glMultMatrix(s_mat_m);
glMatrixMode(GL_MODELVIEW);

and the VS shader


[VERTEX SHADER]
varying vec4 shadowTexCoord;
void main (void)
{
   // vertex calculation
   gl_Position = ftransform();
   
   // shadow texture coordinates generation
   shadowTexCoord = gl_TextureMatrix[0] * gl_ModelViewMatrix * gl_Vertex;
}

Why are they computing InvertMatrix(&s_mat_m)
and then in the shader
shadowTexCoord = gl_TextureMatrix[0] * gl_ModelViewMatrix * gl_Vertex;

Isn’t it simpler to not apply the inverse in the C++ and in the shader

shadowTexCoord = gl_TextureMatrix[0] * gl_Vertex;

2

good question :slight_smile:

They also say:


light_mat_modelview = glGet(GL_MODELVIEW_MATRIX);

so


glMultMatrix(light_mat_modelview);
var s_mat_m = glGet(GL_MODELVIEW_MATRIX);
InvertMatrix(&s_mat_m);
glMultMatrix(s_mat_m);

would also be redundant, but it says

Please note that light_mat_proj and light_mat_modelview are always the same if the light does not move; in that case you do not need to retrieve them at every frame.

So I guess the values for modelviewmatrix are not the same when the functions/shaders are called.

N.

3

So I guess they end up with a identity matrix.

4

Looks like it, maybe they wrote it this way for the sake of clarity…

N.

5

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