Shadow volume shader optimization (GLSL) - optimization

Question

I wondering if there is a way to optimize this vertex shader.
This vertex shader projects (in the light direction) a vertex to the far plane if it is in the shadow.
The aim of this shader is to create a shadow volume object that enclose the shadow of the object itself.
void main(void) {
vec3 lightDir = (gl_ModelViewMatrix * gl_Vertex
- gl_LightSource[0].position).xyz;
// if the vertex is lit
if ( dot(lightDir, gl_NormalMatrix * gl_Normal) < 0.01 ) {
// don't move it
gl_Position = ftransform();
} else {
// move it far, is the light direction
vec4 fin = gl_ProjectionMatrix * (
gl_ModelViewMatrix * gl_Vertex
+ vec4(normalize(lightDir) * 100000.0, 0.0)
);
if ( fin.z > fin.w ) // if fin is behind the far plane
fin.z = fin.w; // move to the far plane (needed for z-fail algo.)
gl_Position = fin;
}
}

Answer 1

If you don't want do touch your principal algorithm (as Michael Daum suggested in his comment) you could replace some parts of your code:
uniform mat4 infiniteProjectionMatrix;
if(...) {
...
} else {
gl_Position = infiniteProjectionMatrix * vec4(lightDir, 0.0);
}
where infiniteProjectionMatrix is a customized projection matrix where the far plane has been set to infinity (see http://www.terathon.com/gdc07_lengyel.pdf on slide 7) and looks something like:
* 0 0 0
0 * 0 0
0 0 -1 *
0 0 -1 0
Since you are projecting to infinity you don't need the "100000.0" scaling factor and the "gl_ModelViewMatrix * gl_Vertex" offset can be neglected (compared to the infinite length of the lightDir vector).