Texture Projection in GLSL - glsl

Question

I'm trying to implement texture projection based directly on this example at oZone3d. My environment is OpenGL ES 2.0 on Android 2.2.
Matrix math has never been my strong point and I suspect that my problem lies somewhere in the texture projection matrix setup (texGenMatrix). I have tried everything imaginable to get this working but the projection doesn't come out correctly. I'd greatly appreciate some extra eyes on my code. Thanks.
Not shown here is my geometry (a horizontal plane through the origin), and some other plumbing like shader uniform/attribute binds.
Camera Setup :
// Set the camera projection matrix
Matrix.frustumM(cameraProjMatrix, 0,
-nearPlaneSize.x, nearPlaneSize.x, // near plane left/right
-nearPlaneSize.y, nearPlaneSize.y, // near plane bottom/top
nearPlaneDistance, farPlaneDistance // near/far plane distance
);
// Set the camera view matrix
Matrix.setLookAtM(cameraViewMatrix, 0,
0, 100, 100, // eye
0, 0, 0, // center
0, 1, 0 // up-vector
);
// Translate the model as needed
Matrix.translateM(modelMatrix, 0, x, y, z);
// Calculate the model * view matrix
Matrix.multiplyMM(modelViewMatrix, 0, cameraViewMatrix, 0, modelMatrix, 0);
// Calculate the inverse camera view matrix (needed in the vert shader)
Matrix.invertM(inverseCameraViewMatrix, 0, cameraViewMatrix, 0);
Texture Projection setup :
// Set a texture projection matrix
Matrix.frustumM(texProjMatrix, 0,
-nearPlaneSize.x, nearPlaneSize.x, // near plane left/right
-nearPlaneSize.y, nearPlaneSize.y, // near plane bottom/top
nearPlaneDistance, farPlaneDistance // near/far plane distance
);
// Set the texture projection to point down at the origin
Matrix.setLookAtM(texViewMatrix, 0,
0, 100, 10, // eye
0, 0, 0, // center
0, 1, 0 // up-vector
);
// scale bias
float[] scaleBiasMatrix = new float[] { 0.5f, 0.0f, 0.0f, 0.0f,
0.0f, 0.5f, 0.0f, 0.0f,
0.0f, 0.0f, 0.5f, 0.0f,
0.5f, 0.5f, 0.5f, 1.0f
};
// calculate the TexGenMatrix
Matrix.multiplyMM(texGenMatrix, 0, scaleBiasMatrix, 0, texProjMatrix, 0);
Matrix.multiplyMM(texGenMatrix, 0, texGenMatrix, 0, texViewMatrix, 0);
The vertex shader :
attribute vec3 aPosition; // vertex x y z
uniform mat4 texGenMatrix;
uniform mat4 modelViewMatrix;
uniform mat4 cameraProjMatrix;
varying vec4 vProjCoord;
void main() {
gl_Position = cameraProjMatrix * modelViewMatrix * aPosition;
vec4 posEye = modelViewMatrix * aPosition;
vec4 posWorld = inverseCameraViewMatrix * posEye;
vProjCoord = texGenMatrix * posWorld;
}
Fragment shader :
uniform sampler2D projectionMap;
varying vec4 vProjCoord;
void main() {
vec3 finalColor = vec3( 0.5, 0.5, 0.5);
if (vProjCoord.q > 0.0) {
vec4 ProjMapColor = texture2DProj(projectionMap, vProjCoord);
finalColor += ProjMapColor.rgb;
}
gl_FragColor = vec4(finalColor, 1.0);
}