Projective texturing

Hi!
I’m having some problems implementing projective texturing using GLSL.

------------ Vertex Shader ------------
uniform mat4 g_projectorMatrix;

void main(void)
{
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
gl_TexCoord[0] = g_projectorMatrix * gl_Vertex;
}

------------ Fragment Shader ------------
uniform sampler2D g_projectorTexture;

void main(void)
{
gl_FragColor = texture2DProj(g_projectorTexture, gl_TexCoord[0]);
}

The scene consists of the following quad:

glVertex3f(-10.0f, -10.0f, -30.0f);
glVertex3f(10.0f, -10.0f, -30.0f);
glVertex3f(10.0f, 10.0f, -30.0f);
glVertex3f(-10.0f, 10.0f, -30.0f);

g_projectorMatrix is setup using the following code:

float projectorFOV = 1.57f;
float projectorAspectRatio = 1.0f;
float projectorNear = 1.0f;
float projectorFar = 1000.0f;

float f = 1.0f / tan(projectorFOV * 0.5f);

Matrix4x4f projection(
f / projectorAspectRatio, 0.0f, 0.0f, 0.0f,
0.0f, f, 0.0f, 0.0f,
0.0f, 0.0f, (projectorFar + projectorNear) / (projectorNear - projectorFar), (-2.0f * projectorFar * projectorNear) / ( projectorNear - projectorFar), 0.0f, 0.0f, -1.0f, 0.0f);

Matrix4x4f scaleAndBias(
0.5f, 0.0f, 0.0f, 0.5f,
0.0f, 0.5f, 0.0f, 0.5f,
0.0f, 0.0f, 0.5f, 0.5f,
0.0f, 0.0f, 0.0f, 1.0f);

Matrix4x4f projectorMatrix = scaleAndBias * projection;

Since the projector is positioned at 0, 0, 0 looking down -Z the projector “view” matrix is ignored.

What am I missing here?

It seems like the q-coordinate of gl_TexCoord[0] equals 0 since the following fragment shader produce a yellow quad:

uniform sampler2D g_projectorTexture;

void main(void)
{
if(gl_TexCoord[0].q == 0.0)
gl_FragColor = vec4(1.0, 1.0, 0.0, 0.0);
else
gl_FragColor = texture2DProj(g_projectorTexture, gl_TexCoord[0]);
}

Any ideas?

Yours sincerely,
Michael

Hi,
i have written the exact same vertex/fragment shaders as you and am in the same trouble, i just can’t get the projective texture to work right i took most of my info from
http://www.gamedev.net/reference/articles/article2138.asp
so maybe it’ll help you, if you haven’t already checked there…if you find anything let me know i’ll sure be interested
good luck

dont know what your matrix struct is like but i do 0.5 on the 12,13,14 elements

well, here’s what i have so far:

  
////////FRAGMENT: 
uniform sampler2D myTexture;

void main (void)
{
  gl_FragColor  = texture2DProj(myTexture, gl_TexCoord[0]);         
}
/////////VERTEX:
uniform mat4 Mprojtex;

void main(void)
{
 gl_TexCoord[0] = Mprojtex * ftransform();
 gl_Position = ftransform();
}		

// Calculating the texture matrix which i called Mprojtex and which i send to my shaders
// doing Mprojtex = Mr*MProj*Model
// Mr being my "scale and bias", Model being Modelview, MProj being Projection matrix

float Mr[16] = { 0.5, 0, 0, 0,
		0, 0.5, 0, 0,
		0, 0, 0.5, 0,
		0.5, 0.5, 0.5, 1 };

float Mprojtex[16], Model[16], MProj[16];
	
glMatrixMode(GL_MODELVIEW);
glGetFloatv(GL_MODELVIEW_MATRIX, Model);
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glGetFloatv(GL_PROJECTION_MATRIX, MProj);
glLoadMatrixf(Mr);     // Here i basically load the Mr matrix and multiply it by the projection matrix and modelview matrix
glMultMatrixf(MProj);
glMultMatrixf(Model);
glGetFloatv(GL_PROJECTION_MATRIX, Mprojtex);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);

glUseProgramObjectARB(ShaderObject);
GLint texLoc   = glGetUniformLocationARB(ShaderObject, "myTexture");
glUniform1iARB(texLoc, 0);
GLint MprojLoc = glGetUniformLocationARB(ShaderObject, "Mprojtex");
glUniform2fvARB(MprojLoc, 16, Mprojtex);

glBindTexture(GL_TEXTURE_2D, render_texture);
glEnable(GL_TEXTURE_2D);

    glBegin( GL_QUADS );
	glVertex3f(0, 0, 30);
	glVertex3f(0,  100, 30);
	glVertex3f( 100,  100, 30);
	glVertex3f( 100, 0, 30);
    glEnd();
 glDisable(GL_TEXTURE_2D);
glUseProgramObjectARB(0);

you see anything wrong that i don’t see???
there seems to be a texture applied, but its like a plain color…maybe the coords are just not right or something

ok, i’ve managed to get my projection working, but without the shaders…so i definitely have the Mprojtex matrix right, now anyone see anything wrong with my shaders?? or maybe the way i send my array to them, im not 100% sure its right
anyway the error can only be somewhere in the shaders or in the way i send it my texture and Mprojtex matrix…

use glUniformMatrix4fvARB(…) to pass the matrix

yeah actually i changed that afterwards but still i get the same result
whats weird is that the projection is alright if i do it by definig the glTexCoord3f without the shaders…however if i try it with shaders its just not happening

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