CODEWITHSUNDEEP
openglglslshader
Milo
Milo

Reputation: 185

GLSL gl_PrimitiveID always 0

I am trying to output gl_PrimitiveID as a color in a fragment shader. However it's always equal to 0.

Here is my fragment shader:

void main(void)
{
    if(gl_PrimitiveID == 0)
        gl_FragColor.rgba = vec4(1, 0, 0, 1); // red for 0
    else
        gl_FragColor.rgba = vec4(0, 0, 1, 1); // blue otherwise
}

and vertex shader

void main(void)
{
    gl_Position = ftransform();
}

The output color for triangles is always red. I am drawing the triangles with glDrawElements and the render is ok when using the default shaders.

What am i doing wrong?

Here is how I draw the triangles:

glBindBuffer(GL_ARRAY_BUFFER, vertexPosBuffer);
glVertexPointer(3, GL_FLOAT, 0, NULL);

glBindBuffer(GL_ARRAY_BUFFER, vertexNormalBuffer);
glNormalPointer(GL_FLOAT, 0, NULL);

glEnableClientState( GL_VERTEX_ARRAY );
glEnableClientState( GL_NORMAL_ARRAY );

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, elementBuffer);
glDrawElements(GL_TRIANGLES, countVertices, GL_UNSIGNED_INT, (void*)0);

Upvotes: 3

Views: 2564

Answers (1)

Milo
Milo

Reputation: 185

After hours of banging my head against the wall, I realized my vertex shader was not attached to the right program id.

Only the fragment shader was attached. It seems gl_PrimitiveID does not auto increment if the vertex shader is not provided.

+1 to Andon and ratchet for pointing me to the right direction.

Upvotes: 3

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