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Journeyman III

In AMD gl_PrimitiveID doesn't work with big mesh, Nvidia does.


We are developing a CAD application where the meshes usually are very big. For improve the performance we are painting in a unique batch all the primitives using glDrawElements.

We are using the follow shader to generate Picking info over a FBO. With small mesh (around 2 million of primitives), all works fine but with mesh greater than of 8 millions of primitives the gl_PrimitiveID became invalid. It looks like if in a previous state, the gl_PrimitiveID was reset to zero. Is gl_PrimitiveID limited to a max number of Primitives? or is just a bug?

The shaders we are using are this:

// Vertex

#version 330

#define POSITION    0

uniform mat4 MVP_Matrix;

layout(location = POSITION) in vec4 VertexPosition;

void main(void)


   gl_Position = MVP_Matrix * VertexPosition;


// Fragment

#version 330

#extension GL_EXT_gpu_shader4 : enable

uniform uint gEntyID; // Object Id

uniform uint gSubEntyID; // Element Id

uvec3 outFragColor;

void main()


    outFragColor = uvec3 (gEntyID,gSubEntyID,gl_PrimitiveID);