Problems outputting gl_PrimitiveID to custom frame buffer object (FBO) - opengl

Question

I have a very basic fragment shader which I want to output 'gl_PrimitiveID' to a fragment buffer object (FBO) which I have defined. Below is my fragment shader:
#version 150
uniform vec4 colorConst;
out vec4 fragColor;
out uvec4 triID;
void main(void)
{
fragColor = colorConst;
triID.r = uint(gl_PrimitiveID);
}
I setup my FBO like this:
GLuint renderbufId0;
GLuint renderbufId1;
GLuint depthbufId;
GLuint framebufId;
// generate render and frame buffer objects
glGenRenderbuffers( 1, &renderbufId0 );
glGenRenderbuffers( 1, &renderbufId1 );
glGenRenderbuffers( 1, &depthbufId );
glGenFramebuffers ( 1, &framebufId );
// setup first renderbuffer (fragColor)
glBindRenderbuffer(GL_RENDERBUFFER, renderbufId0);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA, gViewWidth, gViewHeight);
// setup second renderbuffer (triID)
glBindRenderbuffer(GL_RENDERBUFFER, renderbufId1);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGB32UI, gViewWidth, gViewHeight);
// setup depth buffer
glBindRenderbuffer(GL_RENDERBUFFER, depthbufId);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT32, gViewWidth, gViewHeight);
// setup framebuffer
glBindFramebuffer(GL_FRAMEBUFFER, framebufId);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, renderbufId0);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_RENDERBUFFER, renderbufId1);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthbufId );
// check if everything went well
GLenum stat = glCheckFramebufferStatus(GL_FRAMEBUFFER);
if(stat != GL_FRAMEBUFFER_COMPLETE) { exit(0); }
// setup color attachments
const GLenum att[] = {GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1};
glDrawBuffers(2, att);
// render mesh
RenderMyMesh()
// copy second color attachment (triID) to local buffer
glReadBuffer(GL_COLOR_ATTACHMENT1);
glReadPixels(0, 0, gViewWidth, gViewHeight, GL_RED, GL_UNSIGNED_INT, data);
For some reason glReadPixels gives me a 'GL_INVALID_OPERATION' error? However if i change the internal format of renderbufId1 from 'GL_RGB32UI' to 'GL_RGB' and I use 'GL_FLOAT' in glReadPixels instead of 'GL_UNSIGNED_INT' then everything works fine. Does anyone know why I am getting the 'GL_INVALID_OPERATION' error and how I can solve it?
Is there an alternative way of outputting 'gl_PrimitiveID'?
PS: The reason I want to output 'gl_PrimitiveID' like this is explained here: Picking triangles in OpenGL core profile when using glDrawElements

Answer 1

glReadPixels(0, 0, gViewWidth, gViewHeight, GL_RED, GL_UNSIGNED_INT, data);
As stated on the OpenGL Wiki, you need to use GL_RED_INTEGER when transferring true integer data. Otherwise, OpenGL will try to use floating-point conversion on it.
BTW, make sure you're using glBindFragDataLocation to set up which buffers those fragment shader outputs go to. Alternatively, you can set it up explicitly in the shader if you're using GLSL 3.30 or above.