OpenCL: Drawing to OpenGL Framebuffer - Framebuffer stays empty - opengl

Question

I am trying to render to the OpenGL Framebuffer via an OpenGL Renderbuffer from an OpenCL kernel. The issue is: Even though I can (propably) render/write to the Renderbuffer from an OpenCL kernel, the screen stays empty (-> Black).
I am getting to my limits of what I can test in finite time, so I am asking someone with much more experience to give a tip, about what I am missing.
I personally suspect that I forgot to Bind a Buffer at the right point, but since I don't see which and where, this is practically impossible to check.
Now for some reduced code (So you don't have to look at all the error checking etc.)(This is the function that is called during the render routine):
void TestBuffer(){
GLubyte *buffer = (GLubyte *) malloc(1000 * 1000 * 4);
glReadBuffer(GL_COLOR_ATTACHMENT0);
error = glGetError();
if(error != GL_NO_ERROR){
printf("error with readBuffer, %i\n", error);
}
glReadPixels(0, 0, 1000, 1000, GL_RGBA, GL_UNSIGNED_BYTE, (GLvoid *)buffer);
error = glGetError();
if(error != GL_NO_ERROR){
printf("error with readpixels\n");
}
for(int i = 0; i < 1000*100; i++){
if(buffer[i] != 0){
printf("buffer was not empty # %i: %u\n", i, buffer[i]);
free(buffer);
return;
}
}
printf("buffer was empty\n");
free(buffer);
}
void runShader(){
glFinish(); //Make sure, that OpenGL isn't using our objects
ret = clEnqueueAcquireGLObjects(command_queue, 1, &cl_renderBuffer, 0, NULL, NULL);
// Execute the OpenCL kernel on the list
size_t global_item_size = 1000 * 1000; // Process the entire lists
size_t local_item_size = 1000; // Divide work items into groups of SceenWidth
ret = clEnqueueNDRangeKernel(command_queue, kernel, 1, NULL, &global_item_size, &local_item_size, 0, NULL, NULL);
ret = clEnqueueReleaseGLObjects(command_queue, 1, &cl_renderBuffer, 0, NULL, NULL);
clFlush(command_queue);
clFinish(command_queue);
// We are going to blit into the window (default framebuffer)
glBindFramebuffer (GL_DRAW_FRAMEBUFFER, 0);
glDrawBuffer (GL_BACK); // Use backbuffer as color dst.
// Read from your FBO
glBindFramebuffer (GL_READ_FRAMEBUFFER, gl_frameBuffer);
glReadBuffer (GL_COLOR_ATTACHMENT0); // Use Color Attachment 0 as color src.
// Copy the color and depth buffer from your FBO to the default framebuffer
glBlitFramebuffer (0,0, 1000, 1000, 0,0, 1000, 1000, GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT, GL_NEAREST);
TestBuffer();
}
My ideas where:
Blit the contents of the renderbuffer to the screenbuffer, in case I messed up with binding the new framebuffer object (created earlier), or attaching the renderbuffer (which you can see in the last few lines of the code)
Check, if I messed up with the double Buffer or sth.: this is the TestBuffer() function
Flushing before Finishing, just in case
The shader/kernel code is simple on purpose, to see if the other stuff actually works (.w should be alpha, which should be opaque, so we can see the result, the rest is just a gray rainbow):
#pragma OPENCL EXTENSION all : enable
#define ScreenWidth 1000
#define ScreenHight 1000
const sampler_t sampler = CLK_NORMALIZED_COORDS_FALSE | CLK_ADDRESS_NONE | CLK_FILTER_NEAREST;
__kernel void rainbow(__write_only image2d_t asd) {
int i = get_global_id(0);
unsigned int x = i%ScreenWidth;
unsigned int y = i/ScreenHight;
uint4 pixel; //I wish, I could access this as an array
pixel.x = i;
pixel.y = i;
pixel.z = i;
pixel.w = 255;
write_imageui(asd, (int2)(x, y), pixel);
}
Some further information:
I am only rendering stuff to the COLOR_ATTACHMENT0, since I don't care about the depth or stencil buffer in my usecase. This could be an issue though. (I didn't even generate buffers for them)
I am compiling for Windows 10
The format of the Renderbuffer is RGBA8, but I think the natural format is RGBA24. It once was just RGBA as you can see in the TestBuffer Routine, but I think this should be fine.
What could cause the screen to stay black/empty?