Sorry, I've been away for a while. I've just started again. This is what I got so far:

Code :
int main() {
	GLuint BUFFER;
	glGenBuffers(1,&BUFFER);
	glBindBuffer(GL_READ_FRAMEBUFFER,BUFFER);
	glBufferData(GL_PIXEL_UNPACK_BUFFER,WIDTH*HEIGHT,NULL,GL_DRAW_BUFFER);
	unsigned char* buffer_map = (unsigned char*) glMapBufferRange(GL_PIXEL_UNPACK_BUFFER,0,WIDTH*HEIGHT,GL_MAP_WRITE_BIT);
 
    bool running = true;
    while (running) {
        if (isFinished())  { running = fallse; }
 
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
	fill_buffer(buffer_map, WIDTH, HEIGHT,objects);
	glBlitFramebuffer(0,0,WIDTH,HEIGHT,0,0,WIDTH,HEIGHT,GL_COLOR_BUFFER_BIT,GL_FILTER);
 
        window.display();
    }
 
    return 0;
}
 
void fill_buffer(unsigned char* buffer, unsigned int width, unsigned int height) {
	for (unsigned int pixelX = 0; i < width; ++pixelX) {
		for (unsigned int pixelY = 0; j < height; ++pixelY) {
			buffer[pixelX*width+pixelY] = calc_pixel(i,j) ? 1 : 0;	// writing a value of 1 to the buffer for test reason
			}
		}
	}
 
	// buffer only contains red values
	glDrawPixels(width,height,GL_RED,GL_UNSIGNED_BYTE,buffer);
}

Note that this is only the essential portion of the code. I don't know if this looks good or if this might work?

I wanted to test it, but I couldn't compile it, becaus I got "unresolved external symbol" for some functions. I know what that means: that there is no implementation available for some of the functions I'm using.
I get this error with: glewBindBuffer, glewBufferData, glewGenBuffers, glewBlitFramebuffer, glewMapBufferRange
So at the moment I'm figuring out why my glew isn't properly working... Any suggestions are welcome, but off course it's a difficult for you to guess what might be wrong.
Any suggestions on the could would be greatly appreciated.