problem saving openGL FBO larger than window

Question

I\'m rendering into an OpenGL offscreen framebuffer object and like to save it as an image. Note that the FBO is larger than the display size. I can render into

Answer 1

Finally I solved the issue.

I have to activate the fbo for saving its contents:

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);
// save code
...
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

while only selecting the fbo for glReadPixels via

glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);

doesn't suffice.

(All other things where correct and tested, eg. viewport sizes, width and height of buffer, image texture etc.)