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 the offscreen buffer and use it as texture, which works. I can ‘scroll’ this larger texture through the display using an offset, which makes me confident, that I render into a larger context than the window.

If I save the offscreen buffer to an image file it always gets cropped. The code fragment for saving is:

void ofFBOTexture::saveImage(string fileName) {      glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);      // get the raw buffer from ofImage     unsigned char* pixels = imageSaver.getPixels();     glReadPixels(0, 0, 1024, 1024, GL_RGB, GL_UNSIGNED_BYTE, pixels);       imageSaver.saveImage(fileName);  }  

Please note, that the image content is cropped, the visible part is saved correctly (which means no error in pixel formats, GL_RGB issues etc.), but the remaining space is filled with one color.

So, my question is – what am I doing wrong?

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.)