OpenGL FrameBuffer Object 101
by Rob 'phantom' Jones

Contents   Introduction   Adding a Depth Buffer   Adding a Texture To Render To   Rendering to Texture   Using The Rendered To Texture   Cleaning Up   Source code   Printable version   Discuss this article The Series   OpenGL Frame Buffer Object 101   OpenGL Frame Buffer Object 201

Rendering to Texture

With all the hard work setting things up done the usage of the FBO from here on out is in fact pretty simple and relies on just one function call: glBindFramebufferEXT().

To render to a FBO you bind it and to stop rendering to it you call the above again with ‘0’ as the final parameter:

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);
glPushAttrib(GL_VIEWPORT_BIT);
glViewport(0,0,width, height);


// Render as normal here
// output goes to the FBO and it’s attached buffers

glPopAttrib();
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

Three lines which probably jumped out at you right away are the glPushAttrib/glPopAttrib pair and the glViewport call. The glViewport call is needed so that we don’t try to render into an area bigger than, or even smaller than, the FBO is setup for. The glPushAtrrib and glPopAttrib are used as a quick way to save the old viewport information, this is needed because the FBO shares all of its states with the main context and as such any changes made affect both the FBO and the main context you would be normally rendering to.

An important point to make here is that you’ll notice that we only bound and then unbound the FBO to draw to it, we didn’t reattach any textures or renderbuffers, this because they stay attached until you detach them yourself or the FBO is destroyed.

Using The Rendered To Texture