Adding a Texture To Render To

At this point we still don’t have a way of writing colour information to the FBO, so that is what we are going to add now. There are two ways of going about it:

1. Attach a colour Renderbuffer to the FBO
2. Attach a texture to the FBO

The former does have some uses; however it is the latter we will be covering here.

Before you can attach a texture you need to create one, this hasn’t changed from the normal way of using textures as you’ll see:

GLuint img;
glGenTextures(1, &img);
glBindTexture(GL_TEXTURE_2D, img);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8,  width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

In this instance we are creating a normal RGBA image of the same width and height as the renderbuffer we created earlier; this is important as ALL attachments to a FBO have to be the same width and height. Note that we don’t upload any data, the space is just reserved by OpenGL so we can use it later.

Having created our texture the next job is to attach it to the FBO so we can render to it:

glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, img, 0);

Again this might look imposing but it isn’t that bad, the GL_COLOR_ATTACHMENT0_EXT tells OpenGL to attach it to the relevent attachment point (FBOs can have more than one colour buffer attached at any given time, each one to a different point), the GL_TEXTURE_2D tells OpenGL the format of the texture we are going to be attaching, img is the texture we’ll be attaching and the ‘0’ refers to the mipmap level of the texture you want to attach to, which you will generally want to leave as 0.

The final job to do in setup is to check that the FBO is ‘complete’. Completeness refers to the state of the FBO being one which, given the current OpenGL state and its attachments, all is correct for you to render to it.

This test is done via a single function which returns the status of the currently bound FBO

GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);

If all has gone well then status will equal GL_FRAMEBUFFER_COMPLETE_EXT and your FBO is ready to be rendered to. Other error codes, as found in the spec, indicate other problems which might well have occurred when you tried to setup the FBO.

Rendering to Texture