Using The Rendered To Texture

At this point our scene has been rendered to the texture and is now ready for us to use it and this operation itself is easy; we just bind the attached texture like any other texture.

glBindTexture(GL_TEXTURE_2D, img);

Having carried that out the texture is now ready to read from as normal.

Depending on the texture’s filtering setup you might also want to generate mipmap information for it. Many people are used to using the gluBuild2DMipmaps() function to build mipmap information at load time and some of you might also be aware of the automatic mipmap generation extension; the FBO extension adds a third way with the GenerateMipmapEXT() function.

This function lets OpenGL build the mipmap information for you, the way it’s done depends on the hardware you are running on, and is the correct way to do so for textures you have rendered to (you shouldn’t use the automatic mipmap generation on a texture you are going to render to for various reasons which are covered in the spec).

To use the function all you have to do is bind a texture as above and then call:

glGenerateMipmapEXT(GL_TEXTURE_2D);

OpenGL will then generate all the required mipmap data for you so that your texture is ready to be used.

It’s important to note that if you intend on using any of the mipmap filters (GL_LINEAR_MIPMAP_LINEAR for example) then you must call glGenerateMipmapEXT() before checking the framebuffer is complete or attempting to render to it.

At setup time you can just do it as follows:

glGenTextures(1, &img);
glBindTexture(GL_TEXTURE_2D, img);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8,  width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glGenerateMipmapEXT(GL_TEXTURE_2D);

At this point your texture is complete and can be rendered to like normal.

Cleaning Up