You can either perform operations on the framebuffer, or use special textures in materials, that contain framebuffer images. For example current makes a screenshot into a texture right before the material is used, snapshot is updated by the user, and lastframe is auto generated after l3dlist drawing is done.
The operations are performed at the end of every l3dlayerid.
Note that the position and sizes are in opengl pixel coordinates like view definitions in l3dview.
o-+ l3dlist o-+ framefx o-- framefxclear o-- framefxmesh o-- framefxsnapshot