To use compositor data here… Do this…
Any parameter that expects an image file.

Instead of entering a filename, use op:path_to_cop_node . This will load the Color and Alpha data from the COP node as an RGBA image.

Warning

This will not work in VOPs. The VEX code generated by VOPs cannot do this kind of dynamic image reference.

Instead, fetch the image in the shader node (SHOP) that encapsulates the VOP shader network.

Trace SOP

Creates curve geometry from the color contours of an image. It has an explicit option to take data from a COP node instead of an image file.

Color POP

Colors particles based on the pixel colors of an image. It has an explicit option to take data from a COP node instead of an image file.

Image CHOP

Converts scanlines of an image from a COP node into channels. You can fetch sub-regions of the image instead of the full image.

Compositing

Basics

Layers

Camera effects

Advanced

Guru level