Why won't the compositor backdrop show the current composite result? Working through the Match Track Blend DVD and hitting every stupid branch on the way down.

I'm just about finished with the Two Point Track video (https://www.youtube.com/watch?v=ewq-rZEdhpg), only with some depressingly crappy luminance masking. I was trying to fix this when I realised that the Compositor backdrop was only showing the video footage, not the combined image (as seen in the image/render port, bottom left).

Does anybody know what I should be doing here? I should point out that I'm using Cycles instead of the Blender Internal (as in the video sequence) - as I'm trying to migrate most of my skills from Internal.

Many thanks in advance,

--Rev
The issue stems from having rendered the animation as a series of .pngs, then loading the .pngs into the video sequence editor to test how the finished work looked.

If Blender has a video/image strip loaded into the VSE, it will not render an image to the Render Layers as it knows that the VSE strip will ultimately overwrite this information.

Instead, the Render Layers node in the compositor will appear empty or blank, as in the image at the bottom of my question.

Hiding the strip in the VSE will not solve this problem. Either
1) in the Render tab of the Properties Editor, open the Post Performance panel and untick 'sequencer' or
2) delete the image strip in the Video Sequence Editor.

Hitting render should now cause your Render Layer to show your expected image.

Annoying, and not especially evident to the user, but it makes sense. =)

I found my solution on this page: http://blenderartists.org/forum/showthread.php?274922-Camera-Tracking-render-layers-blank-source

Happy Blending!

--Rev