# Composite mist pass after-the-fact to finished rendered video using the original blend

Say you have 500 stills comprising a video, and you also have the original blend file that created the stills. Is there a way to do a render that skips the time-consuming Image pass but still provides various other passes (like Mist, Shadow, etc.)? And such that those passes are stored in a way that can be input to the compositor to apply the result to the stills? I have done very skimpy passes (low bounces, etc.) that sort of achieve this, and stored them as multilayer EXRs, but haven't found a way to apply the output to more than one frame at a time.

I think the way to do this is as you said: low bounces, no caustics, etc.

You can bring the already rendered image sequences into the compositor with the image node:

1. Add an image node (ShiftA> Input > Image)

2. Open the first frame of your image sequence in the image node

3. Change the source type to Image Sequence:

4. Set the Frames, Offset and Start frame values.

• Frames is the number of frames to use.
• Start frame is the scene frame on which to display the first frame
• Offset is the amount to offset the current frame. (e.g. with Offset = 5, the image for frame 6 will be read instead of the image for frame 1)

You can load your sequence of Multilayer images alongside your original render into the compositor in this fashion. If you rendered the Multilayer images with the render passes enabled you should be able to use the passes from the Image node.

You can also use this alongside the Renderlayer node to composite with the render directly, e.g. with a 150 frame image sequence:

• gandalf3 - Thanks, then I guess that really is as good as it gets. I was hoping for a way to do both 1) Run the PNG/JPEG file Image Sequence side-by-side with the live animation render 2) completely turn-off the main RGB pass in the animation render while still doing other passes that don't require it. The lesson I take from this: Always render to multilayer EXRs with as many passes checked-off as you could conceivably regret missing :) – Plenus Franckly Nov 18 '13 at 12:38
• @PlenusFranckly It's possible the disable to RGB (combined) pass, however this is pointless as cycles still needs to path trace the scene to generate a result for the other passes (to prove this try disabling Use surfaces in the renderlayers panel, all the passes will have no data.) you want, which takes the same amount of time as it would otherwise. In other words, to get a rendered pass, you need to render it. As for 1, if I understand correctly, you should be able to do that with the image sequence node. Have you tried it? – gandalf3 Nov 18 '13 at 21:47
• To answer your question, yes, I've been able to composite a PNG image sequence side-by-side with a multilayer EXR sequence; the approach that didn't work was running the PNG image sequence concurrently with "live frames" from the Render Layers output (what I meant by "live animation render"). I can do one-frame-at-a-time, but not batch. I was interested only because it seemed like a more flexible approach, since I could play with just how many bounces & samples i could do-away-with to still get convincing mist. – Plenus Franckly Nov 19 '13 at 14:13
• @PlenusFranckly Hmm.. It should work.. See my edit. – gandalf3 Nov 19 '13 at 22:55