I have ~800 frames which are technically still. In these 800 frames, only a small section of it moves, is it possible to only render the first frame fully, then only re-render the small moving part of the 799 frames left and reuse the rest of the image from the first rendered frame?

I want this rendered for 800 frames

I want to render this for 800 frames

Only this part moves for those 800 framesenter image description here


1 Answer 1


You can solve this by separating the foreground and background elements and rendering them separately, then using the compositor to re-combine them.

Result of this approach

First, render only the very background - the object(s) that is behind everything else, and save to file: Background

Then, make sure you set the background to transparent.

Then render the static foreground elements (the elements above everything else that aren't animated) and save to file: Static foreground

Now you can render your dynamic, animated element separately from the rest. Make sure you render to an image sequence of PNGs with transparent background: Animated part

Now all you need to do is open the compositor and use alpha over nodes to composite the static and dynamic elements, then render it all out together to one video file (or a new image sequence): Node setup

Alternative node setup using Z combine: enter image description here

You can find the blend file that was used for this answer here. Good luck!

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    $\begingroup$ Please use a more permanent blend file storage like blend-exchange.giantcowfilms.com ; the links do not expire there whereas pasteall links do after 5 months. $\endgroup$ Oct 11, 2015 at 12:48
  • $\begingroup$ Will rendering the background image once, and putting the swirling DNA is another layer and using Z Combine to join the static image and the swirling DNA together also work? $\endgroup$
    – bi0phaz3
    Oct 11, 2015 at 12:50
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    $\begingroup$ Goot point @RayMairlot, updated.@bi0phaz3: you're welcome, wish I started out doing amazing 3D as early as you, good luck! :) $\endgroup$
    – TLousky
    Oct 11, 2015 at 13:02
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    $\begingroup$ @bi0phaz3 - Yup, sure will: bioblog3d.files.wordpress.com/2015/10/z_combine.jpg $\endgroup$
    – TLousky
    Oct 11, 2015 at 13:36
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    $\begingroup$ Sounds like a good idea @bi0phaz3. And yes, higher values mean further into the background. $\endgroup$
    – TLousky
    Oct 11, 2015 at 13:47

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