I have this scene with a hand and a knife and I want to get an animation of blood sorta dripping (spraying) from it, but because of SSS I use in palm material it takes forever to render (over 4 minutes!), so I decided to render this image and use it as a background to free calculating space.

subsurface takes it forever to render!

I managed to shorten the time of frame render to 25 secs by using the pre-rendered image of a hand and knife as background image and translucent render of current frame mixed by the alpha mixer.


It looked great and it worked just fine. But then I thought I can render only the region where the animation of blood flushing will be (marked it with red rectangle), so I turned on Render Region


Cycles cropped the background image in final render, though in the compositing viewer I see it how I thought it would look: whole and not cropped like this!

Could you kindly explain me what the heck am I doing wrong, is it a bug?

  • 1
    $\begingroup$ Are you trying to disable the cropping region ? The shortcut is CTRL + ALT + B $\endgroup$ – Gorgious Jan 16 '20 at 21:49
  • $\begingroup$ No, i am trying to render a region and then merge this region with the background image using alpha over. $\endgroup$ – poumalchan Jan 17 '20 at 5:52

I had the same problem. I decided to render only one layer and only "render region" without checking "crop the render region" (output menu -> dimensions) to png so the output is transparent.

After rendering separate frames of animation I put it together with alpha over node in the compositor. I wonder if there is a better way.

  • $\begingroup$ @RayMairlot, fair enough, thanks for the heads up. It was actually in the original post so I didn't know if I could remove it or not. Now I know ! cheers $\endgroup$ – Gorgious Jan 23 '20 at 19:00
  • $\begingroup$ @Gorgious Ah that's my mistake, I didn't see that it was in the original. $\endgroup$ – Ray Mairlot Jan 23 '20 at 19:31

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