Cropping background image when rendering region 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.



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?
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.