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This is a general question about how Blender processes renders.

I use Blender to create animations of logos on transparent backgrounds for compositing. I am wondering why, even when my geometry is small in the center of the frame with nothing around it, Cycles still takes time to process all of the blank frame area. It seems as though rendering blank areas of the frame should be near instantaneous. Is there a way to have Blender detect areas of the frame that do not need to be processed in order to speed up render time?

Apologies if I haven't explained this well or if this question has already been asked and answered.

Thanks for any help : )

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  • $\begingroup$ My understanding is that cycles, also considers and calculates the light paths of the object surroundings. Adding a alpha channel should (enable film in render setting) should solve this. $\endgroup$
    – Nand 27
    Jun 5, 2021 at 5:32
  • $\begingroup$ Thanks for the quick response, Nand. I do have Transparency checked in the film settings and I'm rendering out .PNGs with RGBA selected in the output properties. That's why I'm curious as to why Cycles would still need to process the alpha channeled pixels. If the software could quickly detect that there is no need to render parts of the frame (i.e. no geometry, volumetrics, etc.), that could really speed up render time, at least for the projects I'm working on. It's not a huge deal, just more of a curiosity. Thanks again : ) $\endgroup$
    – pictureboy
    Jun 5, 2021 at 6:23
  • $\begingroup$ nice to know this :D I never used that feature $\endgroup$
    – Nand 27
    Jun 5, 2021 at 9:19
  • $\begingroup$ That might be it, Jachym Michal. I'll give it a try. Thanks very much! $\endgroup$
    – pictureboy
    Jun 5, 2021 at 9:35

1 Answer 1

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Cycles in a raytracer in that it traces light rays through your scene to render what is there. This means that typically it has to render all tiles to determine whether it's actually blank or not (there could be some geometry crossing that area at any point in the scene from the camera all the way out to infinity) so it can only work by actually rendering those tiles.

There is a workaround for this in that you can set a 'Render Border' which indicates to Blender which areas need to be rendered. This can only be a rectangular section and can be applied by going to Camera view and hitting Ctrl+B. This will allow you to drag a rectangle of the area of interest and anything outside those bounds will automatically be considered 'background' (so will be transparent if you have your Film settings set that way).

Hit Ctrl+Alt+B to remove the Render Border if it is no longer required.

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    $\begingroup$ Thanks for the explanation Rich. That's good to know. : ) $\endgroup$
    – pictureboy
    Jun 6, 2021 at 18:47

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