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This is a bit of a trick question. We know that HOLDOUT will subtract objects from renders. My question is slightly more elaborate. I had a render that I spent a fair amount of money on (render farm), only to find out that a material was set to the wrong settings. So I'm re-rendering the animation with every other object BUT the one I need using the HOLDOUT feature.

However, Blender is crawling all over the rendering even though 80% of the frame is 100% transparent. I'd love a way to have Blender only render the area where the object exists. I know about setting a render area method, but the problem arises where I need the object to be in the EXACT location that its predecessor was that had the material problem, so rendering the entire 4k frame is very helpful for a clean overlay.

enter image description here

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    $\begingroup$ Hello :). Since 2.9 there's adaptive sampling available. It should make transparent areas render very fast, since they don't need any samples. $\endgroup$ Commented Oct 1, 2020 at 5:35
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    $\begingroup$ Thank you for this. I had seen this but didn't realize it affected this area. Much appreciated. $\endgroup$
    – Dr Tyrell
    Commented Oct 1, 2020 at 6:12
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    $\begingroup$ @JachymMichal Thanks! I hadn't twigged to that, either. $\endgroup$
    – Robin Betts
    Commented Oct 1, 2020 at 7:00
  • $\begingroup$ This took rendering times from 45 mins to 12 mins. Really amazing. Thank you again. $\endgroup$
    – Dr Tyrell
    Commented Oct 1, 2020 at 22:15

1 Answer 1

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Since 2.9 you can enable Adaptive Sampling.
Transparent areas (or pure HDRI) are then rendered with 1 sample only.

enter image description here

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    $\begingroup$ I had problems with 2.9.x corrupting my objects, so I pulled back to 2.8.3.6. I'm now using copies of my files to simply render. This was the key! 2.75 hour renders down to 30 mins. $\endgroup$
    – Dr Tyrell
    Commented Oct 3, 2020 at 0:45

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