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I've encountered a very weird phenomenon which I have trouble with: When I render my scene (using the Cycles engine), the initial render time of the first frame is at around ~2 seconds. However, with each frame that is rendered after that, the render time increases by a few hundredths of seconds, sometimes a tenth of a second. After some time, the render time for a single frame has reached almost 1 minute.

The scene itself is not very complex, save for a (baked) particle system and some drivers (which cannot be the source of the problem, since I've created a version of the scene WITHOUT those objects for test purposes - and the same problem remains).

I'm rendering with version 2.79b on a Lenovo Thinkpad T550, which is sufficient since the scene is - as mentioned - not very complex.

Any help/suggestions would be appreciated!

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    $\begingroup$ motion blur can have significant effect on render time on frame-by-frame basis.. $\endgroup$ Jun 27, 2018 at 9:21
  • $\begingroup$ Thank you for your response, Jaroslav. The scene does not use motion blur, so the problem must originate somewhere else $\endgroup$
    – alterdings
    Jun 27, 2018 at 9:26
  • $\begingroup$ Hmm, that was my best bet. Can you share more information about the scene/animation or render settings? The symptoms don't indicate a concrete problem. For example: are the frames the same visually or does geometry coverage of the frame area change? $\endgroup$ Jun 27, 2018 at 9:35
  • $\begingroup$ I won't be able to post the file, but the visual nature of the frames stays very consistent (i.e. no extreme changes in geometry), there are just some elements fading in and out (animated materials). There is a very large amount of transparency bounces though (min: 25, max: 30), but I already tried decreasing those and it did not change anything. A few months back, another user reported a similiar issue, which was not solved: blender.stackexchange.com/questions/105199/… $\endgroup$
    – alterdings
    Jun 27, 2018 at 9:48
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    $\begingroup$ Could you reverse your timeline and check if this is still the case, if so I think this would prove its not complexity increasing in each frame as one would expect it to decrease in this scenario. $\endgroup$
    – Neil
    Jun 27, 2018 at 13:35

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I think I've found the culprit: Through reverse-engineering, removal of geometry and exclusion of certain RenderLayers, I could determine that the increase in render time was caused by Curve objects (Bezier Circles). Once I'd converted those to regular meshes and did a re-render, the problem disappeared.

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    $\begingroup$ Can you simplify the scene, and see the persistence of the problem? If so, maybe it's worth posting a minimal version for others to try in different Blender versions, with different hardware, to see whether it should be submitted as a bug report? $\endgroup$ Jun 28, 2018 at 10:52
  • $\begingroup$ A good idea. I'll give it a try $\endgroup$
    – alterdings
    Jun 28, 2018 at 12:09
  • $\begingroup$ Hey Blender 3.x here with kind of similar Problem and a Bezier Curve for camera motion. Did you filed a bug report? $\endgroup$
    – MaKaNu
    Feb 7 at 22:21

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