Huge FPS difference when render region over whit

[mattjgerard]

I am rendering a touch button, with LED's behind cloudy plastic (AGAIN, its the bane of my existence) and I am noticing an annoying thing. I have a whole scene setup with pick bins full of parts, lots of geometry, over 18MM triangles. So, I will solo the product I'm focusing on, and my FPS will go way up. So, its a 1000x1000 window, with just the product in the middle covering about 25% of the real time viewer. Background is white. this will run at about ~129 fps. If I turn on the region render, and box the product in a 200x200 box, the fps will skyrocket to 240fps.

Note that all this is doing is ignoring the white background. There is nothing to render, its all hidden. Just the simple act of cropping out the built in white background color is causing my frame rate to nearly double. This seems pretty silly to me, why does it have to dedicate so much to render the white background? When the product is solo'd, there are no reflections, no illumitaion coming/going to or from the hidden objects, they stop interacting with the sceneI can tell because the shadows cast on the product from the bin racking and other items in the scene go away. Seems like a super easy performance gain if KS didn't have to constantly render the white background? Can't show the scene, product not released yet but I'll try to see if I can set up an example.

[INNEO_MWo]

This is interesting.
(I just leave a comment to see the reactions)

[Eric Summers]

I'm also interested in hearing a response on this.

[mattjgerard]

Here are the examples. Nothing changed, its the product lighting preset default. Only thing different is that the render region is turned on or off. White bkgd color, no floor shadows. We do tons of these images.

1) Tower light near conveyor belt, some reflections, etc.- 18.4fps
2) Tower Light Solo. All influence of the conveyor system gone.- 154.9fps
3) Render Region on, same scene, Tower Light solo'd- 317.4

IF things were all right in the world, # 2 and #3 should be the same, since the render region is only cropping out the transparent parts of the image. The white is just being put in by KS, if the scene was rendered as a still, it would show checkerboard in photoshop, so there is literally nothing there to render, so why the big boost in FPS when the RR is cropping out supposedly nothing but transparent areas of the image?

Woul;d be some pretty low hanging fruit for a performance bump if they could figure this one out. I use the RR every day to isolate areas of my images to let them render up to see how noise behaves and if my materials resolve nicely at higher samples than lower (and then just denoising in photoshop)

[DT]

I've noticed black backgrounds render faster but I attributed it to my experience with other rendering software such as V-ray in which black always renders faster than white and colors.