How do I optimize render tiles of empty squares?

Hi guys,

I’m rendering a fish on a black background. I’d like to render a short animation but I noticed that rendering just the black background actually takes about 1/3 the total render time. Why is this? And how can I optimize? (I tried film>transparant but that made no difference)

Here is the render:

in your scene,do you have a plane which is black or is it going through to nothing? If its a plane, delete the plane and have nothing there.

what are you rendering on? cpu or gpu? what are your tile sizes? what are your samples?

Thanks for the reply, bishop!

It’s only the world background color. I’m rendering at 800 samples on cpu, tile size 24x24. It doesn’t take super long for the black tiles to render, about 1 second per tile, but that’s a big sum in the end, that I think should be able to render basically immediately.

I’m rendering on cpu because nVidia hasn’t put out CUDA drivers for High Sierra yet. :confused:

In your camera view - CTRL-B and draw a box around your fish - cropping it in as tight as you can and leaving little space around the edges.

In the render panel - ensure “border” is checked (it should be by default).

Your render will then only render the part that you put the box around.

Import this image into GIMP/Photoshop and add the black border back in manually.

Yeah I thought about that too, but that solves the problem only partially for only one frame. I have no idea how to batch process that for an animation. I’d just like to know why rendering a ray that doesn’t bounce takes so long? And how to fix it of course, but I guess there’s no solution there.

I think there is an addon on blendermarket that animate render borders .
You can try to lower the samples and use denoiser too, 800 seems a bit high. I generally get good result with 100 / 250 samples , but it depends a lot on the case.
Nice render BTW

As sozap said, 800 samples is quite high. I would stick to around 100 and enable the denoiser… will get you about 99% of the quality for 800 samples, but be approximately 8 times quicker :slight_smile:

Thanks guys, I’m already using the denoiser. There’s some SSS and transparency going on, that’s what I need all the samples for.

I’ll have a look at that add-on, sozap, thanks for the tip!

The only way to avoid sampling areas that don’t need it, would be if Cycles had adaptive sampling. There were some builds going around with it, but it has not been implemented. The renderer still needs to throw samples there anyway, it really isn’t avoidable.


Interesting! I think the (old) new Luxrender core has this, right? Wish it was a little more user friendly and more tightly integrated with Blender. I’d use it all the time.