Why is there a setting for transparency in light paths cycles

Hi!

Well the title kinda says it but here we go. In the properties editor under the render settings there is a “Lights Paths” section where a setting of min and max for transparency is located. My question is why this setting exists at all.

I have done some tests with images with alpha mapped to planes and distributed as a particle system to get an area for testing. With a low value like 4, I get a lot of black artifacts where the rays basicly went through 4 alpha planes and then hit the fifth and got rendered black. However if I do a couple of more tests I come to the conclusion that all artifacts are gone if I set the max and min value (why is there a min value?) to about 15. However the rendertime doesnt change if i set both values to something insane like 300.

So the only reason that I can see for this setting is if I want artifacts I can decide how much that would be. The alternative for me is to just set this to something crazy high and forget it for the rest of my life and I will never lose ether quality or rendertime when using alphas. In what scenario is this setting not good to just have at a super high value?

Why would the render time change? That value is telling Cycles after how many “levels” of transparency to terminate the tracing of the rays. If you only have about 15 levels of transparency stacked behind each other, increasing that value further will indeed have no effect at all, as Cycles can’t render what isn’t there.

Try testing those values again with a scene in which you actually have 300 levels of transparency and you will see a significant increase in render times with increasing render depth for transparency.

https://dl.dropboxusercontent.com/u/27650383/05-10-2016%2022-09-18.png

aha, well I guess that those trees got translucent in the shader as well, making some of the light pass through meaning alot more bounces and potentialy alot more layers of transparency. If you only had diffuse and transparency I guess you would never come to such extreme levels of transparency. I cloudnt see how I could ever reach that many layers of trasparency because there would only be the edges of for instance a leaf that would br transparent. The rest would be covered with diffuse and glossy, but translucensy or subsurf or any volume shader I guess, would make those 250+ layers very real.

Same test with just diffuse/transparent leaves:

Min 8 / Max 8 = 38.35 s
Min 8 / Max 16 = 57.77 s
Min 8 / Max 250 = 1:24.12 min

Thanks alot for giving me this data!