Chaos Corona for 3ds Max > [Max] Tutorials & Guides

Path tracing samples VS passes

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maru:
In Corona, when using using pt+pt or pt+hd, you have two main parameters responsible for image quality: "path tracing samples" and "passes". Generally, we can say that total number of samples used to render each pixel is PTS*passes. We know that each pass consists of sending a primary ray and then calculating secondary rays. That's why rendering an image with such settings: PTS: 100, passes: 1 will be faster than rendering an image like this: PTS: 1, passes: 100. Probably there won't be much difference in GI noise because number of used samples will be the same (100) but there will be significant difference in quality of geometry edges, thin lines, textures, generally antialiasing. So passes work basically like supersampling in conventional renderers. Each pixel is sampled with and averaged, this works like "subdividing" a pixel X times and averaging the output. So here is my question: does it make sense to render a final image with more than 16-32 passes? I know it depends on what kind of scene you have and what resolution you are rendering at. But generally, isn't rendering with 100 or more passes an overkill? The less passes you use, the faster the rendering will finish with the same GI quality, right? We want as little passes as possible. So here is my workflow:

1. I saved the HD cache solution to have the same results all the time. Then I determined (more or less, this is just a test ;) ) how many samples are required to render the GI noiseless - I set passes to 1 and increased PTS until the noise was acceptable. This time I decided it was 2048, but  it would really require even more.

2048 PTS x 1 PASS - time: 21:31:


2. Ok, so total 2048 samples are required to render nice cleanGI. Now - how much pixel subdivision is required for noisy edges to look nice? I did some test while still maintaining 2048 total samples. I increased passes (2, 4, 8, etc) and divided PTS by them (2048/2=1024, 2048/4=512, 2048/8=256 etc) so the GI quality was the same and antialiasing better. I found out that 16 passes are enough for very thin elements to look ok:

128 PTS x 16 passes - time: 24:40:


So in this case this is the optimal value. Increasing passes and decreasing PTS wouldn't increase quality, only time.

You can see that rendering the same image with 16PTS x 128 passes produces a very similar result but takes more time to render:

16 PTS x 128 passes - time: 28:50:


Hope it makes sense and maybe clarifies some Corona settings to new users.
Of course I encourage everybody to argue with me, shout at me and even disagree. ;)

racoonart:
Yes, it's pretty much the way I do my renderings.

Btw, you can easily explain it when you compare it to vray...
Let's say you used to render your images with 1:10 min:max adaptive DMC image sampler. Now you just need to do the square of your max dmc to have the "maximum passes rendered" in vray 10^2 = 100 passes), the maximum amount of eye-rays Vray will do to render your image. Now imagine that the brute force subdivision, reflection subdivisions and light sudivisions are divided by the number of max DMC subdivs and work as multipliers just like it is in corona (it's not that way in reality but pretty similar). If you set a maximum of 100 passes you will just have to calculate your sampling-multipliers for lt and pt the way you did it. Less wasting time for unnecessary AA, more time for gi and light sampling :)

Juraj:
This thing was quite dilemma for me. Now, for average picture I do, with no DOF I set the PT samples at 32 but it easily goes over 100 passes eventually. Now you made me double think if I should not consider raising the PT even higher (64) if I have enough space for long renders (I do, as each picture renders full night) in some occasions, as it's really just GI that needs to clear out in my scenes, not AA.

Do you guys think it's worth adjusting Light Samples concerning pure HDR lighting inside Environment slot ? Only IBL for scene.

racoonart:
Btw, I found the balance between lights multiplier and pt samples way more important than passes vs. sampling level (especially for interiors with hdcache), That's one of the reasons why I was begging for separate lighting and gi render elements - to be able to see each of them ;)

I have some scenes where it's necessary to set both pt and lt samples to equal values around 20 or more to get a good balance of both elements. It's actually pretty simple: if you have a lot small windows it can be way more complicated to calculate direct light (bright sky, hdri, ...) than the actual GI with pt+hdcache. Both types are shooting rays without bounces; lt at lightsources and environment and pt at the geometry which is mapped with the hdcache. So, sometimes, it's way harder to render the direct lighting than the gi and that should be taken into account when you set your lt/pt values.

[Edit] Hahaha, Juraj_Talcik, that's timing ;)

VadoZe:
So as DeadClown said: Passes are analogue of Vray DMC image samples, and PT samples - VRay material subdivs?
Means Passes response for AA, MBlur, DOF, and PTs - noise!

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