grain in water

[Dune]

I have to come back to this thread: https://planetside.co.uk/forums/index.php/topic,25484.0.html
I can't get rid of the grain in water (volume density=0.5, color=0.3), so I'd like some advice. Perhaps I forgot something obvious again 
Here's what I tried so far, self-explanatory. All PT, and there's hardly any difference in grain with different settings. In the lowest, I forgot to mention that I set atmo samples to 64.

[Hannes]

I'm afraid, you'll have to increase AA, if you're using the path tracer. I did some tests, and with AA of 12 the grain is quite OK, I'd say. But of course rendertimes are way longer. Just for fun I rendered it with the raytracer, and of course it renders almost instantly.

[WAS]

0.02 pixel noise threshold is still pretty high, actually. You are looking for <= 0.01

Even with clouds 0.02 - 0.025 which I used recently to demonstrate noise in clouds, still had noise.

PS we recently (probably years now lol) discussed how PT water is more grainy than the rest of a scene (including clouds) so it's probably accentuated here. I think it has to do with subsurface scattering introducing ray or sample loss, and thus more noise.

Perhaps PT water could use a AA multiplier. So you could multiply the current AA when it's rendering water. Or maybe a sample multiplier. Something that effects the rendering to improve result, but only on the water, so it doesn't drastically hit everything else being rendered.

[Dune]

Thanks for your time, guys. I'll try an even lower pixel noise threshold and experiment a bit more. But I think this is really a problem that needs solving, as it's a pity to need such high AA only for the water. Your idea, Jordan, would be a nice solution, if implementable.
Indeed RT is really fast, but not so good on the veggies. A little grain is acceptable, because it makes a render look photographic , but not too much.
I even tried a non displacable sphere for the water planet, but that didn't work (at all).

[Dune]

I did some experiments, but didn't find a suitable (less grainy) solution for PT water murk, while still maintaining a reasonably speedy render.
The sphere is obviously the most realistic option, but a (non-displacable) card certainly is faster. In these samples the water itself was colored by density 0.5 and color 0.3 (grey).
The fastest were water renders without integral murk, but with a preceding 50% (and 0.3 grey color) surface shader, again, faster on a card. But less sophisticated, and not even very much smoother.
Only with an AA of 12 did the grain get a bit less.

[WAS]

I would like to note you're using 25 samples for the PT. That really won't help things. Low samples will inherently create grain from density of samples per pixel, or whatever you'd like to call it or perceive it.

But yes, like I mentioned we really need a way to boost the quality of water alone, without effecting atmosphere, clouds, terrain, etc. This because of the inherent quality factors for PT controls. You can't get truly nice close-up water volume, or subsurface without extreme quality settings which hit everything.

[Dune]

The samples didn't do much if raised a bit, unless perhaps 81 or so. Didn't test that.

[Dune]

The grain is really bothering me, also in the glass of this building. Set at architecture, and AA=6. What settings would further be required to get grainless transparent windows? AA=12? Atmo raised (because of reflection)? Subsurface didn't do anything. Doublesided?

[Hannes]

I didn't expect grain in the glass. Weird.
However, I'd say AA 6 is definitely not enough. Usually I render my (potentially) final images with AA 9.
Maybe you could create a GI cache file for the whole image, and render only the areas with water (or glass) with a higher AA. Most of the time, when you render with the path tracer, and there are no clouds in the scene, you don't even need a GI cache file.

[Dune]

Good ideas, Hannes, thanks. I usually use AA=6, which is good enough for 'busy' parts with veggies. This apparently needs more. I once learned that AA is best used with even numbers, but I don't know if it still stands and what for (I guess some sort of efficiency).

[WAS]

Ok. That glass looks terrible. What is going on there? I don't think I've seen it that bad before.

[Dune]

I will do some experimenting today. It's quite horrible indeed with default settings.

[Dune]

I am not very satisfied. It looks like even quite extreme settings don't eliminate all grain. All crops enlarged 200% for easier viewing. Titles give some specifics. I wonder what else to try.....

[Hannes]

I just had an idea. Is the glass part of the model? I recommend to make it a separate part set to not cast shadows. I have the impression, that it's not the glass itself, that's grainy, but the stuff behind it, which is then of course somehow indirectly illuminated. And this indoor stuff is always problematic and noisy with the path tracer.
Of course with the glass not casting shadows it's still interior, but who knows? Maybe it helps...

[Dune]

Ah, that sounds like an interesting idea, let's try it! Thanks Hannes.