PhoenixFD + Vray issues

[TomGore]

And by the way - I rendered the smoke in Vray GPU with Aerial Perspective on... And it works well, even in volumetric mode. Lighting is different, so no direct compositing of GPU smoke with CPU city is possible.Too bad that GPU doesn't like multiple 16k textures (128GB cpu ram vs 8GB gpu ram), because it would be the way to go. This is the first time that vray GPU has the feature that CPU doesn't.

[Svetlin.Nikolov]

Yup, we are working on render elements for V-Ray GPU which will make the entire volumetric geometry thing obsolete one day.

I replicated all V-Ray settings visible in the screenshots, with GI off, and now render times with my caches are roughly proportional - 1 grid for 5 minutes, 3 grids for 18 minutes, but the progress bar manages to start filling up here.

Something else that I can think of would be that maybe there is something wrong with the opacity - can you check if setting all grids in simple smoke makes any difference? I can't see the bottom of the smoke curve, but is the first leftmost point at 0,0? If so, can you try to move it a tiny bit to the right? On some CPUs this will add a small error and add some opacity in voxels where there isn't any, so this might cause a slowdown.

If this doesn't reveal any useful information, then the final thing I can ask about is what Paul said - please send over the scene in the state of point (5) - with just the simulators, and only one cache for that frame. You can also send it over to [my forum name] at chaosgroup dot com and I will see what you can do to work around this and catch your deadlines.

Cheers!

[TomGore]

Okay, I am sending you a wetransfer with the scene (125kb) and with 4 grids cache single frames. Scene starts at frame 100, some caches have different timeline origin so don't freak out - it should work just after loading. There are total 4 grids and the sun. Try to render it as it loaded, with current camera view. What I get now is render bar starts, but it is very slow in relation to what is visible onscreen (small puffs of smoke). Then switch to volumetric mode and see how fast it is. Then switch to volumetric geometry back, you can hide some grids and start testing on a heaviest grid (middle one). Higher opacities threshold render faster ("Minimum visible opacity" parameter works as expected) but it still doesn't solve the problem. I need very thin smoke for real look

Isn't there an easy way to add zdepth element to volumetric mode as well as PhoenixFD element? It would solve all problems. FumeFx has this possibility, so I think it is possible to implement in phoenix as well.
best,
norman

[TomGore]

One more thing: Just decreased the resolution to 320x180 to see if there is any improvement. Not really - the progress bar doesnt show up and rendertimes are still absurd for this resolution and content... I've canceled after 15 minutes. Your scene should look like this:




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EDIT: This 320x180 render with volumetric geo on is still rendering after 1hour and 30min now. It is unbelievable. You should test this scene for sure.

[Svetlin.Nikolov]

Got the files and started experimenting with the scene as well. One of the simulators was in resimulation mode so the cache did not match, but I'm using the 'newpc_corr' cache in place of the LQ cache.

It should not be very hard to implement zdepth, but it won't happen in a day or a week, so gotta figure what's wrong ASAP and send you a workaround.

Will ping you as soon as I find anything!

[Svetlin.Nikolov]

Okay, here's a thing I found - disable the Phoenix Light Cache for all Phoenix simulators - right now 2 of the 4 sims have it enabled. There is a warning message about using progressive sampler + Phoenix light cache but you might have muted it and forgotten about it.

[Svetlin.Nikolov]

Another thing - as I mentioned above, check all smoke curves and make sure that if there are any points on position 0,0, you move them to the right, so they become something like X=0.001; Y=0. Often due to numerical inaccuracy this causes all voxels to have 'some' opacity, so you will get a message saying that no voxels in a certain grid are completely transparent. This would cause the 'optimizing volumetrics' phase to get skipped, so rendering of such grids would be slower.

[TomGore]

I am aware of both of these issues (Light Cache and 0.0 opacity) but after testing, it doesn't decrease render times in any meaningful way. What render-times with the camera view do you currently achieve?

[Svetlin.Nikolov]

Definitely don't leave the setup like it is now - both of these changes are very important - after applying these, I am at pass 53 after 1 hour of rendering at 1080p, using V-Ray 4.1 official and the Phoenix 3.12 nightly from today.

I'll keep looking, but start with these changes.

[Svetlin.Nikolov]

We are definitely not done though, since I reach pass 53 with volumetric mode after 40 seconds.

[TomGore]

exactly...

[TomGore]

Can scene scale have the effect on precision errors or something? I am not good in 3dsmax scale system (who is?). Perhaps scaling the whole scene, grids, up or down would change rendertime? Phoenix calculated that my grids have to be in scale of 0.007 in grid settings (based on the dense smoke preset).

[Svetlin.Nikolov]

I think this shouldn't be a factor, but nothing's completely out of the question.

Just found another thing I can do very quickly in the code - managed to speed up rendering roughly two times when you have the smoke opacity modulated by a texture. Gonna push this in the nightlies and if I don't come up with any more ideas for code changes in the next hour or so, will force new nightlies so you can grab that update and use it.

[TomGore]

Thanks for your effort. What do you mean by speeding up render modulated by opacity texture? In volumetric mode or volumetric geo mode? Right now I gave up with testing (because deadlines) and decided to go with pure volumetric mode without any elements. I will try to extract alpha from phoenix smoke and try to add artificial fog to it in post production. There is no easy way to save alpha only from the smoke - I need to render it over a plane with alpha contribution to 0.0 in vray properities. But then any occlusion by buildings comes into a play and so on, so on... Perhaps this is something that could be easily "automated" and put to nightlies?

Now, the image below renders in ~4 minutes, hard to believe how fast cpu vray next is, but it's true. Too bad than any requirement of volume elements forces you to use volumetric geo, which goes into 1 hour for the same quality.

[Svetlin.Nikolov]

Yup, the texture speedup for Smoke Opacity with a texture with checked Modulate would benefit both volumetric and volumetric geometry modes.

I keep looking for anything that can be done and will ping you when I find anything.