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Swimming pool / Underwater / Caustics

Posted: Mon Apr 29, 2019 9:56 am
by fabi
Hello,

Here is an image inspired by a holiday picture. I like it although I find the green in the water reflection at the top is too dark.

However, this took a long time to render with Bidir (default parameters), are there tricks to speed up the rendering (2000*1500px image, 500 samples, 1h 40 min on Intel core i3-6100, 300K tris) ?

Have a good day.

Fabi

Re: Swimming pool / Underwater / Caustics

Posted: Mon Apr 29, 2019 10:02 am
by CodeHD
Hi and welcome to the forum :D

I have to say first that the image already looks very nice to me :)

It is usually a good idea to post a blender scene file with your questions so that others can investigate your problem in detail.

Am I right to assume that you have set up a pool of water and your camera is submerged in the water? In that case, the part that is "too dark" is the reflection on the water surface? It doesn't look bad to me at first glance, as your angles could be below total internal reflection. (hard to tell without a blend file ;) )

Re: Swimming pool / Underwater / Caustics

Posted: Mon Apr 29, 2019 1:45 pm
by Fox
:o It's so beautiful.

Re: Swimming pool / Underwater / Caustics

Posted: Mon Apr 29, 2019 4:11 pm
by fabi
Here is the .blend file, so simple.

You are right, the camera is under the water. I agree the green shall be total reflection, but on my reference photo the caustics are also reflected which makes the green brighter.

Re: Swimming pool / Underwater / Caustics

Posted: Mon Apr 29, 2019 5:00 pm
by B.Y.O.B.
fabi wrote:
Mon Apr 29, 2019 4:11 pm
but on my reference photo the caustics are also reflected which makes the green brighter.
Caustics seen through perfectly specular reflections are so-called "SDS paths" (specular-diffuse-specular), and Bidir can't render these. You would need SPPM or BidirVM for them, or you could try the new PhotonGI caustic cache in the latest daily builds.

Nice scene by the way, could we make it into an example scene? Are the textures under a license that allows redistribution?

edit: looking into your .blend, the shading on the walls is wrong - enable auto smooth in the mesh options.

Re: Swimming pool / Underwater / Caustics

Posted: Mon Apr 29, 2019 5:01 pm
by CodeHD
fabi wrote:
Mon Apr 29, 2019 4:11 pm
but on my reference photo the caustics are also reflected which makes the green brighter.
Aaaah yes, I didn't spot that!
This is because that would be an SDS path (specular-diffuse-specular, here refraction on the water surface, diffuse pool wall and specular reflection on the water surface), which is generally very hard to render for normal ray-trracing algorithms.

Re: Swimming pool / Underwater / Caustics

Posted: Mon Apr 29, 2019 5:45 pm
by fabi
You can use the scene for your example scenes if you like, with pleasure. The textures come from sketchuptextureclub.com, I don't know about their redistribution policy, the textures I took are free, and you have to register for download.

I will download v2.2 beta to test PhotonGI, and post the result as I got it working.

Thank you for your support.

Re: Swimming pool / Underwater / Caustics

Posted: Mon Apr 29, 2019 5:52 pm
by B.Y.O.B.
fabi wrote:
Mon Apr 29, 2019 5:45 pm
You can use the scene for your example scenes if you like, with pleasure. The textures come from sketchuptextureclub.com, I don't know about their redistribution policy, the textures I took are free, and you have to register for download.
Thanks! I'll replace the textures.
fabi wrote:
Mon Apr 29, 2019 5:45 pm
I will download v2.2 beta to test PhotonGI, and post the result as I got it working.
I have played around with the caustic cache a bit, but it's pretty frustrating. You can always see the individual photons, I couldn't figure out settings that fix that.
Anyway, here some pictures that illustrate how PhotonGI can render SDS paths while Bidir can not. I also did a render with the experimental BidirVM engine which can also render SDS paths, but is not in a usable state (you won't find it in Blender for this reason).

I think Bidir + Metropolis is the way to go.
You can try to use the new OIDN denoiser that is available in the v2.2 alpha to get away with less samples.

Re: Swimming pool / Underwater / Caustics

Posted: Tue Apr 30, 2019 1:59 pm
by FarbigeWelt
B.Y.O.B. wrote:
Mon Apr 29, 2019 5:52 pm
I have played around with the caustic cache a bit, but it's pretty frustrating. You can always see the individual photons,
Who d’ you tell? 😉

Re: Swimming pool / Underwater / Caustics

Posted: Tue Apr 30, 2019 2:54 pm
by lighting_freak
Hi all,

One more question from my side. You said SDS paths are impossible to render with BiDir...
But is it really impossible or just hard to render?
I'm wondering because the images look quite good, what are you missing?

Physics based render is always connected to some patience... remembering older days where such kind of scene took some days to render...

BR