glass caustics

[wimmm]

Hi everyone

I'm having problems rendering caustics. I have a simple object with the glass preset material applied to it. When I check the caustics in the render tab, the object just creates a shadow of the object, when I uncheck it, I do seem to get a caustic, but I can't control it. When I change the settings in the wrapper, both for the object recieving as for the one generating, It has no effect. When I check caustics and change the caustic samples of my sun from 1000 to 20000, the calculation takes much longer, but I see no change. When I look at the caustics render channel, it's completely black.

The first image is with caustics turned on, the second with caustics turned off. I was expecting it to work the other way around.




What am I doing wrong?

Thanks in advance for all help and ideas!

[rws]

Re: glass caustics

When caustics are off you get fake caustics like your second image.

Share your scene (or make sample scene) so we can see problem, could be of many things.

[Wolf3D]

Re: glass caustics

Is the affect shadows activated in the material refraction?

[wimmm]

Re: glass caustics

Hi,

Thanks for the advice. I'm not behind my own comp now, but will try to post an example scene tomorrow. "Affect Shadows" was activated. Or should it be off?

thanks

[rws]

Re: glass caustics

With the glass preset, it should work without changing (affect shadows will be on).

[IkariShinji]

Re: glass caustics

I'm having some difficulties with caustics in vray/blender, too. See this example:



As you can see I get (faint) reflective caustics, but no refractive caustics. Troubleshooting this effect was quite cumbersome, as the results were somewhat unpredictable - e. g. raising the number of caustic photons fired from the light source (directional light) did not increase the definition of the caustic effect, but made it disappear entirely...

Blender version is vrayblender-2.69-60904-windows-x86_64.exe from this site,
vray version is the standalone from the latest vray_adv_24001_maya2014_x64.zip available at the Chaos Group server.

The glass material is the Pure Glass preset, "Affect Shadows" is on for the refractions. The scene is a demo scene for 3dsmax's vray.
Simplified demo scene - https://dl.dropboxusercontent.com/u/.../Glasses.blend

Is this a known problem/limitation of vray/blender?
Or is the problem - as most of the time - sitting in front of the PC...

EDIT 1:
Oh, and there's another thing:
The "Display in sRGB" checkbox under Exporter -> Advanced doesn't seem to have any effect: The frame buffer always starts with "Display colors in sRGB space" enabled.

EDIT 2:
On another PC I have an older version of both vray/blender and the vray standalone installed (the standalone from vray_adv_20004_maya2012_x64.exe, vrayblender-2.68-60217-windows-x86_64.exe). The very same file produces this render result on that other machine:



But as soon as the number of caustic photons is increased to 10.000, the result is this (clamping is off):

[IkariShinji]

Re: glass caustics

Reverted my version of vray standalone back to the one from vray for maya 2.30.01:
- Refractive caustics are back,
- issue with "Display colors in sRGB" in the frame buffer is solved.

I guess there are compatibility issues with vray/blender + 2.40.01 standalone for now.

Still not solved for me is the mystery of caustics turning black in the render when the number of caustic subdivisions is increased. I'm a bit lost about what settings to check to get that right - any help is appreciated.

[bdancer]

Re: glass caustics

IkariShinji, thanks for all the reports, will check it.

[IkariShinji]

Re: glass caustics

Thanks for looking into this.

Here is another very simple test scene: https://dl.dropboxusercontent.com/u/..._vray_test.zip
It seems as if pixels that exceed a certain brightness are somehow "cut off"? This occurs on all combinations of vray/blender and vray standalone I tested here (except with standalone 2.40.01, which did not show refractive caustics at all - as reported above).

This might all be a user error on my part - I just don't know which settings to check any more...

[yolov]

Re: glass caustics

Hi guys,

As far as caustics settings go, there are some things you need to be aware of. The default settings will practically never work for you, because the settings that will help you get the result you're after vary greatly from scene to scene.
In general the max photons value has to be increased and you'll have to play around with the search distance. Those are the two key settings for vray caustics.
Say I want something similar to what you're posting, I'd start to experiment with anywhere between 300 and 1000 max photons and 0.1 to 1 search distance. Then I'll see where this is getting me and decide what I want to change.
Refractive objects should have "affect shadows" disabled. Also it'll be better to disable "reflective caustics" in the GI settings, because it will slow down your renders and those are pretty different from the vray caustics.

(just a reminder - you'll also need quite some caustics subdivs on your lights, but that also depend. I'll always start adjusting things at ~3000-4000, which in most cases would be far less than what is needed and then I'll get to something like 10,000 and more).

Didn't check to make sure what's the situation in 2.4 but I think this should at least point you in the right direction.

[IkariShinji]

Re: glass caustics

Thanks for chiming in. It was indeed the "max photons" value, which I had to increase to 2.500 in order to get rid of the black spots.

But somehowe that's not how that value is supposed to work, is it? This is from the vray help docs:

Max photons - this is the maximum number of photons that will be considered when rendering the caustics effect on a surface. Smaller values cause less photons to be used and the caustics will be sharper, but perhaps noisier. Larger values produce smoother, but blurrier caustics. The special value of 0 means that V-Ray will use all the photons that it can find inside the search area.

That fits to the effect that can be seen in this tutorial: http://cienel.net/3d_studio_max-tuto...d-studio-max/3

From that docs I get the impression that the "max photons" value has influence on the sharpness/blurriness of the caustics, but should not result in black spots when set to a low (or even the default) value?

[yolov]

Re: glass caustics

I'm no caustics specialists, but I can tell you one thing - max photons and search distance are tightly related - if you change one, everything changes and you'll have to change the other too.
What that means, like mentioned in the help, is that using those two values, you define how many photons (0=all) you want to use for a given area (around any found photon) to have a caustic effect with.

What I can do is make some tests tomorrow to see what is different between what is shown in this tutorial and the current blender/vray versions. I doubt that there have been changes related to caustics in vray.

Also, I'll have to check what units does the "search distance" parameter use. It's possible it always uses cm and you blender scene is set to use other units, or it uses scene units and there are some differences in the way max and blender (and any other 3d app for that matter) handle their scenes.

[IkariShinji]

Re: glass caustics

Thanks again for joining the discussion - this is most helpful.
I don't have access to 3dsMax now, but could talk a colleague into letting me play with his Cinema 4D + vray for C4D...

With vray for C4D I get plausible results by using values in the range as shown in that vray for 3dsMax tutorial (see attached render). So both of these vray implementations seem to be somewhat congruent.

vray/blender, however, is the "deviationist" in the team: With the same settings the caustics render pitch black. All objects have real world scale and blender is set to a metric unit system: Cinema 4D and blender both show e. g. the search distance as 12.7 cm, but the beam radius in blender on the other hand shows no unit at all (I set it to 2.54, as 254 gave other kinds of render artefacts and had no influence on the caustic problem). Again I had to increase the max photons value in blender to several thousands to totally get rid of that black areas.

The lights in blender seem to miss the "photon subdivisions" setting from vray for C4D (at least I can't find it) - not sure if this is of importance?

Feel free to ask for any of the test scenes (.c4d or .blend) - I just did not want to post them here unnecessarily as the geometry in those scenes was not created by me (I think I downloaded it here: http://www.cgdigest.com/vray-caustics-tutorial/) and I'm not sure about the copyright implications.

[yolov]

Re: glass caustics

I'll be doing some tests today and I'm hoping to find out precisely what's different.
Lights in blender have the "Caustics" param, which is what you need to bump up so your lights will shoot more photons for the caustic map.

[yolov]

Re: glass caustics

Edit to the above post - I may not have enough time to do this today, since I have more pressing matter that needs to be dealt with. Will update when I have something useful to share.