Vray HDR Exposure Blowouts

[jujubee]

I've created a bunch of HDRs, but I am experiencing issues with the Vray interpretation of their exposure values. I can't my HDRs correct when it comes to low values. Either there is another method which I am unaware of, or Vray's interpretation of HDR acts incorrectly. I've outlined my steps/experiment below:

In this example, I've loaded the plain HDR via the splutterfish plugin into the environment. The colors look on target:


In this example, the same HDR is loaded into VrayHDR. The colors are blown out. Reducing the multiplier to extremely low values still renders it bright while the background disappears:


In this example, the same plain HDR is loaded into the Vrayskydome. But it is still too bright and the multiplier doesn't fix things:


Not wanting these overbright values, I resaved the original HDR with clamped colors via the Splutterfish (default Max) I/O plugin. I tried numerous methods to try to clamp these values but no other program I could find would do it. So I set the white point to the left and resaved. Then I click 'view', view the image and resave the new HDR through the Max dialog:


This is the result of the new (clamped) HDR being loaded into the Vraydome. It looks on target again:


This is the result of the new HDR with the multiplier set to 1.5. It behaves as it should by brightening in specific areas (per pixel stored information - the values which make an HDR an HDR):


This is the result of the new HDR with the multiplier set to .5. Below 1.0, the HDR loses its dark exposure values - the scene gets dark equally as a whole. This is incorrect. The sun should be a bright pin while the sky darkens around it. No changes/resaving within the Splutterfish plugin makes a difference:



As a separate example of the above lighting, here are the clamped colors out of the Splutterfish plugin as viewed in HDRShop:

default exposure:


+2 stops exposure:


-2 stops exposure:


This is -2 stops exposure on the original unclamped HDR. Compare the sun to the one above. This is correct:




As I stated in the beginning. The original HDR works perfectly with the default Splutterfish plugin and in all other 3d applications. There appears to be nothing wrong with this. Vray displays this HDR incorrectly by comparison. Resaving the HDR via Splutterfish HDR I/O corrects the overbright values, but end up clamping the dark values unwantedly.

I'm at a loss and could really use someone's help after spending a few weeks working on HDRs. I've been on the Splutterfish IRC channel as well as Debevec's forum and don't have any answers. If you would like to test the unclamped HDR and try to 'fix' this scenario so Vray can properly display this HDR, I would greatly appreciate it.

This HDR can be found here. (11 MB)

[jujubee]

After 4 hours of sleep, 2 cups of coffee, and 2 cigarettes I think I've finally figured something out. I'm clamping the white point too far to the left which causes it not to get darker. The further right I clamp, I can successfully dial down the dark values but at a cost of developing splotches which is an entirely different issue altogether.

In comparison with other HDRs, these splotches occur less. In fact, the majority of HDRs I tested produced significantly more splotches at the same Vray settings. However, when I clamped to the left, the splotches tended to disappear, but at the initial loss of darker HDR exposure.

When comparing the two I/O methods (Splutterfish vs. Vray), the new skydome excels in reducing these artifacts with 'properly' clamped HDR in Vray.

The biggest factor in reducing splotches appears to be higher resolution of HDR (hence more projected light samples), at the penalty of incurring larger memory usage. The other way to help reduce HDR artifacts is to use higher quality Vray settings.

[vlado]

You are not using the HDR in both the dome light and GI environment, are you? If you use the dome light with a HDR, make sure the GI environment is perfectly black.

Best regards,
Vlado

[jujubee]

Regarding the splotches, they are even further reduced if not instanced/copied to the environment. But - what should you do if you want it to display in the background?

[instinct]

In that case simply put the HDR in the MAx environment slot and check the "Override Max Environment" box in the VRAY Environment settings and change the bright blue colour to black.

Regards,
Thorsten

[jujubee]

Thanks Thorsten. Works even better. I have to add that wasn't very intuitive...

[Morbid Angel]

Vlado, he's problem comes from the hdri color clamping, I have the same thing. Whilst I dont have the new vray so only through the eviroment we get negatives values not working.
Juju I had an idea: why not save the hdri from negative exposure instead of midpoint, save it from -3, then one can assume the range from -3 to ~.
Vlado, I also noticed same thing with debevec's sky hdris, where I had to set vlaue to 0.001 ot 0.0005 to get the correct brightness.

[jujubee]

Yeah - that was the solution but I had to figure out how to obtain that while making it look normal at 1.0.

[vlado]

Thanks Thorsten. Works even better. I have to add that wasn't very intuitive...

Maybe, but it's logical. This works the same way as when you normally use a dome light with GI. When you use the same HDR map both in a dome light and as a GI environment, it will be added twice to the scene - once by the dome light, and once by the GI. That's why you need black GI environment.

Best regards,
Vlado

[vlado]

Vlado, he's problem comes from the hdri color clamping, I have the same thing. Whilst I dont have the new vray so only through the eviroment we get negatives values not working.
Juju I had an idea: why not save the hdri from negative exposure instead of midpoint, save it from -3, then one can assume the range from -3 to ~.
Vlado, I also noticed same thing with debevec's sky hdris, where I had to set vlaue to 0.001 ot 0.0005 to get the correct brightness.

This is because the sun and sky are typically very very bright. The situation is very similar with the standard 3dsmax Daylight and Sunlight systems - by default they produce extremely bright lighting. This is also the case with the new sun and sky system in newer versions of V-Ray.

Best regards,
Vlado

[jujubee]

In that case simply put the HDR in the MAx environment slot and check the "Override Max Environment" box in the VRAY Environment settings and change the bright blue colour to black.

Well- this method works to smooth out splotches better on a clamped HDR. However, this method still blows out the scene with a bright white when using the original HDR like the first few examples.

As a side-note, I don't think I should have to clamp the brights down to begin with as this should be able to handle floating-point (wrong term?) - I'm also not sure how this equates to generating more splotches the darker I go.

I should probably add that the main problem isn't related to the Vrayskydome (I just prefer using it at this point cause it's awesome...), so just about anyone could test these using the standard VrayHDRI.

[Dschaga]

I had similar problems with my terragen hdris or faked hdrs with vray.
One really usefull solution is to resize the skyhdri to 256x256px and blur it.
This gives you impressive rendertimes and acceptable shadows.

@vlado:
The Sky is normaly not really bright.
The Sky have about 5000cd/m2 and the sun ~85000lx (it's a spotlight).

So, most lofi hdri's don't have the real range (no real sun) and vray doesn't have any problems with them, but when you use a hdri with a huge range (from terragen) , vray seems to habe serious problems. .. maybe with the way the renderer samples the rays from now two different light emiters (sky and sun)

[jujubee]

Thanks Dschaga. That's a really good link. I wouldn't call these HDRs 'fake' as they do contain HDR (multiple exposure) information. 'Synthetic/fake environment' is probably more appropriate.

Undoubtedly they are not as realistic as a photographed sky HDR - however most real skies contain other unwanted information such as backgrounds, background dimensions, the photographer, blur, distortion, chromatic abberation, and a lack of samples. This can actually lead to a lack of finer precision. The Spheron are great, but they're often prohibitive on a system's resources.

I'm actually getting superb results at this point and I'm pretty excited about how they work - the lighting is arguably much better than any of the other libraries/HDRs which I've tested. There is more control/refinement over your lighting (albeit unnatural) than anything in the real world as you can position and control time of day, sky clarity, color, terrain, backgrounds, etc.

I think you are absolutely correct in all of your statements, especially this one:

but when you use a hdri with a huge range, vray seems to habe serious problems.