what's the 'clamp output'?

[themaxxer]

what means the clamp output option in the color mapping rollout? I don't see a difference when unchecked... maybe I'm blind.

best regards
themaxxer

[vlado]

The difference is only when you have very bright parts in your scene and you use the fixed or adaptive qmc AA samplers. In that case, if the output is clamped, the resulting image may be darker than the correct result. Also, this will ensure that VRay antialiases very bright parts of the image too - this is useful if you want to work with the hdr data later on.

Best regards,
Vlado

[cpnichols]

Does this mean that Vray will do a better job of AA areas with color brighter then 1, but can only do this if the color is clamped back to 1? Would we see aliasing in overbright colors otherwise?

[vlado]

Does this mean that Vray will do a better job of AA areas with color brighter then 1, but can only do this if the color is clamped back to 1? Would we see aliasing in overbright colors otherwise?

Yep, this is the case. Here are two examples that demonstrate the effect of this option.

Both examples use Adaptive QMC AA with Min 1, and Max 20 subdivs, with 0.0001 Noise threshold. This means that maximum of 400 rays are traced per pixel. This is quite enough to compute the pixel values with very good accuracy.

On this image, the dragon is rendered with the "Clamp output" option ON (the default). While this is a good image, it is not correct. The background is very bright, and in reality it would burn out some of the motion-blurred areas.


On this image, the dragon is rendered with "Clamp output" OFF. Notice how the background burns the blurred edges - this is the correct result.


On this image, the sphere is rendered with "Clamp output" ON. The sphere is very bright (it has a VRayLight material with a multiplier of 5.0). The antialiasing is very good.


On this image, the sphere is rendered with "Clamp output" OFF. Notice how it looks jagged. This is not because there is something wrong with the AA - simply the sphere is so bright that it immediately burns out any pixels that it touches.


A good question: is it possible to get the correct result (like in the unclamped dragon case) while keeping good AA (like the clamped sphere)?

And the answer is: no, you can't have both. This is not a limitation of VRay - that's just how things are.

The situation changes if you use color mapping though, but that's a different topic...

Best regards,
Vlado

[flipside]

Very interesting, and a good option too! I've had problems before like in the sphere example.