Speckles on irmap rawgi pass

[tricky]

What could be the cause of the fine speckles on this rawgi pass? We often use the rawgi in post to add a bit of 'oomph' to our work, but with this, we couldn't do that.



My settings are:

[tricky]

I wonder if it could be detail enhancement (which uses BF I believe)? Will try unchecking this.

[joconnell]

Vray uses the physics energy conservation law which means that a surface can not reflect more light than originally shines on it, and the amount of diffuse, reflectivity and refractivity all add up to this. For example you can't have a 100% reflective surface that's also 100% refractive, nor can you have a surface that's 100% diffuse and also 100% reflective - it'd look totally unnatural. In vray if you have a solid material that's 100% diffuse, no spec and no reflectivity but then make it 50% reflective, vray will take away 50% of the diffuse to make everything balance out and natural. If you've make it 90% reflective, then it'll cut the diffuse down to 10% so the physics balances out correctly. In your element here, vray is kind of doing this conservation stuff with it's passes too. Is there a chance that the black pixels in that render are 100% reflective or refractive in certain areas? If they're 100% reflective like in really bright speckles on the couch, then there is no diffuse surface left for light (and in this case GI light) to fall on - the 100% reflective pixels are totally knocking it out. A second possibility is that those pixels are 100% black in their diffuse. No matter how much light you shine on a pure black surface, it'll absord all of it and still appear black. In the case of your element, it's the raw amount of GI light that's bouncing around your scene which is then going to get multiplied by the colour of the surface (diffuse filter) to make the final appearance of the global illumination. If those pixels are falling on a surface that's 100% black in those areas, then vray won't care about them - any amount of raw GI light values multiplied by a 100% black value in your diffuse filter will always end up at black anyway. Vray's early termination is designed to see those areas where sampling won't actually contribute anything to the final look of the image and just stop sampling. You'll see it quite often in the raw reflection pass too. If you've got a surface that's using fresnel and a very weak reflection amount, the very front of your object might only be 0.01% reflective. It makes so little difference to the final pixels of that object, vray won't put too much effort into sampling it. When you look at this in the raw ref pass though, it can appear like your raw gi element - some bits of reflective pixels, some plain black pixels with no data. It's the danger of looking at the raw passes only and using them as an absolute guide - you've gotta take into account the amount that the raw pass is going to be multiplied down by to get a true picture of the final result.

Ultimately vray is trying to be as efficient as possible. It's skipping over parts of the sampling that don't matter in the final render to get speed benefits. If it spent the same amount of time sampling something that contributes 0.1% towards the final look as it did with something contributing 100%, then you'd spend ages sampling something you barely see anyway!

[trixian]

Just replying to commend you on your new black belt in the ancient art of "wall of text"
Please don't take offense......