Righty. You can have a look at this in combination with Akin’s calculator to see how this is working. I’m going to do some incorrect maths just cos it’s handier for me to not think of the subdivs being squared so just look at the idea but not the numbers as being accurate.
- Say you have aa set to 1 / 5 and a material set to 50 subdivs, the aa’s max value of 5 divides our 50 value so we get 10 samples for each time an AA sample is used. Lets pretend in this render that our materials are doing something like driving glossiness by a detailed bitmap so they’re still noisy, so we need to up our material samples to clean this up.
- Next we’ll keep our aa 1 / 5 but increase our material subdivs to 500. Again our max aa of 5 comes in and divides our material samples but we’re now getting 100 samples for every aa ray shot. Lets say with this render our materials are now really clean. Lets say to throw a spanner in the works we now decide we need to render with 3d motion blur or depth of field. This is a geometry issue so that means we’ve to increase our max aa samples to clean this up.
- Say if we now go up to aa of 1 / 50, again the aa’s max value divides our 500 value again but now we’re only getting 10 material samples for every AA sample used. This is kind of back down to a similar amount as what we started off with and lets pretend that our motion blur or depth of field is nice and smooth due to our 50 aa samples but our material has become a bit dirty again, here’s where the aa division of material samples becomes a bit annoying. If we need to get back up to our 100 samples for every aa ray shot, then we’d have to set our materials to be 50 x 100 = 5000 (Again a warning, not actual proper numbers, just ideas here!). It’s actually hard to do that since the spinner for all of the subdiv settings are locked to a max of 1000, and it’s a bit of a pain in the ass to do anyway.
- Now on to our min shading rate! What this rather handy control will do is not give a shit about the division stuff going on, and it’ll set a minimum amount of secondary samples that vray must take for every aa ray. If we continue with the above example where we’ve got our aa at 1 / 50 and we’d have to set our material at 5000 to get our previous number of 100 secondary samples per aa ray, we can just set our min shading rate to 100 instead. What it’ll do is tell vray for every time you use an AA ray, make sure that you take 100 secondary samples every time if the amount of secondary samples you were going to use was less than 100. With this what you can do is freely set your AA to whatever you want, and then instead of having to use really high numbers in each of the secondary samplers controls, you’ve got one control to gradually up all of these things without having to give a crap about any of the division stuff.
The previous issue with the universal stuff was that the 1 / 100 settings would sometimes make it difficult to get some of the secondary sampling clean - the AA was fine but materials and light could be a bit grainy. The min shading rate gives you a handy control to force up the standards of that.
You’re pretty much correct on the colour threshold - the lower down that goes, the closer all of the results have to be in vray before it’ll consider the rendering good enough so ultimately it’s the boss of absolutely everything. Vray doesn’t really give a shit where the clean quality is coming from whether AA or material sampling, it’s just constantly aiming to hit that colour threshold.