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And further to that, a huge factor in deciding whether an object needs cleaning up via AA sampling OR shading sampling is it's surface area.
Say for example you've got a wide flat plane with a glossy material. Each pixel of the plane sitting next to each other will have slight variation because A.) different parts of the object will reflect different parts of your scene and B.) When your sampling is quite low, you won't have taken enough samples for your glossy material for the final result to be accurate, so you'll get huge variation from one pixel to another. We see this in our renders as "noise" or "grain". So once this shading sampling is done, vray hands back those results to be then processed by the image sampler. If it sees lots of variation between the pixels, it presumes that it's noise and will need to be cleaned up, so we get the AA sampler starting to do lots of work to try and smooth out the bad results our shading sampling has done. This theory works for large, flat, simple things.
Now lets say you've got an object that has loads of small, fine detail like fur or grass or trees. A lot of the details in these objects might end up only being a pixel or two wide in your final image. The quality of shape / outline / silhouette of your object will be determined by your anti aliasing sampling, so no matter what, if you've got loads of fine detail or geometric edges, you'll end up using lots of aa samples here to get smooth results. If we think of our shading noise from the earlier example, a big difference between this and a blade of grass or a bit of fur is that our fur object might only be one, two or three pixels wide in our final image. Previously we had a lot of pixels beside each other, all with highly varying results from our glossy material causing noise. In the case of grass, say if it's only three pixels wide in our final render, that gives you one line of pixels for the left edge of the grass, and since it's an edge it'll get high aa sampling, then you get the middle row of pixels which might have some noise from a glossy material, then you'll have the final right edge row of pixels which has reallyheavy aa and again will be very smooth. So on a blade of grass, it's only really the middle line of pixels that doesn't get massive aa, and since vray's sampling is always judging quality based on pixels beside each other, this middle line of pixels has nothing to compare itself too for starters, and second of all doesn't even cover a large enough amount of pixels or have a large enough surface area in our final render to visually detect any noise or grain so you might as well not bother with lots of shading sampling on these thin objects and just let aa handle.
Long story short - big, wide, flat surfaces lots of shading sampling, especially if glossy. If big, wide, flat surface has a fine bump or displacement though, it's kind of like having lots of fine edges so slightly less shading sampling, more AA.
Thin, detailed, fine geometric surfaces, lots of aa, less shading sampling.
Say for example you've got a wide flat plane with a glossy material. Each pixel of the plane sitting next to each other will have slight variation because A.) different parts of the object will reflect different parts of your scene and B.) When your sampling is quite low, you won't have taken enough samples for your glossy material for the final result to be accurate, so you'll get huge variation from one pixel to another. We see this in our renders as "noise" or "grain". So once this shading sampling is done, vray hands back those results to be then processed by the image sampler. If it sees lots of variation between the pixels, it presumes that it's noise and will need to be cleaned up, so we get the AA sampler starting to do lots of work to try and smooth out the bad results our shading sampling has done. This theory works for large, flat, simple things.
Now lets say you've got an object that has loads of small, fine detail like fur or grass or trees. A lot of the details in these objects might end up only being a pixel or two wide in your final image. The quality of shape / outline / silhouette of your object will be determined by your anti aliasing sampling, so no matter what, if you've got loads of fine detail or geometric edges, you'll end up using lots of aa samples here to get smooth results. If we think of our shading noise from the earlier example, a big difference between this and a blade of grass or a bit of fur is that our fur object might only be one, two or three pixels wide in our final image. Previously we had a lot of pixels beside each other, all with highly varying results from our glossy material causing noise. In the case of grass, say if it's only three pixels wide in our final render, that gives you one line of pixels for the left edge of the grass, and since it's an edge it'll get high aa sampling, then you get the middle row of pixels which might have some noise from a glossy material, then you'll have the final right edge row of pixels which has reallyheavy aa and again will be very smooth. So on a blade of grass, it's only really the middle line of pixels that doesn't get massive aa, and since vray's sampling is always judging quality based on pixels beside each other, this middle line of pixels has nothing to compare itself too for starters, and second of all doesn't even cover a large enough amount of pixels or have a large enough surface area in our final render to visually detect any noise or grain so you might as well not bother with lots of shading sampling on these thin objects and just let aa handle.
Long story short - big, wide, flat surfaces lots of shading sampling, especially if glossy. If big, wide, flat surface has a fine bump or displacement though, it's kind of like having lots of fine edges so slightly less shading sampling, more AA.
Thin, detailed, fine geometric surfaces, lots of aa, less shading sampling.
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