Image Samplers

[RayOfLight]

first of all, this might not yet be the correct place to start this topic, but I'm planning on working towards a render theory...

I'd like to start with the question about the subdivs in the image sampler.
a subdiv of 1 uses 4 samples per pixel. Where are those samples taken?

[vlado]

It depends on what image sampler you are using.

For the fixed rate and Adaptive DMC sampler, the samples are distributed inside each pixel.

For the Adaptive Subdivision sampler, the samples are in the vertices of a triangular grid based on the corners of pixels (you can get a glimpse of this if you use the 'Show samples' option) for rates <=0, and inside the pixels for rates >=1.

Best regards,
Vlado

[RayOfLight]

so, if you use fixed or DMC it will apply sampling to each pixel, while ASD (adaptive subdivision) can skip easy pixels.
In the manual it says that ASD isn't good for glossy/blurry surfaces, DMC is better at it, and fixed is perfect for it (if you have a lot of it).
Is this only because fixed will only do what it has to and then move on?
Why is ASD so bad at blurries? Can't it do almost the same as DMC?

[vlado]

DMC is currently on par with the fixed sampler as far as speed and quality goes (e.g. it will not be slower or worse than a fixed sampling of the same max. subdivs). Currently I only use the fixed sampler as a quick switch to turn AA off (e.g. take just 1 sample per pixel).

The adaptive subdivision sampler is worse at blurry effects, because it will always take the full amount of samples for the blurry values for every sub-pixel sample. By contrast, the adaptive DMC sampler can interleave the blurry samples with the AA samples.

For example, let's say that there is a glossy reflection that takes 16 reflection rays. Imagine that there is a pixel on a sharp shadow edge which needs 8 samples to get good AA. With the adaptive subdivision sampler, there will be a total of 8 camera rays, and 8*16=128 reflection rays. With the adaptive DMC sampler, there will still be 8 camera rays, but the 16 reflection rays will be interleaved between the camera ones so each camera ray will only get 2 reflection rays. In the end, with the DMC sampler, you will get 8 camera rays and 16 reflection rays.

So in the end, the adaptive subdivision sampler will take 8+8*16=136 rays for the pixel, and the adaptive DMC sampler - only 8+16=24 rays.

This is why, for blurry effects, the DMC sampler performs best.

Best regards,
Vlado

[RayOfLight]

Does DMC give the same result in that case? With only 24 rays? Or will the actual reflection be get less good AA (whether you can see it or not is another question I think)

[ene.xis]

also bear in mind that the adaptive subdivision sampler will take more memory during rendertime, wich in some cases can be an issue!

[RayOfLight]

I know, that's why I was thinking about a new image sampler, which can have the best of all three combined...like a sampler that works like DMC but also has the possibility of undersampling when there is no need for AA on a pixel..

[vlado]

Well, I've been thinking about this, so it may appear at some point, thus removing the need for the three different samplers at all, but we'll see.

Best regards,
Vlado

[m_hinks]

Sounds nice! and hard! Good luck with that Vlado.

[RayOfLight]

that sounds promising!
On the same subject, while undersampling, is there a need for settings below -1?
Or would be equally fast if a pixel would be skipped for AA when it doens't need AA?

perhaps with lower values you can check AA per bucket instead of per pixel, but this already happens in subdivision I guess...

[ramy02]

I made a chart that compares material reflection glossy samples and image sampler min max values. Check out the pdf. You be the judge!

[Fekta.studio]

verry nice, I allways set min/max to 1-4 , mat. sub 50, Saa/Nt 0.85/0.01, and get reasonable render times with allmost no noise, but sometimes try lowering noice treshhold to about 0.005 and even 0.001, is this SP2 ? i read Lele write that new SP2 samplier is mutch more noise free than SP1.

[Kyle_Grace]

thanks for the chart, that's a great resource!

[stilgarna]

Hello,

a simple test between Max2008 Vray1.50.SP1 and Max2009 Vray1.50.SP2

SP1 :


SP2 :



Noise is sightly lower on SP2 and Glossy reflection is far better...

Here the scene settings :

[stilgarna]

Hello,

Here another test with Clr Tresh set to 0.001

SP1 :


SP2 :