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DingOunan

problems about sampling tech

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DingOunan    136
I am reading the chapter 5 in the book "Ray tracing from the ground up" the author said "it's important to use different sets(of samples) with adjacent pixels...(If we don't do this)...results can be aliasing artifacts..." I don't understand what would happened when I applied same sets of samples onto the adjacent pixels(the author give an example picture with this problem, but I think it is just noise due to the glossy reflection with little samples) [IMG]http://uploader.polorix.net//files/258/private/swap/Figure05.05%28a%29.jpg[/IMG] thanks [Edited by - DingOunan on March 13, 2010 7:03:40 PM]

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haegarr    7372
Alias can be seen where the black grid is adjacent to the white tiles. The blue area perhaps shows a sampling scheme with jitter, where the jitter may be exaggerated for the purpose of demonstration.

To avoid alias, you have to drive oversampling so that the sample rate is twice as high as the highest detail frequency. However, this is a bit vague w.r.t. a 3D scene. So you can sample at a given rate and low-pass filter the resulting image later on.

However, a method to hide alias a bit is to avoid regular sampling by applying some jitter, so that alias is blended with noise. Doing so naively means to jitter the sample in its target pixel w/o considering neighbour samples. In that case you may get too close what looks like a reduced sample rate. It is best to consider the Poisson Disc Distribution (IIRC), but that is very costly. A compromise is to jitter a ray independently but only over an inner area of the pixel, so that a minimum distance of samples is guaranteed. Other schemes exist as well.


BTW: Please don't use [IMG] but regular HTML tags for images.

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Programmer101    199
The reason you want to use different sample sets is the same as the reason that you would jitter samples in the first place. Using different sets eliminates repeating patterns in the render. Look at the figure on the next page for a better description of the problem (fig 5.6).

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