Filtering

Filtering

Filtering is where the renderer assembles the pixel samples to produce images for display. There are several options here.

The most important one is the pixel filter, which is responsible for antialiasing. There are a number of choices of filters available and most of them take a size in pixels. The choice of the filter and size affects the amount of antialiasing and the image sharpness. Note that some sharpening filters may cause negative color values. The default is a 2x2 pixel Gaussian. This is a relatively soft filter that has good antialiasing with a light blurring.

images/filtGaussian.png

2x2 Gaussian filter

images/filtMitchell.png

4x4 Mitchell filter

images/filtSinc.png

4x4 sinc filter

Although non-integer filter sizes are allowed, we recommend integer sizes since they tend to produce less image noise in some situations.

Another option is whether or not to use the adaptive noise suppressor. This compares each new sample to the other previous ones nearby to try to determine if it's consistent with them or not. If not, the sample is rejected before it would have been sent to the pixel filter and added to the image. This is helpful for removing samples with extreme values, known as spike noise or "fireflies", that can cause a render to take a long time to converge to being noise-free.

Remapping is an option that can help in dealing with very bright specular and glossy highlights as well as directly visible light sources. Depending on how bright these are, they can sometimes dominate the average of the samples in neighboring pixels leading to these highlights appearing aliased. They can also contribute to noisy glossy reflections and caustic speckles on diffuse surfaces that can take a long time to converge.

Remapping solves this by performing the pixel filtering in a non-linear space. It compresses the samples down to a limited, finite range before applying filtering and then undoes the compression on the pixels before display. Where there is no variation in the samples this has no effect (even if they are still quite bright). Where there is significant variation, the dimmer samples will have more weight.

images/remapOff.png

No remapping

images/remap110.png

Remapping [1 10 1]

images/remap001.png

Remapping [0 1 1]

For needs that go beyond any of these, a pixel sample imager can be used. These are shaders that can inspect and modify the shaded samples before they are tested against the adaptive noise suppressor or accumulated into the image with the pixel filter. They can be used for things like vignetting, tone mapping, and other effects.