[Pixman] [cairo] Image resampling [PATCH 0/6]

[Bill Spitzak]

Siarhei Siamashka wrote:

> Bilinear scaling uses weighted average of 2 pixels (in one direction),
> with weights calculated on the go. Separable convolution uses weighted
> average of N pixels, with weights obtained by table lookups. Both use
> subpixel positions (7 phase bits or 128 phases for current bilinear
> implementation) to lookup or calculate weights. Bilinear filter is
> naturally a subset of separable convolution.

Also note that the separable filters need different "widths" as well as 
different phases. This is the distance from the center to the first 0 
crossing in a sync filter, note that the non-zero part of the filter is 
several times the width (3 for a triangle, 5 for most, and 7 for 
lancos3). If the image is being scaled by 1/N the width is max(1,N).

There probably should be a pre-calculated table of all combinations of 
width and phase. The minimum width is 1, but there will have to be a 
maximum chosen. Pixman would use the maximum if a larger size is wanted, 
which would mean that scales smaller than a certain value would be 
noisy. However if Cairo prescales by a power of 2 then the maximum scale 
may be 4.

In Nuke the pre-calculated table is just a very wide filter (I think the 
width is 64) and that is the maximum filter size. Smaller filters and 
different offsets are chosen by subsampling this large filter at even 
intervals (Nuke chose the closest sample, but bilinear sampling would be 
better). The table was "normalized" so that the width of 1 produced 
samples that sum to 1, all larger filters needed to a normalization 
factor calculated by doing 1/sum of the samples.