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Document BitmapStateAutoMapper.

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BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1815373002

Review URL: https://codereview.chromium.org/1815373002
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herb 2016-03-19 08:46:24 -07:00 committed by Commit bot
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src/core/SkBitmapProcState.h
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@ -200,6 +200,21 @@ void ClampX_ClampY_nofilter_affine(const SkBitmapProcState& s,
uint32_t xy[], int count, int x, int y);
// Helper class for mapping the middle of pixel (x, y) into SkFractionalInt bitmap space.
// Discussion:
// Overall, this code takes a point in destination space, and uses the center of the pixel
// at (x, y) to determine the sample point in source space. It then adjusts the pixel by different
// amounts based in filtering and tiling.
// This code can be broken into two main cases based on filtering:
// * no filtering (nearest neighbor) - when using nearest neighbor filtering all tile modes reduce
// the sampled by one ulp. If a simple point pt lies precisely on XXX.1/2 then it forced down
// when positive making 1/2 + 1/2 = .999999 instead of 1.0.
// * filtering - in the filtering case, the code calculates the -1/2 shift for starting the
// bilerp kernel. There is a twist; there is a big difference between clamp and the other tile
// modes. In tile and repeat the matrix has been reduced by an additional 1/width and 1/height
// factor. This maps from destination space to [0, 1) (instead of source space) to allow easy
// modulo arithmetic. This means that the -1/2 needed by bilerp is actually 1/2 * 1/width for x
// and 1/2 * 1/height for y. This is what happens when the poorly named fFilterOne{X|Y} is
// divided by two.
class SkBitmapProcStateAutoMapper {
public:
SkBitmapProcStateAutoMapper(const SkBitmapProcState& s, int x, int y,