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Add comment explaining the derivation of our Mitchell coefficients

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BUG=skia:

Change-Id: I8866df425ee9837e75f0b2f76777f7e5d68fb21d
Reviewed-on: https://skia-review.googlesource.com/5624
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
This commit is contained in:
Brian Osman 2016-12-06 11:53:06 -05:00 committed by Skia Commit-Bot
parent 886cf53447
commit 864f6bde37
1 changed files with 16 additions and 0 deletions
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16
src/gpu/effects/GrBicubicEffect.cpp
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@ -15,6 +15,22 @@
#define DS(x) SkDoubleToScalar(x)
/*
* Filter weights come from Don Mitchell & Arun Netravali's 'Reconstruction Filters in Computer
* Graphics', ACM SIGGRAPH Computer Graphics 22, 4 (Aug. 1988).
* ACM DL: http://dl.acm.org/citation.cfm?id=378514
* Free : http://www.cs.utexas.edu/users/fussell/courses/cs384g/lectures/mitchell/Mitchell.pdf
*
* The authors define a family of cubic filters with two free parameters (B and C):
*
* { (12 - 9B - 6C)|x|^3 + (-18 + 12B + 6C)|x|^2 + (6 - 2B) if |x| < 1
* k(x) = 1/6 { (-B - 6C)|x|^3 + (6B + 30C)|x|^2 + (-12B - 48C)|x| + (8B + 24C) if 1 <= |x| < 2
* { 0 otherwise
*
* Various well-known cubic splines can be generated, and the authors select (1/3, 1/3) as their
* favorite overall spline - this is now commonly known as the Mitchell filter, and is the source
* of the specific weights below.
*/
const SkScalar GrBicubicEffect::gMitchellCoefficients[16] = {
DS( 1.0 / 18.0), DS(-9.0 / 18.0), DS( 15.0 / 18.0), DS( -7.0 / 18.0),
DS(16.0 / 18.0), DS( 0.0 / 18.0), DS(-36.0 / 18.0), DS( 21.0 / 18.0),