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Expose SkColorSpaceTransferFn inversion function

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Also adds tolerance to checks against zero

BUG=skia:

Change-Id: I2ad5737c6eef7e3ed52a685dceb347a434607336
Reviewed-on: https://skia-review.googlesource.com/9643
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
This commit is contained in:
Brian Osman 2017-03-14 11:27:13 -04:00 committed by Skia Commit-Bot
parent dce25909ac
commit 73e6270748
3 changed files with 51 additions and 47 deletions
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6
include/core/SkColorSpace.h
View File

@ -48,6 +48,12 @@ struct SK_API SkColorSpaceTransferFn {
float fD;
float fE;
float fF;
/**
* Produces a new parametric transfer function equation that is the mathematical inverse of
* this one.
*/
SkColorSpaceTransferFn invert() const;
};
class SK_API SkColorSpace : public SkRefCnt {

44
src/core/SkColorSpace.cpp
View File

@ -632,3 +632,47 @@ bool SkColorSpace::Equals(const SkColorSpace* src, const SkColorSpace* dst) {
serializedSrcData->size());
}
}
SkColorSpaceTransferFn SkColorSpaceTransferFn::invert() const {
// Original equation is: y = (ax + b)^g + e for x >= d
// y = cx + f otherwise
//
// so 1st inverse is: (y - e)^(1/g) = ax + b
// x = ((y - e)^(1/g) - b) / a
//
// which can be re-written as: x = (1/a)(y - e)^(1/g) - b/a
// x = ((1/a)^g)^(1/g) * (y - e)^(1/g) - b/a
// x = ([(1/a)^g]y + [-((1/a)^g)e]) ^ [1/g] + [-b/a]
//
// and 2nd inverse is: x = (y - f) / c
// which can be re-written as: x = [1/c]y + [-f/c]
//
// and now both can be expressed in terms of the same parametric form as the
// original - parameters are enclosed in square brackets.
SkColorSpaceTransferFn inv = { 0, 0, 0, 0, 0, 0, 0 };
// find inverse for linear segment (if possible)
if (!transfer_fn_almost_equal(0.f, fC)) {
inv.fC = 1.f / fC;
inv.fF = -fF / fC;
} else {
// otherwise assume it should be 0 as it is the lower segment
// as y = f is a constant function
}
// find inverse for the other segment (if possible)
if (transfer_fn_almost_equal(0.f, fA) || transfer_fn_almost_equal(0.f, fG)) {
// otherwise assume it should be 1 as it is the top segment
// as you can't invert the constant functions y = b^g + c, or y = 1 + c
inv.fG = 1.f;
inv.fE = 1.f;
} else {
inv.fG = 1.f / fG;
inv.fA = powf(1.f / fA, fG);
inv.fB = -inv.fA * fE;
inv.fE = -fB / fA;
}
inv.fD = fC * fD + fF;
return inv;
}

48
src/core/SkColorSpaceXform_A2B.cpp
View File

@ -88,52 +88,6 @@ static inline bool gamma_to_parametric(SkColorSpaceTransferFn* coeffs, const SkG
return false;
}
}
static inline SkColorSpaceTransferFn invert_parametric(const SkColorSpaceTransferFn& fn) {
// Original equation is: y = (ax + b)^g + e for x >= d
// y = cx + f otherwise
//
// so 1st inverse is: (y - e)^(1/g) = ax + b
// x = ((y - e)^(1/g) - b) / a
//
// which can be re-written as: x = (1/a)(y - e)^(1/g) - b/a
// x = ((1/a)^g)^(1/g) * (y - e)^(1/g) - b/a
// x = ([(1/a)^g]y + [-((1/a)^g)e]) ^ [1/g] + [-b/a]
//
// and 2nd inverse is: x = (y - f) / c
// which can be re-written as: x = [1/c]y + [-f/c]
//
// and now both can be expressed in terms of the same parametric form as the
// original - parameters are enclosed in square brackets.
// find inverse for linear segment (if possible)
float c, f;
if (0.f == fn.fC) {
// otherwise assume it should be 0 as it is the lower segment
// as y = f is a constant function
c = 0.f;
f = 0.f;
} else {
c = 1.f / fn.fC;
f = -fn.fF / fn.fC;
}
// find inverse for the other segment (if possible)
float g, a, b, e;
if (0.f == fn.fA || 0.f == fn.fG) {
// otherwise assume it should be 1 as it is the top segment
// as you can't invert the constant functions y = b^g + c, or y = 1 + c
g = 1.f;
a = 0.f;
b = 0.f;
e = 1.f;
} else {
g = 1.f / fn.fG;
a = powf(1.f / fn.fA, fn.fG);
b = -a * fn.fE;
e = -fn.fB / fn.fA;
}
const float d = fn.fC * fn.fD + fn.fF;
return {g, a, b, c, d, e, f};
}
SkColorSpaceXform_A2B::SkColorSpaceXform_A2B(SkColorSpace_A2B* srcSpace,
SkColorSpace_XYZ* dstSpace)
@ -279,7 +233,7 @@ SkColorSpaceXform_A2B::SkColorSpaceXform_A2B(SkColorSpace_A2B* srcSpace,
} else {
SkColorSpaceTransferFn fn;
SkAssertResult(gamma_to_parametric(&fn, gammas, channel));
this->addTransferFn(invert_parametric(fn), channel);
this->addTransferFn(fn.invert(), channel);
}
}
}