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Increase accuracy of float -> fixed in ICC code

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Add a comment to SkFixed explaining the accuracy issues of the macros.

Bug: skia:
Change-Id: Ibfecb16821fefe87822cc3acd1cf8498df10a492
Reviewed-on: https://skia-review.googlesource.com/85742
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
This commit is contained in:
Brian Osman 2017-12-15 11:50:20 -05:00 committed by Skia Commit-Bot
parent 2d53d98425
commit ef8dda227b
2 changed files with 23 additions and 10 deletions
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5
include/private/SkFixed.h
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@ -30,6 +30,11 @@ typedef int32_t SkFixed;
#define SK_FixedTanPIOver8 (0x6A0A)
#define SK_FixedRoot2Over2 (0xB505)
// NOTE: SkFixedToFloat is exact. SkFloatToFixed seems to lack a rounding step. For all fixed-point
// values, this version is as accurate as possible for (fixed -> float -> fixed). Rounding reduces
// accuracy if the intermediate floats are in the range that only holds integers (adding 0.5f to an
// odd integer then snaps to nearest even). Using double for the rounding math gives maximum
// accuracy for (float -> fixed -> float), but that's usually overkill.
#define SkFixedToFloat(x) ((x) * 1.52587890625e-5f)
#define SkFloatToFixed(x) sk_float_saturate2int((x) * SK_Fixed1)

28
src/core/SkICC.cpp
View File

@ -281,25 +281,33 @@ static constexpr uint32_t kICCTagTable[3 * kICCNumEntries] {
SkEndian_SwapBE32(kTAG_cprt_Bytes),
};
// This is like SkFloatToFixed, but rounds to nearest, preserving as much accuracy as possible
// when going float -> fixed -> float (it has the same accuracy when going fixed -> float -> fixed).
// The use of double is necessary to accomodate the full potential 32-bit mantissa of the 16.16
// SkFixed value, and so avoiding rounding problems with float. Also, see the comment in SkFixed.h.
static SkFixed float_round_to_fixed(float x) {
return sk_float_saturate2int((float)floor((double)x * SK_Fixed1 + 0.5));
}
static void write_xyz_tag(uint32_t* ptr, const SkMatrix44& toXYZ, int col) {
ptr[0] = SkEndian_SwapBE32(kXYZ_PCSSpace);
ptr[1] = 0;
ptr[2] = SkEndian_SwapBE32(SkFloatToFixed(toXYZ.getFloat(0, col)));
ptr[3] = SkEndian_SwapBE32(SkFloatToFixed(toXYZ.getFloat(1, col)));
ptr[4] = SkEndian_SwapBE32(SkFloatToFixed(toXYZ.getFloat(2, col)));
ptr[2] = SkEndian_SwapBE32(float_round_to_fixed(toXYZ.getFloat(0, col)));
ptr[3] = SkEndian_SwapBE32(float_round_to_fixed(toXYZ.getFloat(1, col)));
ptr[4] = SkEndian_SwapBE32(float_round_to_fixed(toXYZ.getFloat(2, col)));
}
static void write_trc_tag(uint32_t* ptr, const SkColorSpaceTransferFn& fn) {
ptr[0] = SkEndian_SwapBE32(kTAG_ParaCurveType);
ptr[1] = 0;
ptr[2] = (uint32_t) (SkEndian_SwapBE16(kGABCDEF_ParaCurveType));
ptr[3] = SkEndian_SwapBE32(SkFloatToFixed(fn.fG));
ptr[4] = SkEndian_SwapBE32(SkFloatToFixed(fn.fA));
ptr[5] = SkEndian_SwapBE32(SkFloatToFixed(fn.fB));
ptr[6] = SkEndian_SwapBE32(SkFloatToFixed(fn.fC));
ptr[7] = SkEndian_SwapBE32(SkFloatToFixed(fn.fD));
ptr[8] = SkEndian_SwapBE32(SkFloatToFixed(fn.fE));
ptr[9] = SkEndian_SwapBE32(SkFloatToFixed(fn.fF));
ptr[3] = SkEndian_SwapBE32(float_round_to_fixed(fn.fG));
ptr[4] = SkEndian_SwapBE32(float_round_to_fixed(fn.fA));
ptr[5] = SkEndian_SwapBE32(float_round_to_fixed(fn.fB));
ptr[6] = SkEndian_SwapBE32(float_round_to_fixed(fn.fC));
ptr[7] = SkEndian_SwapBE32(float_round_to_fixed(fn.fD));
ptr[8] = SkEndian_SwapBE32(float_round_to_fixed(fn.fE));
ptr[9] = SkEndian_SwapBE32(float_round_to_fixed(fn.fF));
}
static bool is_3x3(const SkMatrix44& toXYZD50) {