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More robust check for sRGB gamma tables

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This is in response to a UMA showing that 5% dst gammas are
unidentified tables.  We want to see if some of these tables
should be marked as sRGB.
https://uma.googleplex.com/p/chrome/histograms?endDate=latest&dayCount=1&histograms=Blink.ColorSpace.Destination&fixupData=true&showMax=true&filters=isofficial%2Ceq%2CTrue&implicitFilters=isofficial

This check is not fast.  If we find that it doesn't help
us recognize sRGB curves, we should delete it.

BUG=skia:5656
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2263233003

Review-Url: https://codereview.chromium.org/2263233003
This commit is contained in:
msarett 2016-08-22 14:58:56 -07:00 committed by Commit bot
parent dcb406c29d
commit 4ff08df15a
1 changed files with 37 additions and 3 deletions
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40
src/core/SkColorSpace_ICC.cpp
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@ -262,6 +262,14 @@ static float read_big_endian_16_dot_16(const uint8_t buf[4]) {
return SkFixedToFloat(read_big_endian_i32(buf));
}
static inline float srgb_fn(float x) {
if (x <= 0.04045f) {
return x * (1.0f / 12.92f);
}
return powf(x * (1.0f / 1.055f) + (0.055f / 1.055f), 2.4f);
}
/**
* @param outData Set to the appropriate value on success. If we have table or
* parametric gamma, it is the responsibility of the caller to set
@ -370,9 +378,35 @@ static SkGammas::Type parse_gamma(SkGammas::Data* outData, SkGammas::Params* out
}
}
// Otherwise, we will represent gamma with a table.
outData->fTable.fSize = count;
return SkGammas::Type::kTable_Type;
// Perform a more robust check for sRGB. See if the table is a close
// match to an sRGB table. This is in addition to the previous sRGB
// checks for a couple reasons:
// (1) It is much slower.
// (2) The 26 entry "sRGB" curve is actually so inaccurate that it fails
// this check. But it still wants to be sRGB.
float x = 0.0f;
float dx = 1.0f / ((float) (count - 1));
for (uint32_t i = 0; i < count; i++) {
float y = srgb_fn(x);
// Convert y to the same format as the table (0.16 fixed point), so we can
// compare values.
uint16_t srgbY = sk_float_round2int(y * (float) (1 << 16));
uint16_t actualY = read_big_endian_u16((const uint8_t*) &table[i]);
// We allow "off by 1" curves to try to not be affected by rounding decisions.
if (SkTAbs((int32_t) srgbY - (int32_t) actualY) > 1) {
// Curve is not sRGB, will use table representation.
outData->fTable.fSize = count;
return SkGammas::Type::kTable_Type;
}
x += dx;
}
outData->fNamed = SkColorSpace::kSRGB_GammaNamed;
return SkGammas::Type::kNamed_Type;
}
case kTAG_ParaCurveType: {
enum ParaCurveType {