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Unified Diff: src/core/SkColorSpace_ICC.cpp

Issue 2263233003: More robust check for sRGB gamma tables (Closed) Base URL: https://skia.googlesource.com/skia.git@master
Patch Set: Clarify logic Created 4 years, 4 months ago
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Index: src/core/SkColorSpace_ICC.cpp
diff --git a/src/core/SkColorSpace_ICC.cpp b/src/core/SkColorSpace_ICC.cpp
index 9ab1da007d16e8dee5aa5dae7b0a358d65effcb9..92dcc0fcf5389985faf0f2d005eefdd7089b7d57 100755
--- a/src/core/SkColorSpace_ICC.cpp
+++ b/src/core/SkColorSpace_ICC.cpp
@@ -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 {
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