More robust check for sRGB gamma tables

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

1 files changed, 37 insertions, 3 deletions

diff --git a/src/core/SkColorSpace_ICC.cpp b/src/core/SkColorSpace_ICC.cpp
index 9ab1da007d..92dcc0fcf5 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 {