Recognize more sRGB gammas

src/core/SkColorSpace.cpp

Index: src/core/SkColorSpace.cpp
diff --git a/src/core/SkColorSpace.cpp b/src/core/SkColorSpace.cpp
index 996158c8c0504fe0bce665221ea7eabed8ff0ed0..f12959f77c6eb154762357cd7e09357057c80e9f 100644
--- a/src/core/SkColorSpace.cpp
+++ b/src/core/SkColorSpace.cpp
@@ -336,15 +336,20 @@ bool SkColorSpace::LoadGammas(SkGammaCurve* gammas, uint32_t numGammas, const ui
                 // calculation necessary.  We encounter identical gamma curves over and
                 // over again, but relatively few variations.
                 if (1024 == count) {
+                    // The magic values were chosen because they match a very common sRGB
+                    // gamma table and the less common Canon sRGB gamma table (which use
+                    // different rounding rules).
                     if (0 == read_big_endian_short((const uint8_t*) &table[0]) &&
-                            3341 == read_big_endian_short((const uint8_t*) &table[256]) &&
-                            14057 == read_big_endian_short((const uint8_t*) &table[512]) &&
+                            3366 == read_big_endian_short((const uint8_t*) &table[257]) &&

[scroggo, 2016/05/13 12:45:24]

Does still correctly identify images that were recognized by the old code?

[msarett, 2016/05/13 12:51:48]

Yes!

+                            14116 == read_big_endian_short((const uint8_t*) &table[513]) &&
                             34318 == read_big_endian_short((const uint8_t*) &table[768]) &&
                             65535 == read_big_endian_short((const uint8_t*) &table[1023])) {
                         gammas[i].fValue = 2.2f;
                         break;
                     }
                 } else if (26 == count) {
+                    // The magic values were chosen because they match a very common sRGB
+                    // gamma table.
                     if (0 == read_big_endian_short((const uint8_t*) &table[0]) &&
                             3062 == read_big_endian_short((const uint8_t*) &table[6]) &&
                             12824 == read_big_endian_short((const uint8_t*) &table[12]) &&
@@ -353,6 +358,17 @@ bool SkColorSpace::LoadGammas(SkGammaCurve* gammas, uint32_t numGammas, const ui
                         gammas[i].fValue = 2.2f;
                         break;
                     }
+                } else if (4096 == count) {
+                    // The magic values were chosen because they match Nikon, Epson, and
+                    // LCMS sRGB gamma tables (all of which use different rounding rules).
+                    if (0 == read_big_endian_short((const uint8_t*) &table[0]) &&

[scroggo, 2016/05/13 12:45:24]

As we add more of these, it makes me wonder if we should lazily create the
SkColorSpace, so clients who won't ever apply it don't need to spend time
reading it and creating it. (That said, the cost is probably cheap compared to
decoding the image.)

[msarett, 2016/05/13 12:51:48]

I think this is a really good idea.

+                            950 == read_big_endian_short((const uint8_t*) &table[515]) &&
+                            3342 == read_big_endian_short((const uint8_t*) &table[1025]) &&
+                            14079 == read_big_endian_short((const uint8_t*) &table[2051]) &&
+                            65535 == read_big_endian_short((const uint8_t*) &table[4095])) {
+                        gammas[i].fValue = 2.2f;
+                        break;
+                    }
                 }
 
                 // Otherwise, fill in the interpolation table.