Recognize more sRGB gammas

src/core/SkColorSpace.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAtomics.h"
 #include "SkColorSpace.h"
 #include "SkOnce.h"
@@ skipping 318 matching lines @@
                 }
 
                 // Check for frequently occurring curves and use a fast approximation.
                 // We do this by sampling a few values and see if they match our expectation.
                 // A more robust solution would be to compare each value in this curve against
                 // a 2.2f curve see if we remain below an error threshold.  At this time,
                 // we haven't seen any images in the wild that make this kind of
                 // 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]) &&
                             31237 == read_big_endian_short((const uint8_t*) &table[18]) &&
                             65535 == read_big_endian_short((const uint8_t*) &table[25])) {
                         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.
                 gammas[i].fTableSize = count;
                 gammas[i].fTable = std::unique_ptr<float[]>(new float[count]);
                 for (uint32_t j = 0; j < count; j++) {
                     gammas[i].fTable[j] =
                             (read_big_endian_short((const uint8_t*) &table[j])) / 65535.0f;
                 }
                 break;
@@ skipping 293 matching lines @@
                                                             toXYZ));
             }
 
         }
         default:
             break;
     }
 
     return_null("ICC profile contains unsupported colorspace");
 }