change SkColorTable to be immutable

src/images/SkImageDecoder_libpng.cpp

Index: src/images/SkImageDecoder_libpng.cpp
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index 4e4106aede85f8266c3b9ffd234e025f9f6a0e02..baab8946c7fa5cd3380556eb84e546e20eddaf3a 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -648,8 +648,6 @@ bool SkPNGImageDecoder::decodePalette(png_structp png_ptr, png_infop info_ptr,
     png_colorp palette;
     png_bytep trans;
     int numTrans;
-    bool reallyHasAlpha = false;
-    SkColorTable* colorTable = NULL;
 
     png_get_PLTE(png_ptr, info_ptr, &palette, &numPalette);
 
@@ -660,21 +658,20 @@ bool SkPNGImageDecoder::decodePalette(png_structp png_ptr, png_infop info_ptr,
         the colortable by 1 (if its < 256) and duplicate the last color into that slot.
     */
     int colorCount = numPalette + (numPalette < 256);
+    SkPMColor colorStorage[256];    // worse-case storage

[scroggo, 2013/10/07 21:58:42]

worst*

[reed1, 2013/10/09 12:19:54]

Done.

+    SkPMColor* colorPtr = colorStorage;
 
-    colorTable = SkNEW_ARGS(SkColorTable, (colorCount));
-
-    SkPMColor* colorPtr = colorTable->lockColors();
     if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
         png_get_tRNS(png_ptr, info_ptr, &trans, &numTrans, NULL);
         *hasAlphap = (numTrans > 0);
     } else {
         numTrans = 0;
-        colorTable->setFlags(colorTable->getFlags() | SkColorTable::kColorsAreOpaque_Flag);
     }
     // check for bad images that might make us crash
     if (numTrans > numPalette) {
         numTrans = numPalette;
     }
+    bool reallyHasAlpha = numTrans > 0;

[scroggo, 2013/10/07 21:58:42]

This is going to change the behavior. Previously, if numTrans was greater than
0, but none of the supposed transparent colors were transparent, this would get
set to false. Now it will remain true.

[reed1, 2013/10/09 12:19:54]

Fixed

[scroggo, 2013/10/09 16:05:23]

Still not fixed. Counterexample:

- numTrans > 0
- none of the "transparent" colors actually have an alpha < FF

The old code would have set reallyHasAlpha to false, but the new code would set
reallyHasAlpha to true. In order to retain the behavior, reallyHasAlpha should
be initialized to false.

[reed1, 2013/10/09 16:16:36]

Done.

 
     int index = 0;
     int transLessThanFF = 0;
@@ -703,8 +700,10 @@ bool SkPNGImageDecoder::decodePalette(png_structp png_ptr, png_infop info_ptr,
     if (numPalette < 256) {
         *colorPtr = colorPtr[-1];
     }
-    colorTable->unlockColors(true);
-    *colorTablep = colorTable;
+
+    SkAlphaType alphaType = reallyHasAlpha ? kPremul_SkAlphaType : kOpaque_SkAlphaType;

[scroggo, 2013/10/07 21:58:42]

You need to check getRequireUnpremultipliedColors to determine the right alpha
type.

[reed1, 2013/10/09 12:19:54]

Fixed

+    *colorTablep = SkNEW_ARGS(SkColorTable,
+                              (colorStorage, colorCount, alphaType));
     *reallyHasAlphap = reallyHasAlpha;
     return true;
 }