Support decoding opaque to *premul

src/codec/SkCodecPriv.h

 /*
  * Copyright 2015 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCodecPriv_DEFINED
 #define SkCodecPriv_DEFINED
 
 #include "SkColorPriv.h"
 #include "SkColorTable.h"
 #include "SkImageInfo.h"
 #include "SkTypes.h"
 #include "SkUtils.h"
 
+#ifdef SK_PRINT_CODEC_MESSAGES
+    #define SkCodecPrintf SkDebugf
+#else
+    #define SkCodecPrintf(...)
+#endif
+
 // FIXME: Consider sharing with dm, nanbench, and tools.
 inline float get_scale_from_sample_size(int sampleSize) {
     return 1.0f / ((float) sampleSize);
 }
 
 inline bool is_valid_subset(const SkIRect& subset, const SkISize& imageDims) {
     return SkIRect::MakeSize(imageDims).contains(subset);
 }
 
 /*
@@ skipping 41 matching lines @@
     // Return false on edge cases
     if (srcCoord < startCoord || get_dst_coord(srcCoord, sampleFactor) >= scaledDim) {
         return false;
     }
 
     // Every sampleFactor rows are necessary
     return ((srcCoord - startCoord) % sampleFactor) == 0;
 }
 
 inline bool valid_alpha(SkAlphaType dstAlpha, SkAlphaType srcAlpha) {
-    // Check for supported alpha types
+    if (kUnknown_SkAlphaType == dstAlpha) {
+        return false;
+    }
+
     if (srcAlpha != dstAlpha) {
         if (kOpaque_SkAlphaType == srcAlpha) {
-            // If the source is opaque, we must decode to opaque
-            return false;
+            // If the source is opaque, we can support any.
+            SkCodecPrintf("Warning: an opaque image should be decoded as opaque "
+                          "- it is being decoded as non-opaque, which will draw slower\n");
+            return true;
         }
 
         // The source is not opaque
         switch (dstAlpha) {
             case kPremul_SkAlphaType:
             case kUnpremul_SkAlphaType:
                 // The source is not opaque, so either of these is okay
                 break;
             default:
                 // We cannot decode a non-opaque image to opaque (or unknown)
                 return false;
         }
     }
     return true;
 }
 
 /*
  * Most of our codecs support the same conversions:
  * - profileType must be the same
- * - opaque only to opaque (and 565 only if opaque)
+ * - opaque to any alpha type
+ * - 565 only if opaque
  * - premul to unpremul and vice versa
  * - always support N32
  * - otherwise match the src color type
  */
 inline bool conversion_possible(const SkImageInfo& dst, const SkImageInfo& src) {
     if (dst.profileType() != src.profileType()) {
         return false;
     }
 
     // Ensure the alpha type is valid
@@ skipping 110 matching lines @@
 inline uint32_t get_int(uint8_t* buffer, uint32_t i) {
     uint32_t result;
     memcpy(&result, &(buffer[i]), 4);
 #ifdef SK_CPU_BENDIAN
     return SkEndianSwap32(result);
 #else
     return result;
 #endif
 }
 
-#ifdef SK_PRINT_CODEC_MESSAGES
-    #define SkCodecPrintf SkDebugf
-#else
-    #define SkCodecPrintf(...)
-#endif
-
 #endif // SkCodecPriv_DEFINED