A8 bitmaps from JPEG are not opaque.

src/images/SkImageDecoder_libjpeg.cpp

 
 /*
  * Copyright 2007 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.
  */
 
 
 #include "SkImageDecoder.h"
@@ skipping 315 matching lines @@
     } else if (SkBitmap::kRGB_565_Config == config && JCS_CMYK != cinfo.out_color_space) {
         cinfo.out_color_space = JCS_RGB_565;
         if (this->getDitherImage()) {
             cinfo.dither_mode = JDITHER_ORDERED;
         }
     }
 #endif
 
     if (1 == sampleSize && SkImageDecoder::kDecodeBounds_Mode == mode) {
         bm->setConfig(config, cinfo.image_width, cinfo.image_height);
-        bm->setIsOpaque(true);
+        bm->setIsOpaque(config != SkBitmap::kA8_Config);
         return true;
     }
 
     /*  image_width and image_height are the original dimensions, available
         after jpeg_read_header(). To see the scaled dimensions, we have to call
         jpeg_start_decompress(), and then read output_width and output_height.
     */
     if (!jpeg_start_decompress(&cinfo)) {
         /*  If we failed here, we may still have enough information to return
             to the caller if they just wanted (subsampled bounds). If sampleSize
             was 1, then we would have already returned. Thus we just check if
             we're in kDecodeBounds_Mode, and that we have valid output sizes.
 
             One reason to fail here is that we have insufficient stream data
             to complete the setup. However, output dimensions seem to get
             computed very early, which is why this special check can pay off.
          */
         if (SkImageDecoder::kDecodeBounds_Mode == mode && valid_output_dimensions(cinfo)) {
             SkScaledBitmapSampler smpl(cinfo.output_width, cinfo.output_height,
                                        recompute_sampleSize(sampleSize, cinfo));
             bm->setConfig(config, smpl.scaledWidth(), smpl.scaledHeight());
-            bm->setIsOpaque(true);
+            bm->setIsOpaque(config != SkBitmap::kA8_Config);
             return true;
         } else {
             return return_false(cinfo, *bm, "start_decompress");
         }
     }
     sampleSize = recompute_sampleSize(sampleSize, cinfo);
 
     // should we allow the Chooser (if present) to pick a config for us???
     if (!this->chooseFromOneChoice(config, cinfo.output_width, cinfo.output_height)) {
         return return_false(cinfo, *bm, "chooseFromOneChoice");
     }
 
     SkScaledBitmapSampler sampler(cinfo.output_width, cinfo.output_height, sampleSize);
     bm->setConfig(config, sampler.scaledWidth(), sampler.scaledHeight());
-    bm->setIsOpaque(true);
+    bm->setIsOpaque(config != SkBitmap::kA8_Config);
     if (SkImageDecoder::kDecodeBounds_Mode == mode) {
         return true;
     }
     if (!this->allocPixelRef(bm, NULL)) {
         return return_false(cinfo, *bm, "allocPixelRef");
     }
 
     SkAutoLockPixels alp(*bm);
 
 #ifdef ANDROID_RGB
@@ skipping 637 matching lines @@
 }
 
 static SkImageEncoder* sk_libjpeg_efactory(SkImageEncoder::Type t) {
     return (SkImageEncoder::kJPEG_Type == t) ? SkNEW(SkJPEGImageEncoder) : NULL;
 }
 
 
 static SkTRegistry<SkImageDecoder*, SkStream*> gDReg(sk_libjpeg_dfactory);
 static SkTRegistry<SkImageDecoder::Format, SkStream*> gFormatReg(get_format_jpeg);
 static SkTRegistry<SkImageEncoder*, SkImageEncoder::Type> gEReg(sk_libjpeg_efactory);