Lazy SKP image decoding in DM.

src/images/SkImageDecoder_libjpeg.cpp

Index: src/images/SkImageDecoder_libjpeg.cpp
diff --git a/src/images/SkImageDecoder_libjpeg.cpp b/src/images/SkImageDecoder_libjpeg.cpp
index d32e2a21c91bea0fe06e472a5e6cce22518f41c5..5eb827122f29e322cb7d20be8b35dcc841dd90d9 100644
--- a/src/images/SkImageDecoder_libjpeg.cpp
+++ b/src/images/SkImageDecoder_libjpeg.cpp
@@ -768,14 +768,31 @@ static SkISize compute_yuv_size(const jpeg_decompress_struct& info, int componen
                          info.cur_comp_info[component]->downsampled_height);
 }
 
+static bool appears_to_be_yuv(const jpeg_decompress_struct& info) {
+    return (info.jpeg_color_space == JCS_YCbCr)
+        && (DCTSIZE == 8)
+        && (info.num_components == 3)
+        && (info.comps_in_scan >= info.num_components)
+        && (info.scale_denom <= 8)
+        && (info.cur_comp_info[0])
+        && (info.cur_comp_info[1])
+        && (info.cur_comp_info[2])
+        && (info.cur_comp_info[1]->h_samp_factor == 1)
+        && (info.cur_comp_info[1]->v_samp_factor == 1)
+        && (info.cur_comp_info[2]->h_samp_factor == 1)
+        && (info.cur_comp_info[2]->v_samp_factor == 1);
+}
+
 static void update_components_sizes(const jpeg_decompress_struct& cinfo, SkISize componentSizes[3],
                                     SizeType sizeType) {
+    SkASSERT(appears_to_be_yuv(cinfo));
     for (int i = 0; i < 3; ++i) {
         componentSizes[i] = compute_yuv_size(cinfo, i, sizeType);
     }
 }
 
 static bool output_raw_data(jpeg_decompress_struct& cinfo, void* planes[3], size_t rowBytes[3]) {
+    SkASSERT(appears_to_be_yuv(cinfo));
     // U size and V size have to be the same if we're calling output_raw_data()
     SkISize uvSize = compute_yuv_size(cinfo, 1, kSizeForMemoryAllocation_SizeType);
     SkASSERT(uvSize == compute_yuv_size(cinfo, 2, kSizeForMemoryAllocation_SizeType));
@@ -861,7 +878,6 @@ bool SkJPEGImageDecoder::onDecodeYUV8Planes(SkStream* stream, SkISize componentS
 #ifdef TIME_DECODE
     SkAutoTime atm("JPEG YUV8 Decode");
 #endif
-
     if (this->getSampleSize() != 1) {
         return false; // Resizing not supported
     }
@@ -888,7 +904,7 @@ bool SkJPEGImageDecoder::onDecodeYUV8Planes(SkStream* stream, SkISize componentS
         return return_false(cinfo, "read_header YUV8");
     }
 
-    if (cinfo.jpeg_color_space != JCS_YCbCr) {
+    if (!appears_to_be_yuv(cinfo)) {
         // It's not an error to not be encoded in YUV, so no need to use return_false()
         return false;
     }
@@ -920,6 +936,12 @@ bool SkJPEGImageDecoder::onDecodeYUV8Planes(SkStream* stream, SkISize componentS
         return return_false(cinfo, "start_decompress YUV8");
     }
 
+    // Seems like jpeg_start_decompress is updating our opinion of whether cinfo represents YUV.

[msarett, 2015/12/21 22:35:57]

Mike, I realize this is a shot in the dark, but is there any chance you know
what failure/test image caused this code to be added?  We should be able to to
tell whether YUV is supported before calling jpeg_start_decompress().

My guess is that "info.comps_in_scan" is a bogus thing to check.  And the only
thing that jpeg_start_decompress() might change.  What we really want to know is
whether the image is multi-scan (progressive) or not.  It doesn't look like we
support multi-scan images.

But it would be nice to confirm.

[mtklein, 2015/12/21 23:44:52]

Not sure what image triggered this, but I'm pretty sure I put it right here
because the assert at the beginning of output_raw_data (the next call) was
triggering.  We could try removing it and see what goes wrong?

+    // Again, not really an error.
+    if (!appears_to_be_yuv(cinfo)) {
+        return false;
+    }
+
     if (!output_raw_data(cinfo, planes, rowBytes)) {
         return return_false(cinfo, "output_raw_data");
     }