Allow decoding JPEG into A8.

include/core/SkImageDecoder.h

Index: include/core/SkImageDecoder.h
diff --git a/include/core/SkImageDecoder.h b/include/core/SkImageDecoder.h
index 191fe53f4c588a0584486fee25b68ee7a2c7ab1f..7bdaaa8eb5f072b26eb7faf63d32fe493b5ff926 100644
--- a/include/core/SkImageDecoder.h
+++ b/include/core/SkImageDecoder.h
@@ -139,14 +139,17 @@ public:
     Chooser* getChooser() const { return fChooser; }
     Chooser* setChooser(Chooser*);
 
-    /** This optional table describes the caller's preferred config based on
+    /**
+        @Deprecated. Use the struct version instead.
+
+        This optional table describes the caller's preferred config based on
         information about the src data. For this table, the src attributes are
         described in terms of depth (index (8), 16, 32/24) and if there is
         per-pixel alpha. These inputs combine to create an index into the
         pref[] table, which contains the caller's preferred config for that
         input, or kNo_Config if there is no preference.
 
-        To specify no preferrence, call setPrefConfigTable(NULL), which is
+        To specify no preference, call setPrefConfigTable(NULL), which is
         the default.
 
         Note, it is still at the discretion of the codec as to what output
@@ -166,6 +169,50 @@ public:
      */
     void setPrefConfigTable(const SkBitmap::Config pref[6]);
 
+    /**
+     *  Optional table describing the caller's preferred config based on
+     *  information about the src data. Each field should be set to the
+     *  preferred config for a src described in the name of the field. The
+     *  src attributes are described in terms of depth (8-index,
+     *  8bit-grayscale, or 8-bits/component) and whether there is per-pixel
+     *  alpha (does not apply to grayscale). If the caller has no preference
+     *  for a particular src type, its slot should be set to kNo_Config.
+     *
+     *  NOTE ABOUT PREFERRED CONFIGS:
+     *  If a config is preferred, either using a pref table or as a parameter
+     *  to some flavor of decode, it is still at the discretion of the codec
+     *  as to what output config is actually returned, as it may not be able
+     *  to support the caller's preference.
+     *
+     *  If a bitmap is decoded into SkBitmap::A8_Config, the resulting bitmap
+     *  will either be a conversion of the grayscale in the case of a
+     *  grayscale source or the alpha channel in the case of a source with
+     *  an alpha channel.
+     */
+    struct PrefConfigTable {
+        SkBitmap::Config fPrefFor_8Index_NoAlpha_src;

[scroggo, 2013/07/11 20:37:29]

New names. Uglier, but also more clear what they stand for.

+        SkBitmap::Config fPrefFor_8Index_YesAlpha_src;
+        SkBitmap::Config fPrefFor_8Gray_src;
+        SkBitmap::Config fPrefFor_8bpc_NoAlpha_src;
+        SkBitmap::Config fPrefFor_8bpc_YesAlpha_src;

[scroggo, 2013/07/11 20:37:29]

The 32/24 duality was because we used the same SrcDepth enum for 24 bit (8 bits
per component RGB) and 32 bit (8 bits per component ARGB). We could split that
up in SrcDepth, but then we have a redundant hasAlpha. 

+    };
+
+    /**
+     *  Set an optional table for specifying the caller's preferred config
+     *  based on information about the src data.
+     *
+     *  The default is no preference, which will assume the config set by
+     *  decode is preferred.
+     */
+    void setPrefConfigTable(const PrefConfigTable&);
+
+    /**
+     *  Do not use a PrefConfigTable to determine the output config. This
+     *  is the default, so there is no need to call unless a PrefConfigTable
+     *  was previously set.
+     */
+    void resetPrefConfigTable() { fUsePrefTable = false; }
+
     SkBitmap::Allocator* getAllocator() const { return fAllocator; }
     SkBitmap::Allocator* setAllocator(SkBitmap::Allocator*);
 
@@ -269,12 +316,8 @@ public:
     /** Decode the image stored in the specified file, and store the result
         in bitmap. Return true for success or false on failure.
 
-        If pref is kNo_Config, then the decoder is free to choose the most natural
-        config given the image data. If pref something other than kNo_Config,
-        the decoder will attempt to decode the image into that format, unless
-        there is a conflict (e.g. the image has per-pixel alpha and the bitmap's
-        config does not support that), in which case the decoder will choose a
-        closest match configuration.
+        @param prefConfig If the PrefConfigTable is not set, prefer this config.
+                          See NOTE ABOUT PREFERRED CONFIGS.
 
         @param format On success, if format is non-null, it is set to the format
                       of the decoded file. On failure it is ignored.
@@ -289,12 +332,8 @@ public:
     /** Decode the image stored in the specified memory buffer, and store the
         result in bitmap. Return true for success or false on failure.
 
-        If pref is kNo_Config, then the decoder is free to choose the most natural
-        config given the image data. If pref something other than kNo_Config,
-        the decoder will attempt to decode the image into that format, unless
-        there is a conflict (e.g. the image has per-pixel alpha and the bitmap's
-        config does not support that), in which case the decoder will choose a
-        closest match configuration.
+        @param prefConfig If the PrefConfigTable is not set, prefer this config.
+                          See NOTE ABOUT PREFERRED CONFIGS.
 
         @param format On success, if format is non-null, it is set to the format
                        of the decoded buffer. On failure it is ignored.
@@ -337,12 +376,8 @@ public:
     /** Decode the image stored in the specified SkStream, and store the result
         in bitmap. Return true for success or false on failure.
 
-        If pref is kNo_Config, then the decoder is free to choose the most
-        natural config given the image data. If pref something other than
-        kNo_Config, the decoder will attempt to decode the image into that
-        format, unless there is a conflict (e.g. the image has per-pixel alpha
-        and the bitmap's config does not support that), in which case the
-        decoder will choose a closest match configuration.
+        @param prefConfig If the PrefConfigTable is not set, prefer this config.
+                          See NOTE ABOUT PREFERRED CONFIGS.
 
         @param format On success, if format is non-null, it is set to the format
                       of the decoded stream. On failure it is ignored.
@@ -441,10 +476,16 @@ protected:
     */
     bool allocPixelRef(SkBitmap*, SkColorTable*) const;
 
+    /**
+     *  The raw data of the src image.
+     */
     enum SrcDepth {
+        // Color-indexed.
         kIndex_SrcDepth,
-        k16Bit_SrcDepth,
-        k32Bit_SrcDepth
+        // Grayscale in 8 bits.
+        k8BitGray_SrcDepth,
+        // 8 bits per component. Used for 24 bit if there is no alpha.
+        k32Bit_SrcDepth,
     };
     /** The subclass, inside onDecode(), calls this to determine the config of
         the returned bitmap. SrcDepth and hasAlpha reflect the raw data of the
@@ -462,7 +503,7 @@ private:
     SkBitmap::Allocator*    fAllocator;
     int                     fSampleSize;
     SkBitmap::Config        fDefaultPref;   // use if fUsePrefTable is false
-    SkBitmap::Config        fPrefTable[6];  // use if fUsePrefTable is true
+    PrefConfigTable         fPrefTable;     // use if fUsePrefTable is true
     bool                    fDitherImage;
     bool                    fUsePrefTable;
     mutable bool            fShouldCancelDecode;