Scanline decoding for bmp

src/codec/SkBmpCodec.h

Index: src/codec/SkBmpCodec.h
diff --git a/src/codec/SkBmpCodec.h b/src/codec/SkBmpCodec.h
index 71006f73153ea3028529b0c51daa999b498bc86e..17973490fea06e31fe4b16137a0d4449c8f028ec 100644
--- a/src/codec/SkBmpCodec.h
+++ b/src/codec/SkBmpCodec.h
@@ -11,6 +11,7 @@
 #include "SkColorTable.h"
 #include "SkImageInfo.h"
 #include "SkMaskSwizzler.h"
+#include "SkScanlineDecoder.h"
 #include "SkStream.h"
 #include "SkSwizzler.h"
 #include "SkTypes.h"
@@ -48,6 +49,13 @@ public:
      */
     static SkCodec* NewFromIco(SkStream*);
 
+    /*
+     * Assumes IsBmp was called and returned true
+     * Creates a bmp scanline decoder
+     * Takes ownership of the stream
+     */
+    static SkScanlineDecoder* NewSDFromStream(SkStream* stream);
+
 protected:
 
     SkBmpCodec(const SkImageInfo& info, SkStream* stream, uint16_t bitsPerPixel,
@@ -77,6 +85,13 @@ protected:
     }
 
     /*
+     * Get the destination row number corresponsing to the encoded row number.
+     * For kTopDown, we simply return y, but for kBottomUp, the rows will be
+     * decoded in reverse order.
+     */
+    int32_t getDstRow(int32_t y, int32_t height);

[scroggo, 2015/08/26 22:40:09]

Can you explain what height means? I suppose it is not the full height of the
image?

[msarett, 2015/08/27 15:00:27]

Adding comments.  It used to mean the full height of the image - but for cases
where we are decoding a subset of the image (ex: 1 row for sampling), it means
the height of the subset we are decoding (so it is 1 a lot).

+
+    /*
      * Get the destination row to start filling from
      * Used to fill the remainder of the image on incomplete input for bmps
      * This is tricky since bmps may be kTopDown or kBottomUp.  For kTopDown,
@@ -104,8 +119,35 @@ private:
      */
     static SkCodec* NewFromStream(SkStream*, bool inIco);
 
-    const uint16_t fBitsPerPixel;
-    const RowOrder fRowOrder;
+    /*
+     * To be overriden by bmp subclasses, which provide unique implementations.
+     * Performs subclass specific setup.
+     */
+    virtual SkCodec::Result prepareToDecode(const SkImageInfo& dstInfo,
+            const SkCodec::Options& options, SkPMColor inputColorPtr[],
+            int* inputColorCount) = 0;
+
+    /*
+     * Decodes the next dstInfo.height() lines.
+     *
+     * onGetPixels() uses this for full image decodes.
+     * SkScaledCodec::onGetPixels() uses the scanline decoder to call this with
+     * dstInfo.height() = 1, in order to implement sampling.
+     * A potential future use is to allow the caller to decode a subset of the
+     * lines in the image.
+     *
+     * @param dstInfo     Contains output information.  Height specifies the
+     *                    number of rows to decode at this time.
+     * @param dst         Memory location to store output pixels
+     * @param dstRowBytes Bytes in a row of the destination
+     */
+    virtual Result decode(const SkImageInfo& dstInfo, void* dst, size_t dstRowBytes,
+            const Options& opts) = 0;
+
+    const uint16_t       fBitsPerPixel;
+    const RowOrder       fRowOrder;
+
+    friend class SkBmpScanlineDecoder;
 
     typedef SkCodec INHERITED;
 };