Bmp Image Decoding

src/codec/SkCodec_libbmp.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkCodec.h"
+#include "SkImageInfo.h"
+#include "SkStream.h"
+#include "SkSwizzler.h"
+#include "SkTypes.h"
+
+// TODO: rename SkCodec_libbmp files to SkBmpCodec
+// TODO: define a wrapper for SkDebugf that doesn't always print
+/*
+ *
+ * This class implements the decoding for bmp images
+ *
+ */
+class SkBmpCodec : public SkCodec {
+public:
+
+    /*
+     *
+     * Bit masks used for color components in mask compression modes
+     *
+     */
+    struct BitMasks {
+        uint32_t redMask;
+        uint32_t greenMask;
+        uint32_t blueMask;
+        uint32_t alphaMask;
+    };
+
+    /*
+     *
+     * Number of trailing zeros on bit masks used in mask compression modes
+     *
+     */
+    struct BitMaskShifts {
+        uint32_t redShift;
+        uint32_t greenShift;
+        uint32_t blueShift;
+        uint32_t alphaShift;
+    };
+
+    /*
+     *
+     * Size of bit masks used in mask compression modes
+     *
+     */
+    struct BitMaskSizes {
+        uint32_t redSize;
+        uint32_t greenSize;
+        uint32_t blueSize;
+        uint32_t alphaSize;
+    };
+
+    /*
+     *
+     * Describes if rows of the input start at the top or bottom of the image
+     *
+     */
+    enum RowOrder {
+        kTopDown_RowOrder,
+        kBottomUp_RowOrder
+    };
+
+    /*
+     *
+     * Checks the start of the stream to see if the image is a bitmap
+     *
+     */
+    static bool IsBmp(SkStream*);
+
+    /*
+     *
+     * Assumes IsBmp was called and returned true
+     * Creates a bitmap decoder
+     * Reads enough of the stream to determine the image format
+     *
+     */
+    static SkCodec* NewFromStream(SkStream*);
+
+protected:
+
+    /*
+     *
+     * Initiates the bitmap decode
+     *
+     */
+    virtual Result onGetPixels(const SkImageInfo& dstInfo, void* dst,
+                               size_t dstRowBytes, SkColor*,

[scroggo, 2015/03/06 18:56:13]

I think this happens to compile because SkColor and SkPMColor are defined to the
same thing. This is incorrect though; SkColor implies a particular order no
matter what, whereas SkPMColor implies an ordering that can be modified with
compile flags.

Originally, SkPMColor also meant PreMultiplied. Currently we still use SkPMColor
when we're talking about unpremultiplied colors (but it still uses the compile
flag specified ordering).

+                               int*) SK_OVERRIDE;
+
+private:
+
+    /*
+     *
+     * Used to define the input format of the bitmap
+     *
+     */
+    enum BitmapInputFormat {
+        kStandard_BitmapInputFormat,
+        kRLE_BitmapInputFormat,
+        kBitMask_BitmapInputFormat,
+        kUnknown_BitmapInputFormat
+    };
+
+    /*
+     *
+     * For a continuous bit mask (ex: 0011100), retrieves the size of the mask
+     * and the number of bits to shift the mask
+     *
+     */
+    void processMasks();
+
+    /*
+     *
+     * Performs the bitmap decoding for bit masks input format
+     *
+     */
+    Result decodeMask(const SkImageInfo& dstInfo, void* dst,
+                      uint32_t dstRowBytes);
+
+    /*
+     *
+     * Set an RLE pixel using the color table
+     *
+     */
+    void setRLEPixel(SkColor* dst, uint32_t dstRowBytes, int height,

[scroggo, 2015/03/06 18:56:14]

SkPMColor (I think)

+                     uint32_t x, uint32_t y, uint8_t index);
+
+    /*
+     *
+     * Performs the bitmap decoding for RLE input format
+     *
+     */
+    Result decodeRLE(const SkImageInfo& dstInfo, void* dst,
+                     uint32_t dstRowBytes);
+
+    /*
+     *
+     * Performs the bitmap decoding for standard input format
+     *
+     */
+    Result decode(const SkImageInfo& dstInfo, void* dst, uint32_t dstRowBytes);
+
+    /*
+     *
+     * Creates an instance of the decoder
+     * Called only by NewFromStream
+     *
+     * @param srcInfo contains the source width and height
+     * @param stream the stream of image data
+     * @param bitsPerPixel the number of bits used to store each pixel
+     * @param format the format of the bmp file
+     * @param bitMasks optional color masks for certain bmp formats
+     * @param colorTable array representing the color table for index-based bmp
+     *                   formats, passes ownership to SkBmpCodec
+     * @param rowOrder indicates whether rows are ordered top-down or bottom-up
+     * @param remainingBytes used only for RLE decodes, as we must decode all
+     *                  of the data at once rather than row by row
+     *                  it indicates the amount of data left in the stream
+     *                  after decoding the headers
+     *
+     */
+    SkBmpCodec(const SkImageInfo& srcInfo, SkStream* stream,
+               uint16_t bitsPerPixel, BitmapInputFormat format,
+               BitMasks bitMasks, SkColor* colorTable,
+               RowOrder rowOrder, uint32_t remainingBytes);
+
+    // Fields
+    const uint16_t                    fBitsPerPixel;
+    const BitmapInputFormat           fInputFormat;
+    BitMasks                          fBitMasks;
+    BitMaskShifts                     fMaskShifts;
+    BitMaskSizes                      fMaskSizes;
+    const SkAutoTDeleteArray<SkColor> fColorTable;     // owned
+    const RowOrder                    fRowOrder;
+    const uint32_t                    fRemainingBytes;
+
+    typedef SkCodec INHERITED;
+};