Bmp Image Decoding

src/codec/SkCodec_libbmp.cpp

+/*
+ * 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_libbmp.h"
+#include "SkColorPriv.h"
+#include "SkColorTable.h"
+#include "SkEndian.h"
+#include "SkStream.h"
+
+#include <algorithm>
+
+/*
+ *
+ * Get a byte from the buffer
+ *
+ */
+uint8_t get_byte(uint8_t* buffer, uint32_t i) {
+    return buffer[i];
+}
+
+/*
+ *
+ * Get a short from the buffer
+ *
+ */
+uint16_t get_short(uint8_t* buffer, uint32_t i) {
+    uint16_t result;
+    memcpy(&result, &(buffer[i]), 2);
+#ifdef SK_CPU_BENDIAN
+    return SkEndianSwap16(result);
+#else
+    return result;
+#endif
+}
+
+/*
+ *
+ * Get an int from the buffer
+ *
+ */
+uint32_t get_int(uint8_t* buffer, uint32_t i) {
+    uint32_t result;
+    memcpy(&result, &(buffer[i]), 4);
+#ifdef SK_CPU_BENDIAN
+    return SkEndianSwap32(result);
+#else
+    return result;
+#endif
+}
+
+/*
+ *
+ * Defines the version and type of the second bitmap header
+ *
+ */
+enum BitmapHeaderType {
+    kInfoV1_BitmapHeaderType,
+    kInfoV2_BitmapHeaderType,
+    kInfoV3_BitmapHeaderType,
+    kInfoV4_BitmapHeaderType,
+    kInfoV5_BitmapHeaderType,
+    kOS2V1_BitmapHeaderType,
+    kOS2VX_BitmapHeaderType,
+    kUnknown_BitmapHeaderType
+};
+
+/*
+ *
+ * Possible bitmap compression types
+ *
+ */
+enum BitmapCompressionMethod {
+    kNone_BitmapCompressionMethod =          0,
+    k8BitRLE_BitmapCompressionMethod =       1,
+    k4BitRLE_BitmapCompressionMethod =       2,
+    kBitMasks_BitmapCompressionMethod =      3,
+    kJpeg_BitmapCompressionMethod =          4,
+    kPng_BitmapCompressionMethod =           5,
+    kAlphaBitMasks_BitmapCompressionMethod = 6,
+    kCMYK_BitmapCompressionMethod =          11,
+    kCMYK8BitRLE_BitmapCompressionMethod =   12,
+    kCMYK4BitRLE_BitmapCompressionMethod =   13
+};
+
+/*
+ *
+ * Checks the start of the stream to see if the image is a bitmap
+ *
+ */
+bool SkBmpCodec::IsBmp(SkStream* stream) {
+    // TODO: Support "IC", "PT", "CI", "CP", "BA"
+    // TODO: ICO files may contain a BMP and need to use this decoder
+    const char bmpSig[] = { 'B', 'M' };
+    char buffer[sizeof(bmpSig)];
+    return stream->read(buffer, sizeof(bmpSig)) == sizeof(bmpSig) &&
+        !memcmp(buffer, bmpSig, sizeof(bmpSig));
+}
+
+/*
+ *
+ * Assumes IsBmp was called and returned true
+ * Creates a bitmap decoder
+ * Reads enough of the stream to determine the image format
+ *
+ */
+SkCodec* SkBmpCodec::NewFromStream(SkStream* stream) {
+    // Header size constants
+    static const uint32_t kBmpHeaderBytes = 14;
+    static const uint32_t kBmpHeaderBytesPlusFour = kBmpHeaderBytes + 4;
+    static const uint32_t kBmpOS2V1Bytes = 12;
+    static const uint32_t kBmpOS2V2Bytes = 64;
+    static const uint32_t kBmpInfoBaseBytes = 16;
+    static const uint32_t kBmpInfoV1Bytes = 40;
+    static const uint32_t kBmpInfoV2Bytes = 52;
+    static const uint32_t kBmpInfoV3Bytes = 56;
+    static const uint32_t kBmpInfoV4Bytes = 108;
+    static const uint32_t kBmpInfoV5Bytes = 124;
+    static const uint32_t kBmpMaskBytes = 12;
+
+    // Read the first header and the size of the second header
+    SkAutoTDeleteArray<uint8_t> hBuffer(
+            SkNEW_ARRAY(uint8_t, kBmpHeaderBytesPlusFour));
+    if (stream->read(hBuffer.get(), kBmpHeaderBytesPlusFour) !=
+            kBmpHeaderBytesPlusFour) {
+        SkDebugf("Error: unable to read first bitmap header.\n");
+        return NULL;
+    }
+    //uint16_t signature = get_short(hBuffer, 0);
+
+    // The total bytes in the bmp file
+    // We only need to use this value for RLE decoding, so we will only check
+    // that it is valid in the RLE case.
+    const uint32_t totalBytes = get_int(hBuffer.get(), 2);
+
+    //uint32_t reserved = get_int(hBuffer, 6);
+
+    // The offset from the start of the file where the pixel data begins
+    const uint32_t offset = get_int(hBuffer.get(), 10);
+    if (offset < kBmpHeaderBytes + kBmpOS2V1Bytes) {
+        SkDebugf("Error: invalid starting location for pixel data\n");
+        return NULL;
+    }
+
+    // The size of the second (info) header in bytes
+    // The size is the first field of the second header, so we have already
+    // read the first four infoBytes.
+    const uint32_t infoBytes = get_int(hBuffer.get(), 14);
+    if (infoBytes < kBmpOS2V1Bytes) {
+        SkDebugf("Error: invalid second header size.\n");
+        return NULL;
+    }
+    const uint32_t infoBytesRemaining = infoBytes - 4;
+    hBuffer.free();
+
+    // Read the second header
+    SkAutoTDeleteArray<uint8_t> iBuffer(
+            SkNEW_ARRAY(uint8_t, infoBytesRemaining));
+    if (stream->read(iBuffer.get(), infoBytesRemaining) != infoBytesRemaining) {
+        SkDebugf("Error: unable to read second bitmap header.\n");
+        return NULL;
+    }
+
+    // The number of bits used per pixel in the pixel data
+    uint16_t bitsPerPixel;
+
+    // The compression method for the pixel data
+    uint32_t compression = kNone_BitmapCompressionMethod;
+
+    // Number of colors in the color table, defaults to 0 or max (see below)
+    uint32_t numColors = 0;
+
+    // Bytes per color in the color table, early versions use 3, most use 4
+    uint32_t bytesPerColor;
+
+    // The image width and height
+    int width, height;
+
+    // Determine image information depending on second header format
+    BitmapHeaderType headerType;
+    if (infoBytes >= kBmpInfoBaseBytes) {
+        // Check the version of the header
+        switch (infoBytes) {
+            case kBmpInfoV1Bytes:
+                headerType = kInfoV1_BitmapHeaderType;
+                break;
+            case kBmpInfoV2Bytes:
+                headerType = kInfoV2_BitmapHeaderType;
+                break;
+            case kBmpInfoV3Bytes:
+                headerType = kInfoV3_BitmapHeaderType;
+                break;
+            case kBmpInfoV4Bytes:
+                headerType = kInfoV4_BitmapHeaderType;
+                break;
+            case kBmpInfoV5Bytes:
+                headerType = kInfoV5_BitmapHeaderType;
+                break;
+            case 16:
+            case 20:
+            case 24:
+            case 28:
+            case 32:
+            case 36:
+            case 42:
+            case 46:
+            case 48:
+            case 60:
+            case kBmpOS2V2Bytes:
+                headerType = kOS2VX_BitmapHeaderType;
+                break;
+            default:
+                // We do not signal an error here because there is the
+                // possibility of new or undocumented bmp header types.  Most
+                // of the newer versions of bmp headers are similar to and
+                // build off of the older versions, so we may still be able to
+                // decode the bmp.
+                SkDebugf("Warning: unknown bmp header format.\n");
+                headerType = kUnknown_BitmapHeaderType;
+                break;
+        }
+        SkASSERT(infoBytesRemaining >= 12);

[scroggo, 2015/02/28 00:25:13]

Could you add a comment explaining why you know this to be true? I had to search
to figure it out.

+        width = get_int(iBuffer.get(), 0);
+        height = get_int(iBuffer.get(), 4);
+        //uint16_t planes = get_short(iBuffer, 8);
+        bitsPerPixel = get_short(iBuffer.get(), 10);
+
+        // Some versions do not have this field, so we check before
+        // overwriting the default value.
+        if (infoBytesRemaining >= 16) {

[scroggo, 2015/02/28 00:25:13]

If this is not true, can we go ahead and return?

[msarett, 2015/03/02 15:00:13]

We can't.  There are valid images without this header field.  It will default to
uncompressed.

+            compression = get_int(iBuffer.get(), 12);
+        }
+        //uint32_t imageBytes = get_int(iBuffer, 16);
+        //uint32_t horizontalResolution = get_int(iBuffer, 20);
+        //uint32_t verticalResolution = get_int(iBuffer, 24);
+
+        // Some versions do not have this field, so we check before
+        // overwriting the default value.
+        if (infoBytesRemaining >= 32) {
+            numColors = get_int(iBuffer.get(), 28);
+        }
+        //uint32_t importantColors = get_int(iBuffer, infoBytes - 4, 32);
+        bytesPerColor = 4;
+    } else if (infoBytes >= kBmpOS2V1Bytes) {
+        // The OS2V1 is treated separately because it has a unique format
+        headerType = kOS2V1_BitmapHeaderType;
+        width = (int) get_short(iBuffer.get(), 0);
+        height = (int) get_short(iBuffer.get(), 2);
+        //uint16_t planes = get_short(iBuffer.get(), 4);
+        bitsPerPixel = get_short(iBuffer.get(), 6);
+        bytesPerColor = 3;
+    } else {
+        // There are no valid bmp headers
+        SkDebugf("Error: second bitmap header size is invalid.\n");
+        return NULL;
+    }
+
+    // Check for valid dimensions from header
+    static const uint32_t kBmpMaxDim = 1 << 16;
+    SkSwizzler::RowOrder rowOrder = SkSwizzler::kBottomUp_RowOrder;
+    if (height < 0) {
+        height = -height;
+        rowOrder = SkSwizzler::kTopDown_RowOrder;
+    }
+    if (width < 0 || width >= kBmpMaxDim || height >= kBmpMaxDim) {
+        // TODO: Decide if we want to support really large bmps.
+        SkDebugf("Error: invalid bitmap dimensions.\n");
+        return NULL;
+    }
+
+    // Create mask struct
+    SkSwizzler::ColorMasks masks;
+    masks.redMask = 0;
+    masks.greenMask = 0;
+    masks.blueMask = 0;
+    masks.alphaMask = 0;
+
+    // Determine the input compression format and set bit masks if necessary
+    uint32_t maskBytes = 0;
+    BitmapInputFormat inputFormat = kUnknown_BitmapInputFormat;
+    switch (compression) {
+        case kNone_BitmapCompressionMethod:
+            inputFormat = kStandard_BitmapInputFormat;
+            // Always respect alpha mask in V4+
+            if (kInfoV4_BitmapHeaderType == headerType ||
+                    kInfoV5_BitmapHeaderType == headerType) {
+                SkASSERT(infoBytesRemaining > 52);

[scroggo, 2015/02/28 00:25:13]

(I think this comment corresponds to this code:)

On 2015/02/27 21:17:28, msarett wrote:
If it's guaranteed, and it must be true in order to do the read, I say assert.
That way if someone else (including future you) modifies this file later, they
know the call to get_int is safe (and if they change things so it's not, the
assert can help us discover that).

Also, please add a comment with your statement about infoBytes matching the
header size. That's not easy to see from looking at this code.

+                masks.alphaMask = get_int(iBuffer.get(), 48);
+            }
+            break;
+        case k8BitRLE_BitmapCompressionMethod:
+            if (bitsPerPixel != 8) {
+                SkDebugf("Warning: correcting invalid bitmap format.\n");
+                bitsPerPixel = 8;
+            }
+            inputFormat = kRLE_BitmapInputFormat;
+            break;
+        case k4BitRLE_BitmapCompressionMethod:
+            if (bitsPerPixel != 4) {
+                SkDebugf("Warning: correcting invalid bitmap format.\n");
+                bitsPerPixel = 4;
+            }
+            inputFormat = kRLE_BitmapInputFormat;
+            break;
+        case kAlphaBitMasks_BitmapCompressionMethod:
+        case kBitMasks_BitmapCompressionMethod:
+            // Load the masks
+            inputFormat = kBitMask_BitmapInputFormat;
+            switch (headerType) {
+                case kInfoV1_BitmapHeaderType: {
+                    // The V1 header stores the bit masks after the header
+                    SkAutoTDeleteArray<uint8_t> mBuffer(
+                            SkNEW_ARRAY(uint8_t, kBmpMaskBytes));
+                    if (stream->read(mBuffer.get(), kBmpMaskBytes) !=
+                            kBmpMaskBytes) {
+                        SkDebugf("Error: unable to read bit masks.\n");
+                        return NULL;
+                    }
+                    maskBytes = kBmpMaskBytes;
+                    masks.redMask = get_int(mBuffer.get(), 0);
+                    masks.greenMask = get_int(mBuffer.get(), 4);
+                    masks.blueMask = get_int(mBuffer.get(), 8);
+                    break;
+                }
+                case kInfoV4_BitmapHeaderType:
+                case kInfoV5_BitmapHeaderType:
+                    SkASSERT(infoBytesRemaining >= 52);
+                    masks.alphaMask = get_int(iBuffer.get(), 48);
+                    // Intentional fall through
+                case kInfoV2_BitmapHeaderType:
+                case kInfoV3_BitmapHeaderType:
+                    SkASSERT(infoBytesRemaining >= 48);
+                    masks.redMask = get_int(iBuffer.get(), 36);
+                    masks.greenMask = get_int(iBuffer.get(), 40);
+                    masks.blueMask = get_int(iBuffer.get(), 44);
+                    break;
+                case kOS2VX_BitmapHeaderType:
+                    // TODO: Decide if we intend to support this.
+                    //       It is unsupported in the previous version and
+                    //       in chromium.  I have not come across a test case
+                    //       that uses this format.
+                    SkDebugf("Error: huffman format unsupported.\n");
+                    return NULL;
+                default:
+                   SkDebugf("Error: invalid bmp bit masks header.\n");
+                   return NULL;
+            }
+            break;
+        case kJpeg_BitmapCompressionMethod:
+            if (24 == bitsPerPixel) {
+                inputFormat = kRLE_BitmapInputFormat;
+                break;
+            }
+        case kPng_BitmapCompressionMethod:
+            // TODO: Decide if we intend to support this.
+            //       It is unsupported in the previous version and
+            //       in chromium.  I think it is used mostly for printers.
+            SkDebugf("Error: compression format not supported.\n");
+            return NULL;
+        case kCMYK_BitmapCompressionMethod:
+        case kCMYK8BitRLE_BitmapCompressionMethod:
+        case kCMYK4BitRLE_BitmapCompressionMethod:
+            // TODO: Same as above.
+            SkDebugf("Error: CMYK not supported for bitmap decoding.\n");
+            return NULL;
+        default:
+            SkDebugf("Error: invalid format for bitmap decoding.\n");
+            return NULL;
+    }
+    iBuffer.free();
+
+    // Check for valid bits per pixel input
+    switch (bitsPerPixel) {
+        // In addition to more standard pixel compression formats, bmp supports
+        // the use of bit masks to determine pixel components.  The bmp standard
+        // format for representing 16-bit colors is 555 (XRRRRRGGGGGBBBBB),
+        // which does not map well to any Skia color formats.  For this reason,
+        // we will always enable mask mode with 16 bits per pixel.
+        case 16:
+            if (kBitMask_BitmapInputFormat != inputFormat) {
+                masks.redMask = 0x7C00;
+                masks.greenMask = 0x03E0;
+                masks.blueMask = 0x001F;
+            }
+            break;
+        case 1:
+        case 2:
+        case 4:
+        case 8:
+        case 24:
+        case 32:
+            break;
+        default:
+            SkDebugf("Error: invalid input value for bits per pixel.\n");
+            return NULL;
+    }
+
+    // Process the color table
+    uint32_t colorBytes = 0;
+    SkPMColor* colorTable = NULL;
+    if (bitsPerPixel < 16) {
+        // Verify the number of colors for the color table
+        const int maxColors = 1 << bitsPerPixel;
+        // Zero is a default for maxColors
+        // Also set numColors to maxColors when input is too large
+        if (numColors <= 0 || numColors > maxColors) {
+            numColors = maxColors;
+        }
+        colorTable = SkNEW_ARRAY(SkPMColor, maxColors);
+
+        // Construct the color table
+        colorBytes = numColors * bytesPerColor;
+        SkAutoTDeleteArray<uint8_t> cBuffer(SkNEW_ARRAY(uint8_t, colorBytes));
+        if (stream->read(cBuffer.get(), colorBytes) != colorBytes) {
+            SkDebugf("Error: unable to read color table.\n");
+            return NULL;
+        }
+        // We must respect the alpha channel for V4 and V5.  However, if it is
+        // all zeros, we will display the image as opaque rather than
+        // transparent.  This may require redoing some of the processing.
+        bool seenNonZeroAlpha = false;
+        uint32_t i = 0;
+        for (; i < numColors; i++) {
+            uint8_t blue = get_byte(cBuffer.get(), i*bytesPerColor);
+            uint8_t green = get_byte(cBuffer.get(), i*bytesPerColor + 1);
+            uint8_t red = get_byte(cBuffer.get(), i*bytesPerColor + 2);
+            uint8_t alpha = 0xFF;
+            if (kInfoV4_BitmapHeaderType == headerType ||
+                    kInfoV5_BitmapHeaderType == headerType) {
+                alpha = (masks.alphaMask >> 24) &
+                        get_byte(cBuffer.get(), i*bytesPerColor + 3);
+                if (!alpha && !seenNonZeroAlpha) {
+                    alpha = 0xFF;
+                } else {
+                    // If we see a non-zero alpha, we restart the loop
+                    seenNonZeroAlpha = true;
+                    i = -1;
+                }
+            }
+            colorTable[i] = SkPreMultiplyColor(SkColorSetARGBInline(alpha,
+                    red, green, blue));
+        }
+        // To avoid segmentation faults on bad pixel data, fill the end of the
+        // color table with black.  This is the same the behavior as the
+        // chromium decoder.
+        for (; i < maxColors; i++) {
+            colorTable[i] = SkPackARGB32(0xFF, 0, 0, 0);
+        }
+    } else {
+        // We will not use the color table if bitsPerPixel >= 16, but if there
+        // is a color table, we may need to skip the color table bytes.
+        if (numColors <= 1 << 8) {

[scroggo, 2015/02/28 00:25:13]

Nit: I find this more legible with parentheses:

(1 << 8)

Also, maybe a note explaining where this number comes from?

+            colorBytes = numColors * bytesPerColor;
+            if (stream->skip(colorBytes) != colorBytes) {
+                SkDebugf("Error: Could not skip color table bytes.\n");
+                return NULL;
+            }
+        }
+    }
+
+    // Ensure that the stream now points to the start of the pixel array
+    uint32_t bytesRead = kBmpHeaderBytes + infoBytes + maskBytes + colorBytes;
+    if (bytesRead <= offset) {

[scroggo, 2015/02/28 00:25:13]

nit: This is more readable if you reverse the if:

if (bytesRead > offset) {
  return NULL;
}

if (stream->skip...

+        if (stream->skip(offset - bytesRead) != offset - bytesRead) {
+            SkDebugf("Error: unable to skip to image data.\n");
+            return NULL;
+        }
+    } else {
+        SkDebugf("Error: pixel data offset less than header size.\n");
+        return NULL;
+    }
+
+    // Remaining bytes is only used for RLE
+    const int remainingBytes = totalBytes - offset;
+    if (remainingBytes <= 0 && kRLE_BitmapInputFormat == inputFormat) {
+        SkDebugf("Error: RLE requires valid input size.\n");
+        return NULL;
+    }
+
+    // Return the codec
+    // Use of image info for input format does not make sense given
+    // that the possible bitmap input formats do not match up with
+    // Skia color types.  Instead we use ImageInfo for width and height,
+    // and other fields for input format information.
+    const SkImageInfo& imageInfo = SkImageInfo::Make(width, height,
+            kN32_SkColorType, kPremul_SkAlphaType);
+    return SkNEW_ARGS(SkBmpCodec, (imageInfo, stream, bitsPerPixel,
+                                   inputFormat, masks, colorTable, rowOrder,
+                                   remainingBytes));
+}
+
+/*
+ *
+ * Creates an instance of the decoder
+ * Called only by NewFromStream
+ *
+ */
+SkBmpCodec::SkBmpCodec(const SkImageInfo& info, SkStream* stream,
+                       uint16_t bitsPerPixel, BitmapInputFormat inputFormat,
+                       SkSwizzler::ColorMasks masks, SkPMColor* colorTable,
+                       SkSwizzler::RowOrder rowOrder,
+                       const uint32_t remainingBytes)
+    : INHERITED(info, stream)
+    , fBitsPerPixel(bitsPerPixel)
+    , fInputFormat(inputFormat)
+    , fBitMasks(masks)
+    , fColorTable(colorTable)
+    , fRowOrder(rowOrder)
+    , fRemainingBytes(remainingBytes)
+{}
+
+/*
+ *
+ * Initiates the bitmap decode
+ *
+ */
+SkCodec::Result SkBmpCodec::onGetPixels(const SkImageInfo& dstInfo,
+                                        void* dst, size_t dstRowBytes,
+                                        SkPMColor*, int*) {
+    // This version of the decoder does not support scaling
+    if (dstInfo.dimensions() != getOriginalInfo().dimensions()) {
+        SkDebugf("Error: scaling not supported.\n");
+        return kInvalidScale;
+    }
+
+    switch (fInputFormat) {
+        case kRLE_BitmapInputFormat:
+            return decodeRLE(dstInfo, dst, dstRowBytes);
+        case kBitMask_BitmapInputFormat:
+        case kStandard_BitmapInputFormat:
+            return decode(dstInfo, dst, dstRowBytes);
+        default:
+            SkDebugf("Error: unknown bitmap input format.\n");
+            return kInvalidInput;
+    }
+}
+
+/*
+ *
+ * Performs the bitmap decoding for standard and bit masks input format
+ *
+ */
+SkCodec::Result SkBmpCodec::decode(const SkImageInfo& dstInfo,
+                                   void* dst, uint32_t dstRowBytes) {
+    // Set constant values
+    const int width = dstInfo.width();
+    const int height = dstInfo.height();
+    const uint32_t pixelsPerByte = 8 / fBitsPerPixel;
+    const uint32_t bytesPerPixel = fBitsPerPixel / 8;
+    const uint32_t unpaddedRowBytes = fBitsPerPixel < 16 ?
+            (width + pixelsPerByte - 1) / pixelsPerByte : width * bytesPerPixel;
+    const uint32_t paddedRowBytes = (unpaddedRowBytes + 3) & (~3);
+    const uint32_t alphaMask = fBitMasks.alphaMask;
+
+    // Get swizzler configuration
+    SkSwizzler::SrcConfig config;
+    switch (fBitsPerPixel) {
+        case 1:
+            config = SkSwizzler::kIndex1;
+            break;
+        case 2:
+            config = SkSwizzler::kIndex2;
+            break;
+        case 4:
+            config = SkSwizzler::kIndex4;
+            break;
+        case 8:
+            config = SkSwizzler::kIndex;
+            break;
+        case 16:
+            config = SkSwizzler::kMask16;
+            break;
+        case 24:
+            if (kBitMask_BitmapInputFormat == fInputFormat) {
+                config = SkSwizzler::kMask24;
+            } else {
+                config = SkSwizzler::kBGR;
+            }
+            break;
+        case 32:
+            if (kBitMask_BitmapInputFormat == fInputFormat) {
+                config = SkSwizzler::kMask32;
+            } else if (!alphaMask) {
+                config = SkSwizzler::kBGRX;
+            } else {
+                config = SkSwizzler::kBGRA;
+            }
+            break;
+        default:
+            SkASSERT(false);
+            return kInvalidInput;
+    }
+
+    // If zeroAlpha is kNormal, it indicates that the image will be
+    // considered as encoded.  If kTransparentAsOpaque, we will respect the
+    // value of the alpha channel if it is nonzero for any of the pixels.
+    // However, if it is always zero, we will consider the image opaque instead
+    // of transparent. This may require redoing some of the decoding.
+    SkSwizzler::ZeroAlpha zeroAlpha = SkSwizzler::kNormal_ZeroAlpha;
+    if (alphaMask) {
+        zeroAlpha = SkSwizzler::kTransparentAsOpaque_ZeroAlpha;
+    }
+
+    // Create swizzler
+    SkSwizzler* swizzler = SkSwizzler::CreateSwizzler(config, fColorTable.get(),
+            dstInfo, dst, dstRowBytes, false, fBitMasks, zeroAlpha,
+            fRowOrder);
+
+    // Allocate space for a row buffer and a source for the swizzler
+    SkAutoTDeleteArray<uint8_t> srcBuffer(SkNEW_ARRAY(uint8_t, paddedRowBytes));
+
+    // Iterate over rows of the image
+    for (uint32_t y = 0; y < height; y++) {
+        // Read a row of the input
+        if (stream()->read(srcBuffer.get(), paddedRowBytes) != paddedRowBytes) {
+            return kIncompleteInput;
+        }
+
+        // Decode the row in destination format
+        swizzler->next(srcBuffer.get());
+    }
+
+    // Finished decoding the entire image
+    return kSuccess;
+}
+
+/*
+ *
+ * Set an RLE pixel using the color table
+ *
+ */
+void SkBmpCodec::setRLEPixel(SkPMColor* dst, uint32_t dstRowBytes, int height,
+                             uint32_t x, uint32_t y, uint8_t index) {
+    if (SkSwizzler::kBottomUp_RowOrder == fRowOrder) {
+        y = height - y - 1;
+    }
+    SkPMColor* dstRow = SkTAddOffset<SkPMColor>(dst, y * dstRowBytes);
+    dstRow[x] = fColorTable.get()[index];
+}
+
+/*
+ *
+ * Performs the bitmap decoding for RLE input format
+ * RLE decoding is performed all at once, rather than a one row at a time
+ *
+ */
+SkCodec::Result SkBmpCodec::decodeRLE(const SkImageInfo& dstInfo,
+                                      void* dst, uint32_t dstRowBytes) {
+    // Set RLE flags
+    static const uint8_t RLE_ESCAPE = 0;
+    static const uint8_t RLE_EOL = 0;
+    static const uint8_t RLE_EOF = 1;
+    static const uint8_t RLE_DELTA = 2;
+
+    // Set constant values
+    const int width = dstInfo.width();
+    const int height = dstInfo.height();
+    const uint32_t pixelsPerByte = 8 / fBitsPerPixel;
+    const uint32_t bytesPerPixel = fBitsPerPixel / 8;
+
+    // Input buffer parameters
+    uint32_t i = 0;
+    SkAutoTDeleteArray<uint8_t> buffer(SkNEW_ARRAY(uint8_t, fRemainingBytes));
+    uint32_t totalBytes = stream()->read(buffer.get(), fRemainingBytes);
+    if (totalBytes < fRemainingBytes) {
+        SkDebugf("Warning: incomplete RLE file.\n");
+    } else if (totalBytes <= 0) {
+        SkDebugf("Error: could not read RLE image data.\n");
+        return kInvalidInput;
+    }
+
+    // Destination parameters
+    uint32_t x = 0;
+    uint32_t y = 0;
+    // If the code skips pixels, remaining pixels are transparent or black
+    // TODO: Skip this is memory was already zeroed.
+    memset(dst, 0, dstRowBytes * height);
+    SkPMColor* dstPtr = (SkPMColor*) dst;
+
+    while (true) {
+        // Every entry takes at least two bytes
+        if (totalBytes - i < 2) {
+            SkDebugf("Warning: incomplete RLE input.\n");
+            return kIncompleteInput;
+        }
+
+        // Read the two bytes and verify we have not reached end of image
+        const uint8_t count = buffer.get()[i++];
+        const uint8_t code = buffer.get()[i++];
+        if ((count || (code != RLE_EOF)) && y >= height) {
+            SkDebugf("Warning: invalid RLE input.\n");
+            return kInvalidInput;
+        }
+
+        // Perform decoding
+        if (RLE_ESCAPE == count) {
+            switch (code) {
+                case RLE_EOL:
+                    x = 0;
+                    y++;
+                    break;
+                case RLE_EOF:
+                    return kSuccess;
+                case RLE_DELTA: {
+                    // Two bytes are needed to specify delta
+                    if (totalBytes - i < 2) {
+                        SkDebugf("Warning: incomplete RLE input\n");
+                        return kIncompleteInput;
+                    }
+                    // Modify x and y
+                    const uint8_t dx = buffer.get()[i++];
+                    const uint8_t dy = buffer.get()[i++];
+                    x += dx;
+                    y += dy;
+                    if (x > width || y > height) {
+                        SkDebugf("Warning: invalid RLE input.\n");
+                        return kInvalidInput;
+                    }
+                    break;
+                }
+                default: { // Absolute mode
+                    // Check that we have enough bytes and that there are
+                    // enough pixels remaining in the row
+                    const uint32_t unpaddedBytes = fBitsPerPixel < 16 ?
+                            (code + pixelsPerByte - 1) / pixelsPerByte :
+                            code * bytesPerPixel;
+                    const uint32_t paddedBytes =
+                            (unpaddedBytes + 1) & (~1);
+                    if (x + code > width || totalBytes - i < paddedBytes) {
+                        SkDebugf("Warning: invalid RLE input.\n");
+                        return kInvalidInput;
+                    }
+                    // Use the color table to set the coded number of pixels
+                    uint8_t num = code;
+                    while (num > 0) {
+                        switch(fBitsPerPixel) {
+                            case 4: {
+                                uint8_t val = buffer.get()[i++];
+                                setRLEPixel(dstPtr, dstRowBytes, height, x++, y,
+                                        val >> 4);
+                                num--;
+                                if (num) {
+                                    setRLEPixel(dstPtr, dstRowBytes, height,
+                                            x++, y, val & 0xF);
+                                    num--;
+                                }
+                                break;
+                            }
+                            case 8:
+                                setRLEPixel(dstPtr, dstRowBytes, height, x++, y,
+                                        buffer.get()[i++]);
+                                num--;
+                                break;
+                            case 24: {
+                                uint8_t blue = buffer.get()[i++];
+                                uint8_t green = buffer.get()[i++];
+                                uint8_t red = buffer.get()[i++];
+                                SkPMColor color = SkPackARGB32(

[scroggo, 2015/02/28 00:25:13]

I think this one should be SkPreMultiplyARGB (Pack doesn't multiply the alpha).

+                                        0xFF, red, green, blue);
+                                SkPMColor* dstRow = SkTAddOffset<SkPMColor>(
+                                        dstPtr, y * dstRowBytes);
+                                dstRow[x++] = color;
+                                num--;
+                            }
+                            default:
+                                SkDebugf("Error: invalid RLE bpp.\n");
+                                return kInvalidInput;
+                        }
+                    }
+                    // Skip a byte if necessary to maintain alignment
+                    if (unpaddedBytes & 1) {
+                        i++;
+                    }
+                    break;
+                }
+            }
+        } else { // Encoded mode
+            // Ensure we do not move past the end of the row
+            const uint32_t endX = SkTMin<uint32_t>(x + count, (uint32_t) width);

[scroggo, 2015/02/28 00:25:13]

Do you still need algorithm? (I assume you had it for min?)

+
+            if (24 == fBitsPerPixel) {
+                // Check that there is enough data
+                if (totalBytes - i < 2) {
+                    SkDebugf("Warning: incomplete RLE input\n");
+                    return kIncompleteInput;
+                }
+
+                // Fill the pixels up to endX with the specified color
+                uint8_t blue = code;
+                uint8_t green = buffer.get()[i++];
+                uint8_t red = buffer.get()[i++];
+                SkPMColor color = SkPackARGB32(0xFF, red, green, blue);
+                SkPMColor* dstRow =
+                        SkTAddOffset<SkPMColor>(dstPtr, y * dstRowBytes);
+                while (x < endX) {
+                    dstRow[x++] = color;
+                }
+            } else {
+                // RLE8 has one color index that gets repeated
+                // RLE4 has two color indexes in the upper and lower 4 bits of
+                // the bytes, which are alternated
+                uint8_t indices[2] = { code, code };
+                if (4 == fBitsPerPixel) {
+                    indices[0] >>= 4;
+                    indices[1] &= 0xf;
+                }
+
+                // Set the indicated number of pixels
+                for (int which = 0; x < endX; x++) {
+                    setRLEPixel(dstPtr, dstRowBytes, height, x, y,
+                            indices[which]);
+                    which = !which;
+                }
+            }
+        }
+    }
+}