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 "SkEndian.h"
+#include "SkStream.h"
+
+/*
+ *
+ * Get a byte from the buffer

[scroggo, 2015/03/11 21:14:26]

Maybe add a comment that these are unsafe, and assume the caller did the real
check?

[msarett, 2015/03/12 14:06:46]

Yes I agree, I made this more clear.

+ *
+ */
+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
+}
+
+static bool premul_and_unpremul(SkAlphaType dst, SkAlphaType src) {

[scroggo, 2015/03/11 21:14:25]

// TODO: Share with other codecs

(unless this is different in some way I've missed?)

[msarett, 2015/03/12 14:06:46]

I have added this method to SkCodec.h (unfortunately it's public because it's
called by conversion_possible, which is not a member function).

I also added get_byte, get_short, and get_int to SkCodec.h because they will be
shared by the Ico decoder that is coming next.

Let me know if this is this follows the design you were looking for.

+    return kPremul_SkAlphaType == dst && kUnpremul_SkAlphaType == src;
+}
+
+/*
+ *
+ * Checks if the conversion between the input image and the requested output
+ * image has been implemented
+ *
+ */
+static bool conversion_possible(const SkImageInfo& dst,
+                                const SkImageInfo& src) {
+    // TODO: Support more conversions
+    if (kN32_SkColorType != dst.colorType()) {
+        return false;
+    }
+    if (dst.alphaType() == src.alphaType()) {
+        return true;
+    }
+    return premul_and_unpremul(dst.alphaType(), src.alphaType())
+            || premul_and_unpremul(dst.alphaType(), src.alphaType());
+}
+
+/*
+ *
+ * Compute row bytes for an image
+ *
+ */
+size_t compute_row_bytes(int width, uint32_t bitsPerPixel) {
+    if (bitsPerPixel < 16) {
+        SkASSERT(0 == 8 % bitsPerPixel);
+        const uint32_t pixelsPerByte = 8 / bitsPerPixel;
+        return (width + pixelsPerByte - 1) / pixelsPerByte;
+    } else {
+        SkASSERT(0 == bitsPerPixel % 8);
+        const uint32_t bytesPerPixel = bitsPerPixel / 8;
+        return width * bytesPerPixel;
+    }
+}
+
+/*
+ *
+ * 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;
+    }
+
+    // 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);
+
+    // 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;
+        }
+        // We check the size of the header before entering the if statement.
+        // We should not reach this point unless the size is large enough for
+        // these required fields.
+        SkASSERT(infoBytesRemaining >= 12);
+        width = get_int(iBuffer.get(), 0);
+        height = get_int(iBuffer.get(), 4);
+        //uint16_t planes = get_short(iBuffer, 8);

[scroggo, 2015/03/11 21:14:26]

Any reason to leave this here?

[msarett, 2015/03/12 14:06:47]

Sorry I must have forgotten to delete this.

+        bitsPerPixel = get_short(iBuffer.get(), 10);
+
+        // Some versions do not have this field, so we check before
+        // overwriting the default value.
+        if (infoBytesRemaining >= 16) {
+            compression = get_int(iBuffer.get(), 12);
+        }
+
+        // Some versions do not have this field, so we check before
+        // overwriting the default value.
+        if (infoBytesRemaining >= 32) {

[scroggo, 2015/03/11 21:14:25]

Maybe this should go inside the previous if statement?

[msarett, 2015/03/12 14:06:47]

Yes that makes sense.

+            numColors = get_int(iBuffer.get(), 28);
+        }
+
+        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);
+        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
+    RowOrder rowOrder = kBottomUp_RowOrder;
+    if (height < 0) {
+        height = -height;
+        rowOrder = kTopDown_RowOrder;
+    }
+    static const int kBmpMaxDim = 1 << 16;
+    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
+    SkMasks::InputMasks inputMasks;
+    memset(&inputMasks, 0, 4*sizeof(uint32_t));
+
+    // 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;
+            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 inputMasks.\n");
+                        return NULL;
+                    }
+                    maskBytes = kBmpMaskBytes;
+                    inputMasks.red = get_int(mBuffer.get(), 0);
+                    inputMasks.green = get_int(mBuffer.get(), 4);
+                    inputMasks.blue = get_int(mBuffer.get(), 8);
+                    break;
+                }
+                case kInfoV2_BitmapHeaderType:
+                case kInfoV3_BitmapHeaderType:
+                case kInfoV4_BitmapHeaderType:
+                case kInfoV5_BitmapHeaderType:
+                    // Header types are matched based on size.  If the header
+                    // is V2+, we are guaranteed to be able at least this size.

[scroggo, 2015/03/11 21:14:26]

able to read*

[msarett, 2015/03/12 14:06:46]

Nice catch.  How does this error keep coming back?

[scroggo, 2015/03/12 15:19:57]

Haha, I don't know, but I know when I rebase I sometimes get confused about
which version is which...

+                    SkASSERT(infoBytesRemaining >= 48);
+                    inputMasks.red = get_int(iBuffer.get(), 36);
+                    inputMasks.green = get_int(iBuffer.get(), 40);
+                    inputMasks.blue = 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;
+            }
+            // Fall through
+        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;
+    }
+
+    // Most versions of bmps should be rendered as opaque.  Either they do
+    // not have an alpha channel, or they expect the alpha channel to be
+    // ignored.  V4+ bmp files introduce an alpha mask and allow the creator
+    // of the image to use the alpha channels.  However, many of these images
+    // leave the alpha channel blank and expect to be rendered as opaque.  For
+    // this reason, we set the alpha type to kUnknown for V4+ bmps and figure
+    // out the alpha type during the decode.
+    SkAlphaType alphaType = kOpaque_SkAlphaType;
+    if (kInfoV4_BitmapHeaderType == headerType ||
+            kInfoV5_BitmapHeaderType == headerType) {
+        // Header types are matched based on size.  If the header is
+        // V4+, we are guaranteed to be able to read at least this size.
+        SkASSERT(infoBytesRemaining > 52);
+        inputMasks.alpha = get_int(iBuffer.get(), 48);
+        if (inputMasks.alpha != 0) {
+            alphaType = kUnpremul_SkAlphaType;
+        }
+    }
+    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 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) {
+                inputMasks.red = 0x7C00;
+                inputMasks.green = 0x03E0;
+                inputMasks.blue = 0x001F;
+                inputFormat = kBitMask_BitmapInputFormat;
+            }
+            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;
+    }
+
+    // Check that input bit masks are valid and create the masks object
+    SkAutoTDelete<SkMasks>
+            masks(SkMasks::CreateMasks(inputMasks, bitsPerPixel));
+    if (NULL == masks) {
+        SkDebugf("Error: invalid input masks.\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 uint32_t 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;
+        }
+
+        // Fill in the color table
+        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 (kOpaque_SkAlphaType != alphaType) {
+                alpha = (inputMasks.alpha >> 24) &
+                        get_byte(cBuffer.get(), i*bytesPerColor + 3);
+            }
+            colorTable[i] = SkPackARGB32NoCheck(alpha, red, green, blue);

[scroggo, 2015/03/11 21:14:25]

So these colors are unpremultiplied? Can you add a comment (maybe by the
declaration of colorTable)?

[msarett, 2015/03/12 14:06:47]

Yes.  I hope that I am following your convention by always using Opaque or
Unpremultiplied alpha types.  I should try to make it more clear that I default
to Unpremul.

[scroggo, 2015/03/12 15:19:57]

But it appears there is no way to get premul. Does onGetPixels later premultiply
the colors if the caller requested premul?

Come to think of it, I just realized that this is different from SkPngCodec.
That Codec reads the color table inside onGetPixels, whereas this one reads it
in NewFromStream.

I can think of advantages to either one:
- In NewFromStream:
  - if the color table is invalid, we can early exit without creating a codec
  - if the caller chooses to rewind and decode again, the color table is already
decoded
- In onGetPixels:
  - when reading the color table, we know whether we want premultiplied data or
not
  - the caller can get the dimensions sooner

(Maybe there are others.)

I thought maybe all codecs should behave the same, but it may be that this model
fits the BMP codec better, while the other fits the PNG model better. Matt, what
are your thoughts?

+        }
+
+        // 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] = SkPackARGB32NoCheck(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.
+        // We will assume that the maximum color table size is the same as when
+        // there are 8 bits per pixel (the largest color table actually used).
+        // Color tables for greater than 8 bits per pixel are somewhat
+        // undocumented.  It is indicated that they may exist to store a list
+        // of colors for optimization on devices with limited color display
+        // capacity.  While we do not know for sure, we will guess that any
+        // value of numColors greater than this maximum is invalid.

[scroggo, 2015/03/11 21:14:25]

If numColors is greater than the maximum, is the rest of the image likely to be
valid? Maybe there's not a good way to determine that...

[msarett, 2015/03/12 14:06:46]

No idea.  Your guess is as good as mine.  I think it's reasonable to continue
because the next step is to try to move the stream to the pixel data. 
Hopefully, this will fail if the rest of the image is invalid and succeed if
there is actually pixel data to decode.

[scroggo, 2015/03/12 15:19:57]

Come to think of it, what do we accomplish with this check? All we do is skip
some bytes. We would skip those bytes anyway down below when we skip to the
offset. It seems like we potentially eliminate images which have invalid color
tables even though the offset is correct. What if instead we just set colorBytes
to 0 in the else case?

+        if (numColors <= (1 << 8)) {
+            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;
+
+    // Check that we have not read past the pixel array offset
+    if(bytesRead > offset) {
+        // This may occur on OS 2.1 and other old versions where the color
+        // table defaults to max size, and the bmp tries to use a smaller color
+        // table.  This is invalid, and our decision is to indicate an error,
+        // rather than try to guess the intended size of the color table and
+        // rewind the stream to display the image.
+        SkDebugf("Error: pixel data offset less than header size.\n");
+        return NULL;
+    }
+
+    // Skip to the start of the pixel array
+    if (stream->skip(offset - bytesRead) != offset - bytesRead) {
+        SkDebugf("Error: unable to skip to image data.\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
+    // We will use ImageInfo to store width, height, and alpha type.  The Skia
+    // color types do not match with the many possible bmp input color types,
+    // so we will ignore this field and depend on other parameters for input
+    // information.
+    const SkImageInfo& imageInfo = SkImageInfo::Make(width, height,
+            kUnknown_SkColorType, alphaType);
+    return SkNEW_ARGS(SkBmpCodec, (imageInfo, stream, bitsPerPixel,
+                                   inputFormat, masks.detach(), 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,
+                       SkMasks* masks, SkPMColor* colorTable,
+                       RowOrder rowOrder,
+                       const uint32_t remainingBytes)
+    : INHERITED(info, stream)
+    , fBitsPerPixel(bitsPerPixel)
+    , fInputFormat(inputFormat)
+    , fMasks(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*) {
+    if (!this->rewindIfNeeded()) {
+        return kCouldNotRewind;
+    }
+    if (dstInfo.dimensions() != this->getOriginalInfo().dimensions()) {
+        SkDebugf("Error: scaling not supported.\n");
+        return kInvalidScale;
+    }
+    if (!conversion_possible(dstInfo, this->getOriginalInfo())) {
+        SkDebugf("Error: cannot convert input type to output type.\n");
+        return kInvalidConversion;
+    }
+
+    switch (fInputFormat) {
+        case kBitMask_BitmapInputFormat:
+            return decodeMask(dstInfo, dst, dstRowBytes);
+        case kRLE_BitmapInputFormat:
+            return decodeRLE(dstInfo, dst, dstRowBytes);
+        case kStandard_BitmapInputFormat:
+            return decode(dstInfo, dst, dstRowBytes);
+        default:
+            SkASSERT(false);
+            return kInvalidInput;
+    }
+}
+
+/*
+ *
+ * Performs the bitmap decoding for bit masks input format
+ *
+ */
+SkCodec::Result SkBmpCodec::decodeMask(const SkImageInfo& dstInfo,
+                                       void* dst, uint32_t dstRowBytes) {
+    // Set constant values
+    const int width = dstInfo.width();
+    const int height = dstInfo.height();
+    const size_t rowBytes = SkAlign4(compute_row_bytes(width, fBitsPerPixel));
+
+    // Allocate space for a row buffer and a source for the swizzler
+    SkAutoTDeleteArray<uint8_t> srcBuffer(SkNEW_ARRAY(uint8_t, rowBytes));
+
+    // Get the destination start row and delta
+    SkPMColor* dstRow;
+    int32_t delta;
+    if (kTopDown_RowOrder == fRowOrder) {
+        dstRow = (SkPMColor*) dst;
+        delta = dstRowBytes;
+    } else {
+        dstRow = (SkPMColor*) SkTAddOffset<void>(dst, (height-1) * dstRowBytes);
+        delta = -dstRowBytes;
+    }
+
+    // Create the swizzler
+    SkMaskSwizzler* swizzler = SkMaskSwizzler::CreateMaskSwizzler(
+            dstInfo, fMasks, fBitsPerPixel);
+
+    // Iterate over rows of the image
+    bool transparent = true;
+    for (int y = 0; y < height; y++) {
+        // Read a row of the input
+        if (stream()->read(srcBuffer.get(), rowBytes) != rowBytes) {
+            SkDebugf("Warning: incomplete input stream.\n");
+            return kIncompleteInput;
+        }
+
+        // Decode the row in destination format
+        SkSwizzler::ResultAlpha r = swizzler->next(dstRow, srcBuffer.get());
+        transparent &= (SkSwizzler::kTransparent_ResultAlpha == r);
+
+        // Move to the next row
+        dstRow = SkTAddOffset<SkPMColor>(dstRow, delta);
+    }
+
+    // Many fully transparent bmp images are intended to be opaque.  Here, we
+    // correct for this possibility.
+    dstRow = (SkPMColor*) dst;
+    if (transparent) {
+        for (int y = 0; y < height; y++) {
+            for (int x = 0; x < width; x++) {
+                dstRow[x] |= 0xFF000000;
+            }
+            dstRow = SkTAddOffset<SkPMColor>(dstRow, dstRowBytes);
+        }
+    }
+
+    // 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 (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();
+
+    // Input buffer parameters
+    uint32_t currByte = 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
+    int x = 0;
+    int y = 0;
+    // If the code skips pixels, remaining pixels are transparent or black
+    // TODO: Skip this if memory was already zeroed.
+    memset(dst, 0, dstRowBytes * height);
+    SkPMColor* dstPtr = (SkPMColor*) dst;
+
+    while (true) {
+        // Every entry takes at least two bytes
+        if (totalBytes - currByte < 2) {
+            SkDebugf("Warning: incomplete RLE input.\n");
+            return kIncompleteInput;
+        }
+
+        // Read the next two bytes.  These bytes have different meanings
+        // depending on their values.  In the first interpretation, the first
+        // byte is an escape flag and the second byte indicates what special
+        // task to perform.
+        const uint8_t flag = buffer.get()[currByte++];
+        const uint8_t task = buffer.get()[currByte++];
+
+        // If we have reached a row that is beyond the image size, and the RLE
+        // code does not indicate end of file, abort and signal a warning.
+        if (y >= height && (flag != RLE_ESCAPE || (task != RLE_EOF))) {
+            SkDebugf("Warning: invalid RLE input.\n");
+            return kIncompleteInput;
+        }
+
+        // Perform decoding
+        if (RLE_ESCAPE == flag) {
+            switch (task) {
+                case RLE_EOL:
+                    x = 0;
+                    y++;
+                    break;
+                case RLE_EOF:
+                    return kSuccess;
+                case RLE_DELTA: {
+                    // Two bytes are needed to specify delta
+                    if (totalBytes - currByte < 2) {
+                        SkDebugf("Warning: incomplete RLE input\n");
+                        return kIncompleteInput;
+                    }
+                    // Modify x and y
+                    const uint8_t dx = buffer.get()[currByte++];
+                    const uint8_t dy = buffer.get()[currByte++];
+                    x += dx;
+                    y += dy;
+                    if (x > width || y > height) {
+                        SkDebugf("Warning: invalid RLE input.\n");
+                        return kIncompleteInput;
+                    }
+                    break;
+                }
+                default: {
+                    // If task does not match any of the above signals, it
+                    // indicates that we have a sequence of non-RLE pixels.
+                    // Furthermore, the value of task is equal to the number
+                    // of pixels to interpret.
+                    uint8_t numPixels = task;
+                    const size_t rowBytes = compute_row_bytes(numPixels,
+                            fBitsPerPixel);
+                    // Abort if setting numPixels moves us off the edge of the
+                    // image.  Also abort if there are not enough bytes
+                    // remaining in the stream to set numPixels.
+                    if (x + numPixels > width ||
+                            totalBytes - currByte < SkAlign2(rowBytes)) {
+                        SkDebugf("Warning: invalid RLE input.\n");
+                        return kIncompleteInput;
+                    }
+                    // Set count number of pixels
+                    while (numPixels > 0) {
+                        switch(fBitsPerPixel) {
+                            case 4: {
+                                uint8_t val = buffer.get()[currByte++];

[scroggo, 2015/03/11 21:14:25]

Do we need to do a check to make sure currByte is still in range? (Or did some
previous check account for this one?) Same questions for below.

Oh, wait, we're guaranteed that we'll only read rowBytes bytes in this while
loop? Maybe add an SkASSERT that we know these reads are safe.

[msarett, 2015/03/12 14:06:47]

I agree that this is confusing.  Hopefully adding asserts will make it more
clear.

+                                setRLEPixel(dstPtr, dstRowBytes, height, x++, y,
+                                        val >> 4);
+                                numPixels--;
+                                if (numPixels != 0) {
+                                    setRLEPixel(dstPtr, dstRowBytes, height,
+                                            x++, y, val & 0xF);
+                                    numPixels--;
+                                }
+                                break;
+                            }
+                            case 8:
+                                setRLEPixel(dstPtr, dstRowBytes, height, x++, y,
+                                        buffer.get()[currByte++]);
+                                numPixels--;
+                                break;
+                            case 24: {
+                                uint8_t blue = buffer.get()[currByte++];
+                                uint8_t green = buffer.get()[currByte++];
+                                uint8_t red = buffer.get()[currByte++];
+                                SkPMColor color = SkPackARGB32NoCheck(
+                                        0xFF, red, green, blue);
+                                SkPMColor* dstRow = SkTAddOffset<SkPMColor>(
+                                        dstPtr, y * dstRowBytes);

[scroggo, 2015/03/11 21:14:26]

y is unchanged in the loop, correct? Can we do this multiply only once?

[msarett, 2015/03/12 14:06:46]

Yes you are correct!

+                                dstRow[x++] = color;
+                                numPixels--;
+                            }
+                            default:
+                                SkASSERT(false);
+                                return kInvalidInput;
+                        }
+                    }
+                    // Skip a byte if necessary to maintain alignment
+                    if (!SkIsAlign2(rowBytes)) {
+                        currByte++;
+                    }
+                    break;
+                }
+            }
+        } else {
+            // If the first byte read is not a flag, it indicates the number of
+            // pixels to set in RLE mode.
+            const uint8_t numPixels = flag;
+            const int endX = SkTMin<int>(x + numPixels, width);
+
+            if (24 == fBitsPerPixel) {
+                // In RLE24, the second byte read is part of the pixel color.
+                // There are two more required bytes to finish encoding the
+                // color.
+                if (totalBytes - currByte < 2) {
+                    SkDebugf("Warning: incomplete RLE input\n");
+                    return kIncompleteInput;
+                }
+
+                // Fill the pixels up to endX with the specified color
+                uint8_t blue = task;
+                uint8_t green = buffer.get()[currByte++];
+                uint8_t red = buffer.get()[currByte++];
+                SkPMColor color = SkPackARGB32NoCheck(0xFF, red, green, blue);
+                SkPMColor* dstRow =
+                        SkTAddOffset<SkPMColor>(dstPtr, y * dstRowBytes);
+                while (x < endX) {
+                    dstRow[x++] = color;
+                }
+            } else {
+                // In RLE8 or RLE4, the second byte read gives the index in the
+                // color table to look up the pixel color.
+                // 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] = { task, task };
+                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;
+                }
+            }
+        }
+    }
+}
+
+/*
+ *
+ * Performs the bitmap decoding for standard 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 size_t rowBytes = SkAlign4(compute_row_bytes(width, fBitsPerPixel));
+    const uint32_t alphaMask = fMasks->getAlphaMask();
+
+    // Get swizzler configuration
+    SkSwizzler::SrcConfig config;
+    switch (fBitsPerPixel) {

[scroggo, 2015/03/11 21:14:25]

It's ironic that here we use fBitsPerPixel to determine the SrcConfig, and
SkSwizzler has a function that takes the SrcConfig and returns the bitsPerPixel.

Would it make sense to make the two independent? Not completely, obviously, but
I wonder if it would make sense to just have kIndex, and let the caller
separately specify the bitsPerPixel. (If so, we can land that as a separate CL.)

[msarett, 2015/03/12 14:06:46]

Yeah I completely agree with you, it doesn't make much sense.  I will make
myself a note to make that change.  I'm glad to do it as a separate CL.  I'm
thinking that we could provide another call to CreateSwizzler where bpp is
provided, otherwise it is calculated?

[scroggo, 2015/03/12 15:19:57]

I'd lean towards going all one direction or the other, rather than having a
separate CreateSwizzler for each.

+        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 24:
+            config = SkSwizzler::kBGR;
+            break;
+        case 32:
+            if (alphaMask == 0) {
+                config = SkSwizzler::kBGRX;
+            } else {
+                config = SkSwizzler::kBGRA;
+            }
+            break;
+        default:
+            SkASSERT(false);
+            return kInvalidInput;
+    }
+
+    // Create swizzler
+    SkSwizzler* swizzler = SkSwizzler::CreateSwizzler(config, fColorTable.get(),
+            dstInfo, dst, dstRowBytes, false);
+
+    // Allocate space for a row buffer and a source for the swizzler
+    SkAutoTDeleteArray<uint8_t> srcBuffer(SkNEW_ARRAY(uint8_t, rowBytes));
+
+    // Iterate over rows of the image
+    bool transparent = true;
+    for (int y = 0; y < height; y++) {
+        // Read a row of the input
+        if (stream()->read(srcBuffer.get(), rowBytes) != rowBytes) {
+            SkDebugf("Warning: incomplete input stream.\n");
+            return kIncompleteInput;
+        }
+
+        // Decode the row in destination format
+        uint32_t row;
+        if (kTopDown_RowOrder == fRowOrder) {
+            row = y;
+        } else {
+            row = height - 1 - y;
+        }
+        SkSwizzler::ResultAlpha r = swizzler->next(srcBuffer.get(), row);
+        transparent &= (SkSwizzler::kTransparent_ResultAlpha == r);

[scroggo, 2015/03/11 21:14:25]

Maybe the parenthetical bit should be a helper function?

If/when we change the way ResultAlpha is stored we can then continue using that
function without modifying callers.

[msarett, 2015/03/12 14:06:47]

Agreed, I will add a helper function to the Swizzler.

+    }
+
+    // Now we adjust the output image with some additional behavior that
+    // SkSwizzler does not support.  Firstly, all bmp images that contain
+    // alpha are masked by the alpha mask.  Secondly, many fully transparent
+    // bmp images are intended to be opaque.  Here, we make those corrections.
+    SkPMColor* dstRow = (SkPMColor*) dst;
+    if (alphaMask != 0) {
+        for (int y = 0; y < height; y++) {
+            for (int x = 0; x < width; x++) {
+                if (transparent) {
+                    dstRow[x] |= 0xFF000000;
+                } else {
+                    dstRow[x] &= alphaMask;
+                }
+                dstRow = SkTAddOffset<SkPMColor>(dstRow, dstRowBytes);
+            }
+        }
+    }
+
+    // Finished decoding the entire image
+    return kSuccess;
+}