xref: /external/webkit/Source/WebCore/platform/image-decoders/bmp/BMPImageReader.cpp

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 * modification, are permitted provided that the following conditions are
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 *
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#include "config.h"
#include "BMPImageReader.h"

namespace WebCore {

BMPImageReader::BMPImageReader(ImageDecoder* parent, size_t decodedAndHeaderOffset, size_t imgDataOffset, bool usesAndMask)
    : m_parent(parent)
    , m_buffer(0)
    , m_decodedOffset(decodedAndHeaderOffset)
    , m_headerOffset(decodedAndHeaderOffset)
    , m_imgDataOffset(imgDataOffset)
    , m_isOS21x(false)
    , m_isOS22x(false)
    , m_isTopDown(false)
    , m_needToProcessBitmasks(false)
    , m_needToProcessColorTable(false)
    , m_tableSizeInBytes(0)
    , m_seenNonZeroAlphaPixel(false)
    , m_seenZeroAlphaPixel(false)
    , m_andMaskState(usesAndMask ? NotYetDecoded : None)
{
    // Clue-in decodeBMP() that we need to detect the correct info header size.
    memset(&m_infoHeader, 0, sizeof(m_infoHeader));
}

bool BMPImageReader::decodeBMP(bool onlySize)
{
    // Calculate size of info header.
    if (!m_infoHeader.biSize && !readInfoHeaderSize())
        return false;

    // Read and process info header.
    if ((m_decodedOffset < (m_headerOffset + m_infoHeader.biSize)) && !processInfoHeader())
        return false;

    // processInfoHeader() set the size, so if that's all we needed, we're done.
    if (onlySize)
        return true;

    // Read and process the bitmasks, if needed.
    if (m_needToProcessBitmasks && !processBitmasks())
        return false;

    // Read and process the color table, if needed.
    if (m_needToProcessColorTable && !processColorTable())
        return false;

    // Initialize the framebuffer if needed.
    ASSERT(m_buffer);  // Parent should set this before asking us to decode!
    if (m_buffer->status() == ImageFrame::FrameEmpty) {
        if (!m_buffer->setSize(m_parent->size().width(), m_parent->size().height()))
            return m_parent->setFailed(); // Unable to allocate.
        m_buffer->setStatus(ImageFrame::FramePartial);
        // setSize() calls eraseARGB(), which resets the alpha flag, so we force
        // it back to false here.  We'll set it true below in all cases where
        // these 0s could actually show through.
        m_buffer->setHasAlpha(false);

        // For BMPs, the frame always fills the entire image.
        m_buffer->setOriginalFrameRect(IntRect(IntPoint(), m_parent->size()));

        if (!m_isTopDown)
            m_coord.setY(m_parent->size().height() - 1);
    }

    // Decode the data.
    if ((m_andMaskState != Decoding) && !pastEndOfImage(0)) {
        if ((m_infoHeader.biCompression != RLE4) && (m_infoHeader.biCompression != RLE8) && (m_infoHeader.biCompression != RLE24)) {
            const ProcessingResult result = processNonRLEData(false, 0);
            if (result != Success)
                return (result == Failure) ? m_parent->setFailed() : false;
        } else if (!processRLEData())
            return false;
    }

    // If the image has an AND mask and there was no alpha data, process the
    // mask.
    if ((m_andMaskState == NotYetDecoded) && !m_buffer->hasAlpha()) {
        // Reset decoding coordinates to start of image.
        m_coord.setX(0);
        m_coord.setY(m_isTopDown ? 0 : (m_parent->size().height() - 1));

        // The AND mask is stored as 1-bit data.
        m_infoHeader.biBitCount = 1;

        m_andMaskState = Decoding;
    }
    if (m_andMaskState == Decoding) {
        const ProcessingResult result = processNonRLEData(false, 0);
        if (result != Success)
            return (result == Failure) ? m_parent->setFailed() : false;
    }

    // Done!
    m_buffer->setStatus(ImageFrame::FrameComplete);
    return true;
}

bool BMPImageReader::readInfoHeaderSize()
{
    // Get size of info header.
    ASSERT(m_decodedOffset == m_headerOffset);
    if ((m_decodedOffset > m_data->size()) || ((m_data->size() - m_decodedOffset) < 4))
        return false;
    m_infoHeader.biSize = readUint32(0);
    // Don't increment m_decodedOffset here, it just makes the code in
    // processInfoHeader() more confusing.

    // Don't allow the header to overflow (which would be harmless here, but
    // problematic or at least confusing in other places), or to overrun the
    // image data.
    if (((m_headerOffset + m_infoHeader.biSize) < m_headerOffset) || (m_imgDataOffset && (m_imgDataOffset < (m_headerOffset + m_infoHeader.biSize))))
        return m_parent->setFailed();

    // See if this is a header size we understand:
    // OS/2 1.x: 12
    if (m_infoHeader.biSize == 12)
        m_isOS21x = true;
    // Windows V3: 40
    else if ((m_infoHeader.biSize == 40) || isWindowsV4Plus())
        ;
    // OS/2 2.x: any multiple of 4 between 16 and 64, inclusive, or 42 or 46
    else if ((m_infoHeader.biSize >= 16) && (m_infoHeader.biSize <= 64) && (!(m_infoHeader.biSize & 3) || (m_infoHeader.biSize == 42) || (m_infoHeader.biSize == 46)))
        m_isOS22x = true;
    else
        return m_parent->setFailed();

    return true;
}

bool BMPImageReader::processInfoHeader()
{
    // Read info header.
    ASSERT(m_decodedOffset == m_headerOffset);
    if ((m_decodedOffset > m_data->size()) || ((m_data->size() - m_decodedOffset) < m_infoHeader.biSize) || !readInfoHeader())
        return false;
    m_decodedOffset += m_infoHeader.biSize;

    // Sanity-check header values.
    if (!isInfoHeaderValid())
        return m_parent->setFailed();

    // Set our size.
    if (!m_parent->setSize(m_infoHeader.biWidth, m_infoHeader.biHeight))
        return false;

    // For paletted images, bitmaps can set biClrUsed to 0 to mean "all
    // colors", so set it to the maximum number of colors for this bit depth.
    // Also do this for bitmaps that put too large a value here.
    if (m_infoHeader.biBitCount < 16) {
      const uint32_t maxColors = static_cast<uint32_t>(1) << m_infoHeader.biBitCount;
      if (!m_infoHeader.biClrUsed || (m_infoHeader.biClrUsed > maxColors))
          m_infoHeader.biClrUsed = maxColors;
    }

    // For any bitmaps that set their BitCount to the wrong value, reset the
    // counts now that we've calculated the number of necessary colors, since
    // other code relies on this value being correct.
    if (m_infoHeader.biCompression == RLE8)
        m_infoHeader.biBitCount = 8;
    else if (m_infoHeader.biCompression == RLE4)
        m_infoHeader.biBitCount = 4;

    // Tell caller what still needs to be processed.
    if (m_infoHeader.biBitCount >= 16)
        m_needToProcessBitmasks = true;
    else if (m_infoHeader.biBitCount)
        m_needToProcessColorTable = true;

    return true;
}

bool BMPImageReader::readInfoHeader()
{
    // Pre-initialize some fields that not all headers set.
    m_infoHeader.biCompression = RGB;
    m_infoHeader.biClrUsed = 0;

    if (m_isOS21x) {
        m_infoHeader.biWidth = readUint16(4);
        m_infoHeader.biHeight = readUint16(6);
        ASSERT(m_andMaskState == None);  // ICO is a Windows format, not OS/2!
        m_infoHeader.biBitCount = readUint16(10);
        return true;
    }

    m_infoHeader.biWidth = readUint32(4);
    m_infoHeader.biHeight = readUint32(8);
    if (m_andMaskState != None)
        m_infoHeader.biHeight /= 2;
    m_infoHeader.biBitCount = readUint16(14);

    // Read compression type, if present.
    if (m_infoHeader.biSize >= 20) {
        uint32_t biCompression = readUint32(16);

        // Detect OS/2 2.x-specific compression types.
        if ((biCompression == 3) && (m_infoHeader.biBitCount == 1)) {
            m_infoHeader.biCompression = HUFFMAN1D;
            m_isOS22x = true;
        } else if ((biCompression == 4) && (m_infoHeader.biBitCount == 24)) {
            m_infoHeader.biCompression = RLE24;
            m_isOS22x = true;
        } else if (biCompression > 5)
            return m_parent->setFailed(); // Some type we don't understand.
        else
            m_infoHeader.biCompression = static_cast<CompressionType>(biCompression);
    }

    // Read colors used, if present.
    if (m_infoHeader.biSize >= 36)
        m_infoHeader.biClrUsed = readUint32(32);

    // Windows V4+ can safely read the four bitmasks from 40-56 bytes in, so do
    // that here.  If the bit depth is less than 16, these values will be
    // ignored by the image data decoders.  If the bit depth is at least 16 but
    // the compression format isn't BITFIELDS, these values will be ignored and
    // overwritten* in processBitmasks().
    // NOTE: We allow alpha here.  Microsoft doesn't really document this well,
    // but some BMPs appear to use it.
    //
    // For non-Windows V4+, m_bitMasks[] et. al will be initialized later
    // during processBitmasks().
    //
    // *Except the alpha channel.  Bizarrely, some RGB bitmaps expect decoders
    // to pay attention to the alpha mask here, so there's a special case in
    // processBitmasks() that doesn't always overwrite that value.
    if (isWindowsV4Plus()) {
        m_bitMasks[0] = readUint32(40);
        m_bitMasks[1] = readUint32(44);
        m_bitMasks[2] = readUint32(48);
        m_bitMasks[3] = readUint32(52);
    }

    // Detect top-down BMPs.
    if (m_infoHeader.biHeight < 0) {
        m_isTopDown = true;
        m_infoHeader.biHeight = -m_infoHeader.biHeight;
    }

    return true;
}

bool BMPImageReader::isInfoHeaderValid() const
{
    // Non-positive widths/heights are invalid.  (We've already flipped the
    // sign of the height for top-down bitmaps.)
    if ((m_infoHeader.biWidth <= 0) || !m_infoHeader.biHeight)
        return false;

    // Only Windows V3+ has top-down bitmaps.
    if (m_isTopDown && (m_isOS21x || m_isOS22x))
        return false;

    // Only bit depths of 1, 4, 8, or 24 are universally supported.
    if ((m_infoHeader.biBitCount != 1) && (m_infoHeader.biBitCount != 4) && (m_infoHeader.biBitCount != 8) && (m_infoHeader.biBitCount != 24)) {
        // Windows V3+ additionally supports bit depths of 0 (for embedded
        // JPEG/PNG images), 16, and 32.
        if (m_isOS21x || m_isOS22x || (m_infoHeader.biBitCount && (m_infoHeader.biBitCount != 16) && (m_infoHeader.biBitCount != 32)))
            return false;
    }

    // Each compression type is only valid with certain bit depths (except RGB,
    // which can be used with any bit depth).  Also, some formats do not
    // some compression types.
    switch (m_infoHeader.biCompression) {
    case RGB:
        if (!m_infoHeader.biBitCount)
            return false;
        break;

    case RLE8:
        // Supposedly there are undocumented formats like "BitCount = 1,
        // Compression = RLE4" (which means "4 bit, but with a 2-color table"),
        // so also allow the paletted RLE compression types to have too low a
        // bit count; we'll correct this later.
        if (!m_infoHeader.biBitCount || (m_infoHeader.biBitCount > 8))
            return false;
        break;

    case RLE4:
        // See comments in RLE8.
        if (!m_infoHeader.biBitCount || (m_infoHeader.biBitCount > 4))
            return false;
        break;

    case BITFIELDS:
        // Only valid for Windows V3+.
        if (m_isOS21x || m_isOS22x || ((m_infoHeader.biBitCount != 16) && (m_infoHeader.biBitCount != 32)))
            return false;
        break;

    case JPEG:
    case PNG:
        // Only valid for Windows V3+.
        if (m_isOS21x || m_isOS22x || m_infoHeader.biBitCount)
            return false;
        break;

    case HUFFMAN1D:
        // Only valid for OS/2 2.x.
        if (!m_isOS22x || (m_infoHeader.biBitCount != 1))
            return false;
        break;

    case RLE24:
        // Only valid for OS/2 2.x.
        if (!m_isOS22x || (m_infoHeader.biBitCount != 24))
            return false;
        break;

    default:
        // Some type we don't understand.  This should have been caught in
        // readInfoHeader().
        ASSERT_NOT_REACHED();
        return false;
    }

    // Top-down bitmaps cannot be compressed; they must be RGB or BITFIELDS.
    if (m_isTopDown && (m_infoHeader.biCompression != RGB) && (m_infoHeader.biCompression != BITFIELDS))
        return false;

    // Reject the following valid bitmap types that we don't currently bother
    // decoding.  Few other people decode these either, they're unlikely to be
    // in much use.
    // TODO(pkasting): Consider supporting these someday.
    //   * Bitmaps larger than 2^16 pixels in either dimension (Windows
    //     probably doesn't draw these well anyway, and the decoded data would
    //     take a lot of memory).
    if ((m_infoHeader.biWidth >= (1 << 16)) || (m_infoHeader.biHeight >= (1 << 16)))
        return false;
    //   * Windows V3+ JPEG-in-BMP and PNG-in-BMP bitmaps (supposedly not found
    //     in the wild, only used to send data to printers?).
    if ((m_infoHeader.biCompression == JPEG) || (m_infoHeader.biCompression == PNG))
        return false;
    //   * OS/2 2.x Huffman-encoded monochrome bitmaps (see
    //      http://www.fileformat.info/mirror/egff/ch09_05.htm , re: "G31D"
    //      algorithm).
    if (m_infoHeader.biCompression == HUFFMAN1D)
        return false;

    return true;
}

bool BMPImageReader::processBitmasks()
{
    // Create m_bitMasks[] values.
    if (m_infoHeader.biCompression != BITFIELDS) {
        // The format doesn't actually use bitmasks.  To simplify the decode
        // logic later, create bitmasks for the RGB data.  For Windows V4+,
        // this overwrites the masks we read from the header, which are
        // supposed to be ignored in non-BITFIELDS cases.
        // 16 bits:    MSB <-                     xRRRRRGG GGGBBBBB -> LSB
        // 24/32 bits: MSB <- [AAAAAAAA] RRRRRRRR GGGGGGGG BBBBBBBB -> LSB
        const int numBits = (m_infoHeader.biBitCount == 16) ? 5 : 8;
        for (int i = 0; i <= 2; ++i)
            m_bitMasks[i] = ((static_cast<uint32_t>(1) << (numBits * (3 - i))) - 1) ^ ((static_cast<uint32_t>(1) << (numBits * (2 - i))) - 1);

        // For Windows V4+ 32-bit RGB, don't overwrite the alpha mask from the
        // header (see note in readInfoHeader()).
        if (m_infoHeader.biBitCount < 32)
            m_bitMasks[3] = 0;
        else if (!isWindowsV4Plus())
            m_bitMasks[3] = static_cast<uint32_t>(0xff000000);
    } else if (!isWindowsV4Plus()) {
        // For Windows V4+ BITFIELDS mode bitmaps, this was already done when
        // we read the info header.

        // Fail if we don't have enough file space for the bitmasks.
        static const size_t SIZEOF_BITMASKS = 12;
        if (((m_headerOffset + m_infoHeader.biSize + SIZEOF_BITMASKS) < (m_headerOffset + m_infoHeader.biSize)) || (m_imgDataOffset && (m_imgDataOffset < (m_headerOffset + m_infoHeader.biSize + SIZEOF_BITMASKS))))
            return m_parent->setFailed();

        // Read bitmasks.
        if ((m_data->size() - m_decodedOffset) < SIZEOF_BITMASKS)
            return false;
        m_bitMasks[0] = readUint32(0);
        m_bitMasks[1] = readUint32(4);
        m_bitMasks[2] = readUint32(8);
        // No alpha in anything other than Windows V4+.
        m_bitMasks[3] = 0;

        m_decodedOffset += SIZEOF_BITMASKS;
    }

    // We've now decoded all the non-image data we care about.  Skip anything
    // else before the actual raster data.
    if (m_imgDataOffset)
        m_decodedOffset = m_imgDataOffset;
    m_needToProcessBitmasks = false;

    // Check masks and set shift values.
    for (int i = 0; i < 4; ++i) {
        // Trim the mask to the allowed bit depth.  Some Windows V4+ BMPs
        // specify a bogus alpha channel in bits that don't exist in the pixel
        // data (for example, bits 25-31 in a 24-bit RGB format).
        if (m_infoHeader.biBitCount < 32)
            m_bitMasks[i] &= ((static_cast<uint32_t>(1) << m_infoHeader.biBitCount) - 1);

        // For empty masks (common on the alpha channel, especially after the
        // trimming above), quickly clear the shifts and continue, to avoid an
        // infinite loop in the counting code below.
        uint32_t tempMask = m_bitMasks[i];
        if (!tempMask) {
            m_bitShiftsRight[i] = m_bitShiftsLeft[i] = 0;
            continue;
        }

        // Make sure bitmask does not overlap any other bitmasks.
        for (int j = 0; j < i; ++j) {
            if (tempMask & m_bitMasks[j])
                return m_parent->setFailed();
        }

        // Count offset into pixel data.
        for (m_bitShiftsRight[i] = 0; !(tempMask & 1); tempMask >>= 1)
            ++m_bitShiftsRight[i];

        // Count size of mask.
        for (m_bitShiftsLeft[i] = 8; tempMask & 1; tempMask >>= 1)
            --m_bitShiftsLeft[i];

        // Make sure bitmask is contiguous.
        if (tempMask)
            return m_parent->setFailed();

        // Since RGBABuffer tops out at 8 bits per channel, adjust the shift
        // amounts to use the most significant 8 bits of the channel.
        if (m_bitShiftsLeft[i] < 0) {
            m_bitShiftsRight[i] -= m_bitShiftsLeft[i];
            m_bitShiftsLeft[i] = 0;
        }
    }

    return true;
}

bool BMPImageReader::processColorTable()
{
    m_tableSizeInBytes = m_infoHeader.biClrUsed * (m_isOS21x ? 3 : 4);

    // Fail if we don't have enough file space for the color table.
    if (((m_headerOffset + m_infoHeader.biSize + m_tableSizeInBytes) < (m_headerOffset + m_infoHeader.biSize)) || (m_imgDataOffset && (m_imgDataOffset < (m_headerOffset + m_infoHeader.biSize + m_tableSizeInBytes))))
        return m_parent->setFailed();

    // Read color table.
    if ((m_decodedOffset > m_data->size()) || ((m_data->size() - m_decodedOffset) < m_tableSizeInBytes))
        return false;
    m_colorTable.resize(m_infoHeader.biClrUsed);
    for (size_t i = 0; i < m_infoHeader.biClrUsed; ++i) {
        m_colorTable[i].rgbBlue = m_data->data()[m_decodedOffset++];
        m_colorTable[i].rgbGreen = m_data->data()[m_decodedOffset++];
        m_colorTable[i].rgbRed = m_data->data()[m_decodedOffset++];
        // Skip padding byte (not present on OS/2 1.x).
        if (!m_isOS21x)
            ++m_decodedOffset;
    }

    // We've now decoded all the non-image data we care about.  Skip anything
    // else before the actual raster data.
    if (m_imgDataOffset)
        m_decodedOffset = m_imgDataOffset;
    m_needToProcessColorTable = false;

    return true;
}

bool BMPImageReader::processRLEData()
{
    if (m_decodedOffset > m_data->size())
        return false;

    // RLE decoding is poorly specified.  Two main problems:
    // (1) Are EOL markers necessary?  What happens when we have too many
    //     pixels for one row?
    //     http://www.fileformat.info/format/bmp/egff.htm says extra pixels
    //     should wrap to the next line.  Real BMPs I've encountered seem to
    //     instead expect extra pixels to be ignored until the EOL marker is
    //     seen, although this has only happened in a few cases and I suspect
    //     those BMPs may be invalid.  So we only change lines on EOL (or Delta
    //     with dy > 0), and fail in most cases when pixels extend past the end
    //     of the line.
    // (2) When Delta, EOL, or EOF are seen, what happens to the "skipped"
    //     pixels?
    //     http://www.daubnet.com/formats/BMP.html says these should be filled
    //     with color 0.  However, the "do nothing" and "don't care" comments
    //     of other references suggest leaving these alone, i.e. letting them
    //     be transparent to the background behind the image.  This seems to
    //     match how MSPAINT treats BMPs, so we do that.  Note that when we
    //     actually skip pixels for a case like this, we need to note on the
    //     framebuffer that we have alpha.

    // Impossible to decode row-at-a-time, so just do things as a stream of
    // bytes.
    while (true) {
        // Every entry takes at least two bytes; bail if there isn't enough
        // data.
        if ((m_data->size() - m_decodedOffset) < 2)
            return false;

        // For every entry except EOF, we'd better not have reached the end of
        // the image.
        const uint8_t count = m_data->data()[m_decodedOffset];
        const uint8_t code = m_data->data()[m_decodedOffset + 1];
        if ((count || (code != 1)) && pastEndOfImage(0))
            return m_parent->setFailed();

        // Decode.
        if (!count) {
            switch (code) {
            case 0:  // Magic token: EOL
                // Skip any remaining pixels in this row.
                if (m_coord.x() < m_parent->size().width())
                    m_buffer->setHasAlpha(true);
                moveBufferToNextRow();

                m_decodedOffset += 2;
                break;

            case 1:  // Magic token: EOF
                // Skip any remaining pixels in the image.
                if ((m_coord.x() < m_parent->size().width()) || (m_isTopDown ? (m_coord.y() < (m_parent->size().height() - 1)) : (m_coord.y() > 0)))
                    m_buffer->setHasAlpha(true);
                return true;

            case 2: {  // Magic token: Delta
                // The next two bytes specify dx and dy.  Bail if there isn't
                // enough data.
                if ((m_data->size() - m_decodedOffset) < 4)
                    return false;

                // Fail if this takes us past the end of the desired row or
                // past the end of the image.
                const uint8_t dx = m_data->data()[m_decodedOffset + 2];
                const uint8_t dy = m_data->data()[m_decodedOffset + 3];
                if (dx || dy)
                    m_buffer->setHasAlpha(true);
                if (((m_coord.x() + dx) > m_parent->size().width()) || pastEndOfImage(dy))
                    return m_parent->setFailed();

                // Skip intervening pixels.
                m_coord.move(dx, m_isTopDown ? dy : -dy);

                m_decodedOffset += 4;
                break;
            }

            default: { // Absolute mode
                // |code| pixels specified as in BI_RGB, zero-padded at the end
                // to a multiple of 16 bits.
                // Because processNonRLEData() expects m_decodedOffset to
                // point to the beginning of the pixel data, bump it past
                // the escape bytes and then reset if decoding failed.
                m_decodedOffset += 2;
                const ProcessingResult result = processNonRLEData(true, code);
                if (result == Failure)
                    return m_parent->setFailed();
                if (result == InsufficientData) {
                    m_decodedOffset -= 2;
                    return false;
                }
                break;
            }
            }
        } else {  // Encoded mode
            // The following color data is repeated for |count| total pixels.
            // Strangely, some BMPs seem to specify excessively large counts
            // here; ignore pixels past the end of the row.
            const int endX = std::min(m_coord.x() + count, m_parent->size().width());

            if (m_infoHeader.biCompression == RLE24) {
                // Bail if there isn't enough data.
                if ((m_data->size() - m_decodedOffset) < 4)
                    return false;

                // One BGR triple that we copy |count| times.
                fillRGBA(endX, m_data->data()[m_decodedOffset + 3], m_data->data()[m_decodedOffset + 2], code, 0xff);
                m_decodedOffset += 4;
            } else {
                // RLE8 has one color index that gets repeated; RLE4 has two
                // color indexes in the upper and lower 4 bits of the byte,
                // which are alternated.
                size_t colorIndexes[2] = {code, code};
                if (m_infoHeader.biCompression == RLE4) {
                    colorIndexes[0] = (colorIndexes[0] >> 4) & 0xf;
                    colorIndexes[1] &= 0xf;
                }
                if ((colorIndexes[0] >= m_infoHeader.biClrUsed) || (colorIndexes[1] >= m_infoHeader.biClrUsed))
                    return m_parent->setFailed();
                for (int which = 0; m_coord.x() < endX; ) {
                    setI(colorIndexes[which]);
                    which = !which;
                }

                m_decodedOffset += 2;
            }
        }
    }
}

BMPImageReader::ProcessingResult BMPImageReader::processNonRLEData(bool inRLE, int numPixels)
{
    if (m_decodedOffset > m_data->size())
        return InsufficientData;

    if (!inRLE)
        numPixels = m_parent->size().width();

    // Fail if we're being asked to decode more pixels than remain in the row.
    const int endX = m_coord.x() + numPixels;
    if (endX > m_parent->size().width())
        return Failure;

    // Determine how many bytes of data the requested number of pixels
    // requires.
    const size_t pixelsPerByte = 8 / m_infoHeader.biBitCount;
    const size_t bytesPerPixel = m_infoHeader.biBitCount / 8;
    const size_t unpaddedNumBytes = (m_infoHeader.biBitCount < 16) ? ((numPixels + pixelsPerByte - 1) / pixelsPerByte) : (numPixels * bytesPerPixel);
    // RLE runs are zero-padded at the end to a multiple of 16 bits.  Non-RLE
    // data is in rows and is zero-padded to a multiple of 32 bits.
    const size_t alignBits = inRLE ? 1 : 3;
    const size_t paddedNumBytes = (unpaddedNumBytes + alignBits) & ~alignBits;

    // Decode as many rows as we can.  (For RLE, where we only want to decode
    // one row, we've already checked that this condition is true.)
    while (!pastEndOfImage(0)) {
        // Bail if we don't have enough data for the desired number of pixels.
        if ((m_data->size() - m_decodedOffset) < paddedNumBytes)
            return InsufficientData;

        if (m_infoHeader.biBitCount < 16) {
            // Paletted data.  Pixels are stored little-endian within bytes.
            // Decode pixels one byte at a time, left to right (so, starting at
            // the most significant bits in the byte).
            const uint8_t mask = (1 << m_infoHeader.biBitCount) - 1;
            for (size_t byte = 0; byte < unpaddedNumBytes; ++byte) {
                uint8_t pixelData = m_data->data()[m_decodedOffset + byte];
                for (size_t pixel = 0; (pixel < pixelsPerByte) && (m_coord.x() < endX); ++pixel) {
                    const size_t colorIndex = (pixelData >> (8 - m_infoHeader.biBitCount)) & mask;
                    if (m_andMaskState == Decoding) {
                        // There's no way to accurately represent an AND + XOR
                        // operation as an RGBA image, so where the AND values
                        // are 1, we simply set the framebuffer pixels to fully
                        // transparent, on the assumption that most ICOs on the
                        // web will not be doing a lot of inverting.
                        if (colorIndex) {
                            setRGBA(0, 0, 0, 0);
                            m_buffer->setHasAlpha(true);
                        } else
                            m_coord.move(1, 0);
                    } else {
                        if (colorIndex >= m_infoHeader.biClrUsed)
                            return Failure;
                        setI(colorIndex);
                    }
                    pixelData <<= m_infoHeader.biBitCount;
                }
            }
        } else {
            // RGB data.  Decode pixels one at a time, left to right.
            while (m_coord.x() < endX) {
                const uint32_t pixel = readCurrentPixel(bytesPerPixel);

                // Some BMPs specify an alpha channel but don't actually use it
                // (it contains all 0s).  To avoid displaying these images as
                // fully-transparent, decode as if images are fully opaque
                // until we actually see a non-zero alpha value; at that point,
                // reset any previously-decoded pixels to fully transparent and
                // continue decoding based on the real alpha channel values.
                // As an optimization, avoid setting "hasAlpha" to true for
                // images where all alpha values are 255; opaque images are
                // faster to draw.
                int alpha = getAlpha(pixel);
                if (!m_seenNonZeroAlphaPixel && !alpha) {
                    m_seenZeroAlphaPixel = true;
                    alpha = 255;
                } else {
                    m_seenNonZeroAlphaPixel = true;
                    if (m_seenZeroAlphaPixel) {
                        m_buffer->zeroFillPixelData();
                        m_seenZeroAlphaPixel = false;
                    } else if (alpha != 255)
                        m_buffer->setHasAlpha(true);
                }

                setRGBA(getComponent(pixel, 0), getComponent(pixel, 1),
                        getComponent(pixel, 2), alpha);
            }
        }

        // Success, keep going.
        m_decodedOffset += paddedNumBytes;
        if (inRLE)
            return Success;
        moveBufferToNextRow();
    }

    // Finished decoding whole image.
    return Success;
}

void BMPImageReader::moveBufferToNextRow()
{
    m_coord.move(-m_coord.x(), m_isTopDown ? 1 : -1);
}

} // namespace WebCore