bmpimagereader.cpp

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            m_isOS22x = true;
        } else if ((biCompression == 4) && (m_infoHeader.biBitCount == 24)) {
            m_infoHeader.biCompression = RLE24;
            m_isOS22x = true;
        } else if (biCompression > 5) {
            // Some type we don't understand.
            m_failed = true;
            return false;
        } else
            m_infoHeader.biCompression = static_cast<CompressionType>(biCompression);
    }

    // Read colors used, if present.
    if (m_infoHeader.biSize >= 36)
        m_infoHeader.biClrUsed = readUint32(data, 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(data, 40);
        m_bitMasks[1] = readUint32(data, 44);
        m_bitMasks[2] = readUint32(data, 48);
        m_bitMasks[3] = readUint32(data, 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 == 0))
        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)
            return false;
        if ((m_infoHeader.biBitCount != 0)
            && (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 == 0)
            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 == 0 || m_infoHeader.biBitCount > 8)
            return false;
        break;

    case RLE4:
        // See comments in RLE8.
        if (m_infoHeader.biBitCount == 0 || m_infoHeader.biBitCount > 4)
            return false;
        break;

    case BITFIELDS:
        // Only valid for Windows V3+.
        if (m_isOS21x || m_isOS22x)
            return false;
        if ((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)
            return false;
        if (m_infoHeader.biBitCount != 0)
            return false;
        break;

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

    case RLE24:
        // Only valid for OS/2 2.x.
        if (!m_isOS22x)
            return false;
        if (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(SharedBuffer* data)
{
    // 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 int 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)))) {
            m_failed = true;
            return false;
        }

        // Read bitmasks.
        if ((data->size() - m_decodedOffset) < SIZEOF_BITMASKS)
            return false;
        m_bitMasks[0] = readUint32(data, 0);
        m_bitMasks[1] = readUint32(data, 4);
        m_bitMasks[2] = readUint32(data, 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]) {
                m_failed = true;
                return false;
            }
        }

        // 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) {
            m_failed = true;
            return false;
        }

        // 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(SharedBuffer* data)
{
    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)))) {
        m_failed = true;
        return false;
    }

    // Read color table.