从 BMP 中提取附加数据

Question

我想从 BMP 图像文件中提取附加数据。

在我的函数中，选择 BMP 图像的数据流作为输入提供。在文件类型之后，我读取文件大小，然后我需要遍历流的其余部分。附加数据被附加到 BMP 文件中，因此在图像标题中编码的文件大小不变。

如何从文件大小字节数组中获取一个值，该值将确定在原始文件结束之前我需要读取多少字节？ （我需要迭代正确数量的字节才能在附加数据之前获取）

private String getBMPAppendedData(DataInputStream in) {

        StringBuilder message = new StringBuilder();

        try {
            // Read the fileType : always 0x4d42 = "BM"
            in.readShort();
            // Read the file size
            byte[] fileSizeBytes = new byte[4];
            in.readFully(fileSizeBytes);

            // Read bytes in the loop 
            loop () {
            in.readByte();
            }

            // Read appended byte by byte if present
            boolean areSecretDataPresent = true;

            while (areSecretDataPresent) {
                try {

                    byte b = in.readByte();
                    message.append(String.format("%02X", b));

                } catch (Exception e) {

                    areSecretDataPresent = false;
                }
            }
        } catch (Exception exception) {
            exception.printStackTrace();
        }

        return message.toString();
    }

Answer 1

维基百科writes:

Offset	Length	Description
2	4 bytes	The size of the BMP file in bytes

所有整数值都以little-endian 格式存储（即最低有效字节在前）。

因此，我可能会这样做：

in.readShort(); // skip Header
int fileLength = in.readUnsignedByte() 
               | in.readUnsignedByte() << 8 
               | in.readUnsignedByte() << 16 
               | in.readUnsignedByte() << 24;

int bytesRead = 6;
while (bytesRead < fileLength) {
    bytesRead += in.skipBytes(fileLength - bytesRead);
}

// proceed to read the extra data

<< 将整数中的位向左移动指定数量的位置，| 然后将这些部分组合成一个数字。

Answer 2

/**
 * Read an extra message, if it exists, from an InputStream containing a BMP image.
 *
 * @param is the InputStream to read from
 * @return the message found
 * @throws IOException if any exception other than EndOfFile is thrown after the BMP image
 */
private static String readExtraMessage(final InputStream is) throws IOException {
    final DataInputStream s = new DataInputStream(is);
    final byte b = 0x42; // 'B'
    final byte m = 0x4d; // 'M'
    final byte sig1 = s.readByte();
    final byte sig2 = s.readByte();
    if (!(sig1 == b && sig2 == m)) {
        throw new IllegalArgumentException("not a BMP file");
    }

    final byte[] fileSizeBuffer = new byte[4];
    s.readFully(fileSizeBuffer);
    final int fileSize = ByteBuffer.wrap(fileSizeBuffer, 0, 4).order(ByteOrder.LITTLE_ENDIAN).getInt();
    int bytesToSkip = fileSize - 6; // 2 byte for the signature, 4 bytes for the filesize.
    while (bytesToSkip > 0) {
        bytesToSkip -= s.skipBytes(bytesToSkip);
    }

    final StringBuilder buf = new StringBuilder();
    while (true) {
        try {
            final byte x = s.readByte();
            buf.append((char) x);
        } catch (final EOFException e) {
            break;
        }
    }

    return buf.toString();
}

Answer 3

/**
 * Read an extra message, if it exists, from a File containing a BMP image.
 *
 * @param f the File to read
 * @return the message found
 * @throws IOException if any exception is thrown while reading
 */
private static String readExtraMessage(final File f) throws IOException {
    final MappedByteBuffer mbb =
            FileChannel.open(f.toPath(), StandardOpenOption.READ).map(FileChannel.MapMode.READ_ONLY, 0, f.length());
    final byte B = 0x42;
    final byte M = 0x4d;
    if (!(mbb.get() == B && mbb.get() == M)) {
        throw new IllegalArgumentException("not a BMP file");
    }

    final int fileSize = mbb.order(ByteOrder.LITTLE_ENDIAN).getInt();
    if (fileSize == f.length()) {
        return "";
    }

    final ByteBuffer extra =  mbb.position(fileSize).slice();
    final byte[] messageBytes = new byte[extra.remaining()];
    extra.get(messageBytes);
    return new String(messageBytes);
}