java - Java 中 JPEG 压缩的自定义量化表

Question

正如标题所说，我正在尝试使用自定义量化表来压缩 JPEG 格式的图像。我的问题是生成的文件无法打开，错误是:

Quantization table 0x00 was not defined

我的代码是这样的:

        JPEGImageWriteParam params = new JPEGImageWriteParam(null);
        if (mQMatrix != null) {
            JPEGHuffmanTable[] huffmanDcTables = {JPEGHuffmanTable.StdDCLuminance, JPEGHuffmanTable.StdDCChrominance};
            JPEGHuffmanTable[] huffmanAcTables = {JPEGHuffmanTable.StdACLuminance, JPEGHuffmanTable.StdACChrominance};
            dumpMatrices(mQMatrix);
            params.setEncodeTables(mQMatrix, huffmanDcTables, huffmanAcTables);
        }

        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();

        Iterator writers = ImageIO.getImageWritersByFormatName("JPEG");
        ImageWriter imageWriter = (ImageWriter) writers.next();

        ImageOutputStream imageOutputStream = ImageIO.createImageOutputStream(outputStream);
        imageWriter.setOutput(imageOutputStream);
        imageWriter.write(null, new IIOImage(mSourceImage, null, null), params);

        mCompressedImageSize = outputStream.size();

        try (FileOutputStream fileOutputStream = new FileOutputStream(mOutFileName)) {
            fileOutputStream.write(outputStream.toByteArray());

        }
        mCompressedImage = ImageIO.read(new ByteArrayInputStream(outputStream.toByteArray()));

我的猜测是它与元数据有关，但我没能找到解决方案。

谢谢，

更新:我使用十六进制查看器确定量化表(DQT - 0xFF、0xDB 部分)未写入输出文件。我假设我必须强制以某种方式编写它。

更新 2:因此在实际调试执行之后，我发现如果在参数对象中设置表，则不会为量化表和 Huffman 表生成元数据。如果缺少元数据，则不会将表写入文件。问题是我没有办法自定义元数据的内容。

Answer 1

非常有趣的问题，不幸的是，它并不平凡......这是我发现的:

首先，使用 JPEGImageWriteParam.setEncodeTables(...) 是行不通的。来自JavaDoc :

Sets the quantization and Huffman tables to use in encoding abbreviated streams.

从JPEG Metadata Format Specification and Usage Notes进一步:

This ordering implements the design intention that tables should be included in JPEGImageWriteParams only as a means of specifying tables when no other source is available, and this can occur only when writing to an abbreviated stream without tables using known non-standard tables for compression.

即，param 选项只能用于编写“缩写流”(没有表格的自定义 JPEG，假设在回读时会提供表格)。结论:我们可以指定要使用 JPEG 编码的表格的唯一方法是将其传递到元数据中。

从上面提到的同一个文档中，元数据中的表将被忽略和替换，除非压缩模式是 MODE_COPY_FROM_METADATA，所以我们需要指定它。

参见 Image Metadata DTD有关元数据结构的文档。重要的部分是带有子节点的 dqt 和 dht 节点，以及它们的“用户对象”(不要与普通 DOM“userData”混淆)。我们需要使用我们要使用的新表来更新这些节点。

这是我想出的代码:

// Obtain qtables
mQMatrix = ...;

// Read source image
ImageInputStream imageInputStream = ImageIO.createImageInputStream(...);
ImageReader reader = ImageIO.getImageReaders(imageInputStream).next();
reader.setInput(imageInputStream);

mSourceImage = reader.read(0);
IIOMetadata metadata = null;

// We need the imageWriter to create the default JPEG metadata
ImageWriter imageWriter = ImageIO.getImageWritersByFormatName("JPEG").next();

if (mQMatrix != null) {
    dumpMatrices(mQMatrix);

    // Obtain default image metadata data, in native JPEG format
    metadata = imageWriter.getDefaultImageMetadata(ImageTypeSpecifier.createFromRenderedImage(m‌​SourceImage), null);
    IIOMetadataNode nativeMeta = (IIOMetadataNode) metadata.getAsTree("javax_imageio_jpeg_image_1.0");

    // Update dqt to values from mQMatrix
    NodeList dqtables = nativeMeta.getElementsByTagName("dqtable");
    for (int i = 0; i < dqtables.getLength(); i++) {
        IIOMetadataNode dqt = (IIOMetadataNode) dqtables.item(i);
        int dqtId = Integer.parseInt(dqt.getAttribute("qtableId"));
        dqt.setUserObject(mQMatrix[dqtId]);
    }

    // For some reason, we need dht explicitly defined, when using MODE_COPY_FROM_METADATA...
    NodeList dhtables = nativeMeta.getElementsByTagName("dhtable");

    // Just use defaults for dht
    JPEGHuffmanTable[] huffmanDcTables = {JPEGHuffmanTable.StdDCLuminance, JPEGHuffmanTable.StdDCChrominance};
    JPEGHuffmanTable[] huffmanAcTables = {JPEGHuffmanTable.StdACLuminance, JPEGHuffmanTable.StdACChrominance};

    // Update dht
    for (int i = 0; i < dhtables.getLength(); i++) {
        IIOMetadataNode dht = (IIOMetadataNode) dhtables.item(i);
        int dhtClass = Integer.parseInt(dht.getAttribute("class")); // 0: DC, 1: AC
        int dhtId = Integer.parseInt(dht.getAttribute("htableId"));

        dht.setUserObject(dhtClass == 0 ? huffmanDcTables[dhtId] : huffmanAcTables[dhtId]);
    }

    // Merge updated tree back (important!)
    metadata.mergeTree("javax_imageio_jpeg_image_1.0", nativeMeta);
}

ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
ImageOutputStream imageOutputStream = ImageIO.createImageOutputStream(outputStream);
imageWriter.setOutput(imageOutputStream);

// See http://docs.oracle.com/javase/6/docs/api/javax/imageio/metadata/doc-files/jpeg_metadata.html#tables
JPEGImageWriteParam params = new JPEGImageWriteParam(null);
params.setCompressionMode(metadata == null ? MODE_DEFAULT : MODE_COPY_FROM_METADATA); // Unless MODE_COPY_FROM_METADATA, tables will be created!

imageWriter.write(null, new IIOImage(mSourceImage, null, metadata), params);
imageOutputStream.close();

mCompressedImageSize = outputStream.size();

try (FileOutputStream fileOutputStream = new FileOutputStream(mOutFileName)) {
    fileOutputStream.write(outputStream.toByteArray());
}

mCompressedImage = ImageIO.read(new ByteArrayInputStream(outputStream.toByteArray()));