JPEG Metadata Format Specification and Usage Notes

JPEG Metadata
Abbreviated Streams
Sources of Tables
Colorspace Transformations and Conventional Markers
Thumbnail Images
Progressive Encoding
Native Metadata Format Tree Structure and Editing
Image Metadata DTD
Stream Metadata DTD

NOTE: It is important to call dispose() on the JPEG reader and writer objects when they are no longer needed, as they consume significant native resources which are not adequately recovered by garbage collection. Both reader and writer call dispose() in their finalizers, but those finalizers may not be called before the native code has exhausted native memory.

The JPEG writer does not support replacing pixels.

JPEG Metadata

JPEG metadata consists of the data contained in marker segments in a JPEG stream. The image metadata object returned from a read describes the contents of the marker segments between the SOI marker and the EOI marker for that image. The image metadata object passed into a write determines the contents of the stream between the SOI marker and the EOI marker for that image, subject to the controls in any ImageWriteParam.

Stream metadata is used only for tables-only images found (or to be placed) at the beginning of a stream containing abbreviated images. Tables-only images are not treated as images and do not consume an image index. The stream metadata object returned from a read describes the contents of the marker segments between the SOI marker and the EOI marker for the single tables-only image at the beginning of the stream, if one is present. If no tables-only image is present at the front of the stream, the getStreamMetadata method of ImageReader returns null. If stream metadata is provided to the writer, a single tables-only image containing the tables from the stream metadata object will be written at the beginning of the stream. If the stream metadata object contains no tables, default tables will be written. As the sole purpose of stream metadata is for specifying tables-only images at the front of abbreviated streams, the stream metadata argument is useful only on the ImageWriter.prepareWriteSequence method. It is ignored on all other methods.

The ImageWriter.getDefaultStreamMetadata method returns an object containing the tables from the ImageWriteParam argument, if it is a JPEGImageWriteParam and contains tables. Otherwise, the returned object will contain default tables.

The ImageWriter.getDefaultImageMetadata method returns a metadata object containing no tables if the ImageWriteParam argument contains tables. Otherwise the returned metadata object will contain default visually lossless tables. Of course, only a JPEGImageWriteParam may contain tables.

If ignoreMetadata is set to true when the input is set on the reader, stream metadata will not be available, but image metadata will.

Abbreviated Streams

Both the reader and the writer retain their tables from one operation to the next, thus permitting the use of abbreviated streams quite naturally, with a few minor restrictions:
  1. Abbreviated streams may contain only one tables-only image, which must come first in the stream. Subsequent tables-only images will cause undefined behavior.
  2. Abbreviated streams must be read fully and in order. No random access is allowed, in either direction. The same image may be read multiple times. If a call is made with an image index that is not the same as or one greater than the most recent call (or 0 if no calls have been made), then an IllegalArgumentException is thrown.
These restrictions mean that streams may contain abbreviated images interspersed with images containing tables. As required by JPEG, any tables appearing in the stream override previous tables, regardless of the source of the previous tables.

Note that once a tables-only image has been read, it's contents is available as stream metadata from the reader until either another tables-only image is read from another stream or the reader is reset. Changing the input does not reset the stream metadata. This is useful for reading the tables from one file, then changing the input to read an abbreviated stream containing a sequence of images. The tables will be used automatically, and will remain available as "stream" metadata.

Abbreviated streams are written using the sequence methods of ImageWriter. Stream metadata is used to write a tables-only image at the beginning of the stream, and the tables are set up for use, using ImageWriter.prepareWriteSequence. If no stream metadata is supplied to ImageWriter.prepareWriteSequence, then no tables-only image is written. If stream metadata containing no tables is supplied to ImageWriter.prepareWriteSequence, then a tables-only image containing default visually lossless tables is written.

Sources of Tables

Images are written with tables if tables are present in their metadata objects or without them if no tables are present in their metadata objects. If no metadata object is present then the tables are written. The tables used for compression are taken from one of the following sources, which are consulted in order:

  1. If there is an ImageWriteParam and the compression mode is set to EXPLICIT, default tables constructed using the quality setting are used. They are written only if the metadata contains tables or if there is no metadata, but they replace the tables in the metadata.
  2. If there is an ImageWriteParam and the compression mode is set to DEFAULT, default visually lossles tables are used. They are written only if the metadata contains tables or if there is no metadata, but they replace the tables in the metadata.
  3. Otherwise the compression mode on the ImageWriteParam must be MODE_COPY_FROM_METADATA, in which case the following are used:
    1. the tables in the image metadata, if present
    2. the tables in the stream metadata, if present
    3. the tables in the JPEGImageWriteParam, if present
    4. default visually lossless tables
    Tables are written only if they are taken from image metadata.
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.

When reading, tables in a JPEGImageReadParam are consulted only if tables have not been set by any previous read. Tables set from a JPEGImageReadParam are overridden by any tables present in the stream being read.

Note that if no image metadata object is specified for a particular image, a default object is used, which includes default tables.

Colorspace Transformations and Conventional Markers

Colorspace transformations are controlled by the destination type for both reading and writing of images. When Rasters are read, no colorspace transformation is performed, and any destination type is ignored. A warning is sent to any listeners if a destination type is specified in this case. When Rasters are written, any destination type is used to interpret the bands. This might result in a JFIF or Adobe header being written, or different component ids being written to the frame and scan headers. If values present in a metadata object do not match the destination type, the destination type is used and a warning is sent to any listeners.

Optional ColorSpace support: Handling of PhotoYCC (YCC), PhotoYCCA (YCCA), RGBA and YCbCrA color spaces by the standard plugin, as described below, is dependent on capabilities of the libraries used to interpret the JPEG data. Thus all consequential behaviors are optional. If the support is not available when decoding, the color space will be treated as unrecognized and the appropriate default color space for the specified number of component channels may be used. When writing, an Exception may be thrown if no suitable conversion can be applied before encoding. But where the support for these color spaces is available, the behavior must be as documented.

When reading, the contents of the stream are interpreted by the usual JPEG conventions, as follows:

Once an encoded colorspace is determined, then the target colorspace is determined as follows:

For writing, the color transformation to apply is determined as follows:

If a subset of the source bands is to be written, no color conversion is performed. Any destination, if set, must match the number of bands that will be written, and serves as an interpretation of the selected bands, rather than a conversion request. This behavior is identical to that for Rasters. If all the bands are to be written and an image (as opposed to a Raster) is being written, any destination type is ignored and a warning is sent to any listeners.

If a destination type is used and any aspect of the metadata object, if there is one, is not compatible with that type, the destination type is used, the metadata written is modified from that provided, and a warning is sent to listeners. This includes the app0JFIF and app14Adobe nodes. The component ids in the sof and sos nodes are not modified, however, as unless a app0JFIF node is present, any values may be used.

When a full image is written, a destination colorspace will be chosen based on the image contents and the metadata settings, according to the following algorithm:

If no metadata object is specified, then the following defaults apply:

Default metadata objects for these image types will reflect these settings.

If a metadata object is specified, then the number of channels in the frame and scan headers must always match the number of bands to be written, or an exception is thrown. app0JFIF and app14Adobe nodes may appear in the same metadata object only if the app14Adobe node indicates YCbCr, and the component ids are JFIF compatible (0-2). The various image types are processed in the following ways:
(All multi-channel images are subsampled according to the sampling factors in the frame header node of the metadata object, regardless of color space.)

Thumbnail Images

Thumbnails are supported by the use of JFIF and JFIF extension marker segments. Thumbnails provided on the write methods determine the thumbnails that will be included. app0JFIF and app0JFXX nodes present in the metadata do not contain any thumbnail pixel data. However, the kinds of thumbnails written depend on the contents of the metadata object, as follows. Any thumbnail which is to be written as an indexed or RGB image and which is larger than 255 by 255 will be clipped, not scaled, to 255 by 255. Thumbnails written as JPEG images may be any size. A warning is sent to any listeners whenever a thumbnail is clipped.

Note that as the only mechanism for storing thumbnails is via the JFIF or JFIF extension marker segments, only grayscale or RGB images may have thumbnails. If thumbnails are present when writing any other type of image, the thumbnails are ignored and a warning is sent to any warning listeners.

Progressive Encoding

Progressive encoding must be enabled on the ImageWriteParam passed in to a write operation, or the image will be written sequentially, regardless of the scan headers included in the metadata object. If progressive encoding is enabled and set to copy from metadata, then the sequence of scan headers from the metadata is used to write the image. If progressive encoding is enabled and set to use a default, then the scans in the metadata are ignored and a default set of scans is used. Progressive encoding always forces optimized Huffman tables to be used. Any Huffman tables present in the metadata will be ignored, and a warning will be sent to any warning listeners. If Huffman-table optimization is requested on the ImageWriteParam, all Huffman tables in the metadata or in the ImageWriteParam itself are ignored, and a warning will be sent to any warning listeners if any such tables are present.

Native Metadata Format Tree Structure and Editing

The DTDs below describe just the trees of metadata objects actually returned by the IIOMetadata object. They do not include nodes corresponding to SOI, EOI, or RST markers, as these parsing delimiters do not carry any meaningful metadata.

The first node is always a JPEGvariety node. In the javax_imageio_jpeg_image_1.0 version of the JPEG metadata format, this node may have one child, an app0JFIF node, indicating that the JPEG stream contains a JFIF marker segment and related data, or no children, indicating that the stream contains no JFIF marker. In future versions of the JPEG metadata format, other varieties of JPEG metadata may be supported (e.g. Exif) by defining other types of nodes which may appear as a child of the JPEGvariety node.

(Note that an application wishing to interpret Exif metadata given a metadata tree structure in the javax_imageio_jpeg_image_1.0 format must check for an unknown marker segment with a tag indicating an APP1 marker and containing data identifying it as an Exif marker segment. Then it may use application-specific code to interpret the data in the marker segment. If such an application were to encounter a metadata tree formatted according to a future version of the JPEG metadata format, the Exif marker segment might not be unknown in that format - it might be structured as a child node of the JPEGvariety node. Thus, it is important for an application to specify which version to use by passing the string identifying the version to the method/constructor used to obtain an IIOMetadata object.)

On reading, JFXX and app2ICC nodes occur as children of an app0JFIF node. This is true regardless of where the JFXX APP0 and APP2 marker segments actually occur in the stream. The ordering of nodes within the markerSequence node corresponds to the ordering of marker segments found in the JPEG stream.

On writing, any JFXX and app2ICC nodes must occur as children of an app0JFIF node, itself a child of a JPEGvariety node, which must always be the first node. (If the stream is not to be JFIF compliant, no app0JFIF node should be provided, and the JPEGvariety node should have no children.) Any JFIF APP0, JFXX APP0, and APP2 marker segments are written first, followed by all Adobe APP14, APPn, COM and unknown segments in the order in which their corresponding nodes appear in the markerSequence node, followed by DQT (and DHT for non-progressive writes) marker segments, followed by the SOF and SOS marker segments. For progressive writes using metadata to control progression, the SOS segments are used in the order in which their corresponding nodes occur in the markerSequence node.

The reset, mergeTree and setFromTree operations have the following semantics for the JPEG plug-in metadata object:

reset - A call to reset will restore the metadata object to the same state it had immediately after creation, whether this came about from reading a stream or by obtaining a default object from the ImageWriter. This is true regardless of how many times the metadata object has been modified since creation.

mergeTree - Native Format
The mergeTree operation accepts valid trees conforming to the DTD below, and merges the nodes using the following ordering rules. In all cases, only data present in the new node is changed in a corresponding existing node, if any. This means that nodes cannot be removed using mergeTree. To remove nodes, use setFromTree. The tree must consist of IIOMetadataNodes.

mergeTree - Standard Format
The mergeTree operation, when given a tree in the standard format, will modify the native tree in the following ways:

setFromTree - Native Format
The setFromTree operation, when given a tree in the native format described below, will simply replace the existing tree in its entirety with the new one. The tree must consist of IIOMetadataNodes.

setFromTree - Standard Format
The setFromTree operation, when given a tree in the standard format, performs a reset followed by a merge of the new tree.

Image Metadata DTD 

<!DOCTYPE "javax_imageio_jpeg_image_1.0" [

  <!ELEMENT "javax_imageio_jpeg_image_1.0" (JPEGvariety, markerSequence)>

    <!ELEMENT "JPEGvariety" (app0JFIF)>
      <!-- A node grouping all marker segments specific to the variety of
              stream being read/written (e.g. JFIF) - may be empty -->

      <!ELEMENT "app0JFIF" (JFXX?, app2ICC?)>
        <!ATTLIST "app0JFIF" "majorVersion" #CDATA "1">
          <!-- The major JFIF version number -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 255 (inclusive) -->
        <!ATTLIST "app0JFIF" "minorVersion" #CDATA "2">
          <!-- The minor JFIF version number -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 255 (inclusive) -->
        <!ATTLIST "app0JFIF" "resUnits" ("0" | "1" | "2") "0">
          <!-- The resolution units for Xdensisty and Ydensity (0 = no
               units, just aspect ratio; 1 = dots/inch; 2 = dots/cm) -->
        <!ATTLIST "app0JFIF" "Xdensity" #CDATA "1">
          <!-- The horizontal density or aspect ratio numerator -->
          <!-- Data type: Integer -->
          <!-- Min value: 1 (inclusive) -->
          <!-- Max value: 65535 (inclusive) -->
        <!ATTLIST "app0JFIF" "Ydensity" #CDATA "1">
          <!-- The vertical density or aspect ratio denominator -->
          <!-- Data type: Integer -->
          <!-- Min value: 1 (inclusive) -->
          <!-- Max value: 65535 (inclusive) -->
        <!ATTLIST "app0JFIF" "thumbWidth" #CDATA "0">
          <!-- The width of the thumbnail, or 0 if there isn't one -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 255 (inclusive) -->
        <!ATTLIST "app0JFIF" "thumbHeight" #CDATA "0">
          <!-- The height of the thumbnail, or 0 if there isn't one -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 255 (inclusive) -->

        <!ELEMENT "JFXX" (app0JFXX)*>
          <!-- Min children: 1 -->

        <!ELEMENT "app0JFXX" (JFIFthumbJPEG | JFIFthumbPalette |
          JFIFthumbRGB)>
          <!-- A JFIF extension marker segment -->
          <!ATTLIST "app0JFXX" "extensionCode" ("16" | "17" | "19")
             #IMPLIED>
            <!-- The JFXX extension code identifying thumbnail type: (16 =
                 JPEG, 17 = indexed, 19 = RGB -->

          <!ELEMENT "JFIFthumbJPEG" (markerSequence?)>
            <!-- A JFIF thumbnail in JPEG format (no JFIF segments
                 permitted) -->

          <!ELEMENT "JFIFthumbPalette" EMPTY>
            <!-- A JFIF thumbnail as an RGB indexed image -->
            <!ATTLIST "JFIFthumbPalette" "thumbWidth" #CDATA #IMPLIED>
              <!-- The width of the thumbnail -->
              <!-- Data type: Integer -->
              <!-- Min value: 0 (inclusive) -->
              <!-- Max value: 255 (inclusive) -->
            <!ATTLIST "JFIFthumbPalette" "thumbHeight" #CDATA #IMPLIED>
              <!-- The height of the thumbnail -->
              <!-- Data type: Integer -->
              <!-- Min value: 0 (inclusive) -->
              <!-- Max value: 255 (inclusive) -->

          <!ELEMENT "JFIFthumbRGB" EMPTY>
            <!-- A JFIF thumbnail as an RGB image -->
            <!ATTLIST "JFIFthumbRGB" "thumbWidth" #CDATA #IMPLIED>
              <!-- The width of the thumbnail -->
              <!-- Data type: Integer -->
              <!-- Min value: 0 (inclusive) -->
              <!-- Max value: 255 (inclusive) -->
            <!ATTLIST "JFIFthumbRGB" "thumbHeight" #CDATA #IMPLIED>
              <!-- The height of the thumbnail -->
              <!-- Data type: Integer -->
              <!-- Min value: 0 (inclusive) -->
              <!-- Max value: 255 (inclusive) -->

        <!ELEMENT "app2ICC" EMPTY>
          <!-- An ICC profile APP2 marker segment -->
          <!-- Optional User object: java.awt.color.ICC_Profile -->

    <!ELEMENT "markerSequence" (dqt | dht | dri | com | unknown |
      app14Adobe | sof | sos)*>
      <!-- A node grouping all non-jfif marker segments -->

      <!ELEMENT "dqt" (dqtable)*>
        <!-- A Define Quantization Table(s) marker segment -->
        <!-- Min children: 1 -->
        <!-- Max children: 4 -->

        <!ELEMENT "dqtable" EMPTY>
          <!-- A single quantization table -->
          <!-- User object: javax.imageio.plugins.jpeg.JPEGQTable -->
          <!ATTLIST "dqtable" "elementPrecision" #CDATA "0">
            <!-- The number of bits in each table element (0 = 8, 1 = 16)
                 -->
            <!-- Data type: Integer -->
          <!ATTLIST "dqtable" "qtableId" ("0" | "1" | "2" | "3") #REQUIRED>

      <!ELEMENT "dht" (dhtable)*>
        <!-- A Define Huffman Table(s) marker segment -->
        <!-- Min children: 1 -->
        <!-- Max children: 4 -->

        <!ELEMENT "dhtable" EMPTY>
          <!-- A single Huffman table -->
          <!-- User object: javax.imageio.plugins.jpeg.JPEGHuffmanTable -->
          <!ATTLIST "dhtable" "class" ("0" | "1") #REQUIRED>
            <!-- Indicates whether this is a DC (0) or an AC (1) table -->
          <!ATTLIST "dhtable" "htableId" ("0" | "1" | "2" | "3") #REQUIRED>
            <!-- The table id -->

      <!ELEMENT "dri" EMPTY>
        <!-- A Define Restart Interval marker segment -->
        <!ATTLIST "dri" "interval" #CDATA #REQUIRED>
          <!-- The restart interval in MCUs -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 65535 (inclusive) -->

      <!ELEMENT "com" EMPTY>
        <!-- A Comment marker segment. The user object contains the actual
             bytes. -->
        <!-- User object: array of [B -->
        <!-- Min length: 1 -->
        <!-- Max length: 65533 -->
        <!ATTLIST "com" "comment" #CDATA #IMPLIED>
          <!-- The comment as a string (used only if user object is null)
               -->
          <!-- Data type: String -->

      <!ELEMENT "unknown" EMPTY>
        <!-- An unrecognized marker segment. The user object contains the
             data not including length. -->
        <!-- User object: array of [B -->
        <!-- Min length: 1 -->
        <!-- Max length: 65533 -->
        <!ATTLIST "unknown" "MarkerTag" #CDATA #REQUIRED>
          <!-- The tag identifying this marker segment -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 255 (inclusive) -->

      <!ELEMENT "app14Adobe" EMPTY>
        <!-- An Adobe APP14 marker segment -->
        <!ATTLIST "app14Adobe" "version" #CDATA "100">
          <!-- The version of Adobe APP14 marker segment -->
          <!-- Data type: Integer -->
          <!-- Min value: 100 (inclusive) -->
          <!-- Max value: 255 (inclusive) -->
        <!ATTLIST "app14Adobe" "flags0" #CDATA "0">
          <!-- The flags0 variable of an APP14 marker segment -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 65535 (inclusive) -->
        <!ATTLIST "app14Adobe" "flags1" #CDATA "0">
          <!-- The flags1 variable of an APP14 marker segment -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 65535 (inclusive) -->
        <!ATTLIST "app14Adobe" "transform" ("0" | "1" | "2") #REQUIRED>
          <!-- The color transform applied to the image (0 = Unknown, 1 =
               YCbCr, 2 = YCCK) -->

      <!ELEMENT "sof" (componentSpec)*>
        <!-- A Start Of Frame marker segment -->
        <!-- Min children: 1 -->
        <!-- Max children: 4 -->
        <!ATTLIST "sof" "process" ("0" | "1" | "2") #IMPLIED>
          <!-- The JPEG process (0 = Baseline sequential, 1 = Extended
               sequential, 2 = Progressive) -->
        <!ATTLIST "sof" "samplePrecision" #CDATA "8">
          <!-- The number of bits per sample -->
          <!-- Data type: Integer -->
        <!ATTLIST "sof" "numLines" #CDATA #IMPLIED>
          <!-- The number of lines in the image -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 65535 (inclusive) -->
        <!ATTLIST "sof" "samplesPerLine" #CDATA #IMPLIED>
          <!-- The number of samples per line -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 65535 (inclusive) -->
        <!ATTLIST "sof" "numFrameComponents" ("1" | "2" | "3" | "4")
           #IMPLIED>
          <!-- The number of components in the image -->

        <!ELEMENT "componentSpec" EMPTY>
          <!-- A component specification for a frame -->
          <!ATTLIST "componentSpec" "componentId" #CDATA #REQUIRED>
            <!-- The id for this component -->
            <!-- Data type: Integer -->
            <!-- Min value: 0 (inclusive) -->
            <!-- Max value: 255 (inclusive) -->
          <!ATTLIST "componentSpec" "HsamplingFactor" #CDATA #REQUIRED>
            <!-- The horizontal sampling factor for this component -->
            <!-- Data type: Integer -->
            <!-- Min value: 1 (inclusive) -->
            <!-- Max value: 255 (inclusive) -->
          <!ATTLIST "componentSpec" "VsamplingFactor" #CDATA #REQUIRED>
            <!-- The vertical sampling factor for this component -->
            <!-- Data type: Integer -->
            <!-- Min value: 1 (inclusive) -->
            <!-- Max value: 255 (inclusive) -->
          <!ATTLIST "componentSpec" "QtableSelector" ("0" | "1" | "2" |
            "3") #REQUIRED>
            <!-- The quantization table to use for this component -->

      <!ELEMENT "sos" (scanComponentSpec)*>
        <!-- A Start Of Scan marker segment -->
        <!-- Min children: 1 -->
        <!-- Max children: 4 -->
        <!ATTLIST "sos" "numScanComponents" ("1" | "2" | "3" | "4")
           #REQUIRED>
          <!-- The number of components in the scan -->
        <!ATTLIST "sos" "startSpectralSelection" #CDATA "0">
          <!-- The first spectral band included in this scan -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 63 (inclusive) -->
        <!ATTLIST "sos" "endSpectralSelection" #CDATA "63">
          <!-- The last spectral band included in this scan -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 63 (inclusive) -->
        <!ATTLIST "sos" "approxHigh" #CDATA "0">
          <!-- The highest bit position included in this scan -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 15 (inclusive) -->
        <!ATTLIST "sos" "approxLow" #CDATA "0">
          <!-- The lowest bit position included in this scan -->
          <!-- Data type: Integer -->
          <!-- Min value: 0 (inclusive) -->
          <!-- Max value: 15 (inclusive) -->

        <!ELEMENT "scanComponentSpec" EMPTY>
          <!-- A component specification for a scan -->
          <!ATTLIST "scanComponentSpec" "componentSelector" #CDATA
             #REQUIRED>
            <!-- The id of this component -->
            <!-- Data type: Integer -->
            <!-- Min value: 0 (inclusive) -->
            <!-- Max value: 255 (inclusive) -->
          <!ATTLIST "scanComponentSpec" "dcHuffTable" ("0" | "1" | "2" |
            "3") #REQUIRED>
            <!-- The huffman table to use for encoding DC coefficients -->
          <!ATTLIST "scanComponentSpec" "acHuffTable" ("0" | "1" | "2" |
            "3") #REQUIRED>
            <!-- The huffman table to use for encoding AC coefficients -->
]>

Stream Metadata DTD 

<!DOCTYPE "javax_imageio_jpeg_stream_1.0" [
  <!ELEMENT "javax_imageio_jpeg_stream_1.0" (dqt |
                      dht |
                      dri |
                      com |
                      unknown)*>

  <!-- All elements are as defined above for image metadata -->
]>