Class JPEGImageReadParam

java.lang.Object
javax.imageio.IIOParam
javax.imageio.ImageReadParam
javax.imageio.plugins.jpeg.JPEGImageReadParam
public class JPEGImageReadParam extends ImageReadParam
This class adds the ability to set JPEG quantization and Huffman tables when using the built-in JPEG reader plug-in. An instance of this class will be returned from the getDefaultImageReadParam methods of the built-in JPEG ImageReader.

The sole purpose of these additions is to allow the specification of tables for use in decoding abbreviated streams. The built-in JPEG reader will also accept an ordinary ImageReadParam, which is sufficient for decoding non-abbreviated streams.

While tables for abbreviated streams are often obtained by first reading another abbreviated stream containing only the tables, in some applications the tables are fixed ahead of time. This class allows the tables to be specified directly from client code. If no tables are specified either in the stream or in a JPEGImageReadParam, then the stream is presumed to use the "standard" visually lossless tables. See JPEGQTable and JPEGHuffmanTable for more information on the default tables.

The default JPEGImageReadParam returned by the getDefaultReadParam method of the builtin JPEG reader contains no tables. Default tables may be obtained from the table classes JPEGQTable and JPEGHuffmanTable.

If a stream does contain tables, the tables given in a JPEGImageReadParam are ignored. Furthermore, if the first image in a stream does contain tables and subsequent ones do not, then the tables given in the first image are used for all the abbreviated images. Once tables have been read from a stream, they can be overridden only by tables subsequently read from the same stream. In order to specify new tables, the setInput method of the reader must be called to change the stream.

Note that this class does not provide a means for obtaining the tables found in a stream. These may be extracted from a stream by consulting the IIOMetadata object returned by the reader.

For more information about the operation of the built-in JPEG plug-ins, see the JPEG metadata format specification and usage notes.