com.twelvemonkeys.io.enc

Class PackBitsDecoder

java.lang.Object

com.twelvemonkeys.io.enc.PackBitsDecoder

All Implemented Interfaces:

Decoder

public final class PackBitsDecoder
extends Object
implements Decoder

Decoder implementation for Apple PackBits run-length encoding.

From Wikipedia, the free encyclopedia
PackBits is a fast, simple compression scheme for run-length encoding of data.

Apple introduced the PackBits format with the release of MacPaint on the Macintosh computer. This compression scheme is one of the types of compression that can be used in TIFF-files.

A PackBits data stream consists of packets of one byte of header followed by data. The header is a signed byte; the data can be signed, unsigned, or packed (such as MacPaint pixels).

Header byte	Data
0 to 127	1 + n literal bytes of data
0 to -127	One byte of data, repeated 1 - n times in the decompressed output
-128	No operation

Note that interpreting 0 as positive or negative makes no difference in the output. Runs of two bytes adjacent to non-runs are typically written as literal data.

See Understanding PackBits

Version:

$Id: //depot/branches/personal/haraldk/twelvemonkeys/release-2/twelvemonkeys-core/src/main/java/com/twelvemonkeys/io/enc/PackBitsDecoder.java#1 $

Author:

Harald Kuhr

Constructor Summary

Constructors

Constructor and Description
PackBitsDecoder() Creates a PackBitsDecoder.
PackBitsDecoder(boolean pDisableNoop) Creates a PackBitsDecoder, with optional compatibility mode.

Method Summary

Methods

Modifier and Type	Method and Description
int	decode(InputStream stream, ByteBuffer buffer) Decodes bytes from the given input stream, to the given buffer.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

PackBitsDecoder

public PackBitsDecoder()

Creates a PackBitsDecoder.

PackBitsDecoder

public PackBitsDecoder(boolean pDisableNoop)

Creates a PackBitsDecoder, with optional compatibility mode.

As some implementations of PackBits-like encoders treat -128 as length of a compressed run, instead of a no-op, it's possible to disable no-ops for compatibility. Should be used with caution, even though, most known encoders never write no-ops in the compressed streams.

Parameters:

pDisableNoop - true if -128 should be treated as a compressed run, and not a no-op

Method Detail

decode

public int decode(InputStream stream,
         ByteBuffer buffer)
           throws IOException

Decodes bytes from the given input stream, to the given buffer.

Specified by:

decode in interface Decoder

Parameters:

stream - the stream to decode from

buffer - a byte array, minimum 128 (or 129 if no-op is disabled) bytes long

Returns:

The number of bytes decoded

Throws:

IOException

DecodeException - if encoded data is corrupt

EOFException - if a premature end-of-file is encountered