Class sbk.image.GifImageEncoder

All Packages  Class Hierarchy  This Package  Previous  Next  Index

Class sbk.image.GifImageEncoder

java.lang.Object
   |
   +----sbk.image.GifImageEncoder

public class GifImageEncoder
extends Object

GifImageEncoder is a class which saves an image to a stream using the GIF file format. However, it doesn't use the patented standard LZW algorithm for compression. Instead it uses run length compression (RLE) with an LZW like coding.

The object is constructed with either an Image (which must be fully loaded) or an array of int.
The next thing to do is creating the palette and the indexes into it with a call to indexColors.
Given an OutputStream the image can now be compressed by calling encodeTo.

Using the command line one may convert an image file readable for java^TM (GIF, JPEG or XBM) with 256 or less colors to a RLE compressed GIF file.
The syntax for doing this:
java sbk.image.GifImageEncoder infile outfile
(assuming java is in your search path)

Disadvantages

The compression ratio is not as good as LZW, especially with the given sort of coding. Usually GifImageEncoder's files are larger, but in rare cases even smaller.
This first release supports only noninterlaced, nontransparent GIF87a.

Advantages

The generated files seem to be fully compatible with todays image processing tools.
Compression using RLE is usually faster than using LZW.
Overall compression (using eg. zip archives or modems) is often better because of the repeated code sequences.
RLE should be a free compression algorithm, at least I don't know any case where someone pays for using it.

History

17 Feb 1997: Enhanced coding => better compression ratio
9 Feb 1997: Initial Release

Version:: 9 Feb 1997
Author:: Thomas Wendt @ SBK Software + Systeme GmbH

Constructor Index

o GifImageEncoder(Image): Constructs the encoder using an image.
o GifImageEncoder(int[], int, int): Constructs the encoder using an array of pixels (top-down, left-right order).

Method Index

o encodeTo(OutputStream): Writes the image out to a stream in GIF87a format (non interlaced, no transparency).
o indexColors(): Converts the RGB-values to indexes into a palette.
o main(String[]): The method of the class which can be called from the command line.

Constructors

o GifImageEncoder

  public GifImageEncoder(Image i) throws Exception

Constructs the encoder using an image.

Parameters:: i - The image to be encoded.
Throws: Exception: Will be thrown if the pixels can't be grabbed (possibly because of low memory).

o GifImageEncoder

  public GifImageEncoder(int p[],
                         int w,
                         int h)

Constructs the encoder using an array of pixels (top-down, left-right order).

Parameters:: p - The pixels to be encoded.; w - The width of the image.; h - The height of the image.

Methods

o indexColors

  public final void indexColors() throws Exception

Converts the RGB-values to indexes into a palette.

Throws: Exception: Will be thrown if there are more than 256 colors.

o encodeTo

  public final void encodeTo(OutputStream o) throws IOException

Writes the image out to a stream in GIF87a format (non interlaced, no transparency).

Parameters:: o - The stream to output to, eg. a FileOutputStream. Is buffered internally.
Throws: IOException: Might be thrown from write operations.

o main

  public final static void main(String args[])

The method of the class which can be called from the command line. Converts an image file readable for java^TM (gif, jpeg or xbm) with 256 or less colors to rle compressed gif.

Parameters:: args - The inputfile as first element and the outputfile as the second one.

All Packages  Class Hierarchy  This Package  Previous  Next  Index