GIFCOMPR.C - GIF Image compression routines
Lempel-Ziv compression based on 'compress'. GIF modifications by
David Rowley (mgardi@watdcsu.waterloo.edu)
Adapted from Jef Poskanzer's Java port by way of J. M. G. Elliott.
K Weiner 12/00
GIF Image compression - modified 'compress'
Based on: compress.c - File compression ala IEEE Computer, June 1984.
By Authors: Spencer W. Thomas (decvax!harpo!utah-cs!utah-gr!thomas)
Jim McKie (decvax!mcvax!jim)
Steve Davies (decvax!vax135!petsd!peora!srd)
Ken Turkowski (decvax!decwrl!turtlevax!ken)
James A. Woods (decvax!ihnp4!ames!jaw)
Joe Orost (decvax!vax135!petsd!joe)
Algorithm: use open addressing double hashing (no chaining) on the
prefix code / next character combination. We do a variant of Knuth's
algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
secondary probe. Here, the modular division first probe is gives way
to a faster exclusive-or manipulation. Also do block compression with
an adaptive reset, whereby the code table is cleared when the compression
ratio decreases, but after the table fills. The variable-length output
codes are re-sized at this point, and a special CLEAR code is generated
for the decompressor. Late addition: construct the table according to
file size for noticeable speed improvement on small files. Please direct
questions about this implementation to ames!jaw.
Inputs:
code: A n_bits-bit integer. If == -1, then EOF. This assumes
that n_bits =< wordsize - 1.
Outputs:
Outputs code to the file.
Assumptions:
Chars are 8 bits long.
Algorithm:
Maintain a BITS character long buffer (so that 8 codes will
fit in it exactly). Use the VAX insv instruction to insert each
code in turn. When the buffer fills up empty it and start over.
Modified by Simon Bridewell, June-August 2009:
Downloaded from
http://www.thinkedge.com/blogengine/post/2008/02/20/Animated-GIF-Encoder-for-NET-Update.aspx
http://www.thinkedge.com/BlogEngine/file.axd?file=NGif_src2.zip
* Corrected FxCop code analysis errors.
* Documentation comments converted to .net XML comments.
* Updated to use .net 2.0 generics - use List instead of byte[]
TODO: maybe this belongs in TableBasedImageData?
Declaration Syntax| C# | Visual Basic | Visual C++ |
public class LzwEncoder
Public Class LzwEncoder
public ref class LzwEncoder
Members| All Members | Constructors | Methods | |||
| Icon | Member | Description |
|---|---|---|
 | LzwEncoder(IndexedPixels) |
Constructor.
|
| Encode(Stream) |
Encodes the data and writes it to the supplied output stream.
|
| Equals(Object) | (Inherited from Object.) |
 | Finalize()()() | Allows an Object to attempt to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.) |
| GetHashCode()()() | Serves as a hash function for a particular type. GetHashCode()()() is suitable for use in hashing algorithms and data structures like a hash table. (Inherited from Object.) |
| GetType()()() | Gets the Type of the current instance. (Inherited from Object.) |
| MemberwiseClone()()() | Creates a shallow copy of the current Object. (Inherited from Object.) |
| ToString()()() | (Inherited from Object.) |
Inheritance Hierarchy| Object | |
 | LzwEncoder |