gifwrite.c File Reference

Write animated GIF files. More...

#include <stdio.h>
#include <stdlib.h>

Go to the source code of this file.

Data Structures
struct	tableentry
	Entry in the compression hash table representing a string. More...
struct	bitstream
	Management for writing codes of variable bitlength to a file. More...

Macros
#define	MAXBITS   12
	Maximum number of bits allowed by GIF for encoded symbols.
#define	MAXCODE   4095
	Maximum code, equals 2^MAXBITS - 1.
#define	TABLESIZE   5003
	Size of the hash table.
#define	HASHSHIFT   4
	Shift value used for hashing.
#define	UNUSED   -1
	Hash value for an unused table entry.

Functions
static void	WriteWordLE (FILE *File, unsigned short Value)
	Write a 16-bit word in big Endian format.
static void	WriteImageData (FILE *File, tableentry *Table, unsigned char *Data, int FrameLeft, int FrameTop, int FrameWidth, int FrameHeight, int ImageWidth)
	Write compressed image data for one frame of a GIF animation.
static void	CropFrame (int *FrameLeft, int *FrameTop, int *FrameWidth, int *FrameHeight, unsigned char *Data, int ImageWidth, int ImageHeight, int TransparentColor)
	Crop the extent of a frame according to its transparency.
int	GifWrite (unsigned char **Image, int ImageWidth, int ImageHeight, int NumFrames, const unsigned char *Palette, int NumColors, int TransparentColor, const int *Delays, const char *OutputFile)
	Write an animated GIF image. More...
static void	FlushBlock (bitstream *Stream)
	Flush the block buffer to GIF file.
static void	FlushBits (bitstream *Stream, int MaxRemaining)
	Flush the bit accumulator to the block buffer.
static void	WriteBits (bitstream *Stream, unsigned short Code)
	Write a code having Stream->BitsPerCode bits.
void	FrameDifference (unsigned char **Image, int ImageWidth, int ImageHeight, int NumFrames, int TransparentColor)
	Optimize animation frames by setting unchanged pixels to transparent. More...

Detailed Description

Write animated GIF files.

Author

Pascal Getreuer getre.nosp@m.uer@.nosp@m.gmail.nosp@m..com

Copyright (c) 2011, Pascal Getreuer All rights reserved.

This program is free software: you can redistribute it and/or modify it under, at your option, the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version, or the terms of the simplified BSD license.

You should have received a copy of these licenses along with this program. If not, see http://www.gnu.org/licenses/ and http://www.opensource.org/licenses/bsd-license.html.

Definition in file gifwrite.c.

Function Documentation

void FrameDifference	(	unsigned char **	Image,
		int	ImageWidth,
		int	ImageHeight,
		int	NumFrames,
		int	TransparentColor
	)

Optimize animation frames by setting unchanged pixels to transparent.

Parameters

Image	array holding the image data for each frame
ImageWidth,ImageHeight	dimensions of the image
NumFrames	number of frames
Palette	(global) color palette used by all the frames
NumColors	number of colors in Palette
TransparentColor	index of which color is transparent
Delays	the delay for each frame in centiseconds
OutputFile	filename of the output GIF file

Definition at line 401 of file gifwrite.c.

int GifWrite	(	unsigned char **	Image,
		int	ImageWidth,
		int	ImageHeight,
		int	NumFrames,
		const unsigned char *	Palette,
		int	NumColors,
		int	TransparentColor,
		const int *	Delays,
		const char *	OutputFile
	)

Write an animated GIF image.

Parameters

Image	array holding the image data for each frame
ImageWidth,ImageHeight	dimensions of the image
NumFrames	number of frames
Palette	(global) color palette used by all the frames
NumColors	number of colors in Palette
TransparentColor	index of which color is transparent
Delays	the delay for each frame in centiseconds
OutputFile	filename of the output GIF file

Returns

1 on success, 0 on failure

This routine writes a sequence of image frames as an animated GIF file. Image should be an array of pointers where Image[k] points to image data in row-major order for the kth frame,

Image[k][x + ImageWidth*y] = pixel at (x,y) in the kth frame.

The value of Image[k][x + ImageWidth*y] is the palette index of the pixel's color. Palette should be ordered such that

Palette[3*i + 0] = red intensity of the ith color,
Palette[3*i + 1] = green intensity of the ith color,
Palette[3*i + 2] = blue intensity of the ith color,

where i = 0, 1, ... NumColors - 1. TransparentColor specifies an index to denote transparent pixels.

This routine always uses the overwrite "frame disposal" method, which means that the current frame is drawn on top of the previous ones. This approach allows animated frames to be transparent everywhere except for the pixels that differ from the preceding frame. Call the FrameDifference() function before GifWrite() to perform this optimization.

Delays is an array specifying how long each frame of the animation is shown in units of centiseconds (1/100th of a second). Specifying Delays as NULL sets a delay of 0.1 seconds per frame.

Only a limited portion of the GIF specification is implemented. Particularly,

• transparency is always used (GIF does not require this)
• local color tables are not supported
• the frame disposal method is hardcoded to overwrite
• the background color index is hardcoded to 0

See [1] for details on the GIF format.

For LZW compression, we follow the hash table algorithm used in GIFCSRC [2] and the UNIX "compress" program [3]. The hash table uses open addressing double hashing (no chaining) on the prefix code / next character combination and a variant of Knuth's algorithm D (vol. 3, sec. 6.4) with G. Knott's relatively-prime secondary probe.

References: [1] http://www.w3.org/Graphics/GIF/spec-gif89a.txt [2] http://www.programmersheaven.com/download/15257/download.aspx [3] http://www.freebsd.org/cgi/cvsweb.cgi/src/usr.bin/compress/

Definition at line 120 of file gifwrite.c.