+-----------------------------------------------------------+
|                                                           |
| Documentation for CGifDecoder class                       |
|                                                           |
|   WRITTEN BY:  Yaztromo the Wizard                        |
|                (yaztromo@digital.squares.net)             |
|                                                           |
|   This is the GIF image loader module for C++ program.    |
|   CGifDecoder read GIF87a image from MEMORY, and decode   |
|     into raw 24 bit RGB image.                            |
|                                                           |
|   YOU MUST USE THIS CLASS AT YOUR OWN RISK!               |
|                                                           |
|   NOTE:                                                   |
|     a) Can read animated gif file, but will only decode   |
|          first one frame.                                 |
|     b) Error assertion is humble. Decoding result of      |
|          corrupted gif image is undefined.                |
|                                                           |
+-----------------------------------------------------------+

Member functions

    long CGifDecoder::Open(unsigned char* src, long len)
        This function reads gif header, calculate decoded image size,
          and initialize all required resources.

        [return value]  Buffer size needed to store raw RGB image. (bytes)
        [src]           Source gif image, perhaps loaded from a file.
        [len]           gif image size (bytes).

    int  CGifDecoder::Decode( unsigned char* dest = 0x0 )
        This function decode gif image ( assigned by CGifDecoder::Open )
          into raw RGB image.
        If dest is set, decoded image is stored into the memory pointed by dest.
          If dest is null, destination buffer is allocated by CGifDecoder.

        [return value]  Is TRUE if decoding process completed successfully.
        [dest]          Destination buffer for decoded image.

    unsigned char* CGifDecoder::GetImageBuffer(void)
        This function can't be called unless CGifDecoder::Decode was already called.
        [return value]  Pointer which indicates raw RGB image.

    long CGifDecoder::GetWidth(void)
        This function can't be called unless CGifDecoder::Open was already called.
        [return value]  Image width ( pixel ).

    long CGifDecoder::GetHeight(void)
        This function can't be called unless CGifDecoder::Open was already called.
        [return value]  Image height ( pixel ).

    void CGifDecoder::Close(void)
        This function clean up all resources allocated by CGifDecoder::Open.
          If destination buffer was allocated by CGifDecoder, this function also
          releases that buffer.

Byte ordering of raw RGB image

    One pixel consists of 3 bytes, unsigned char blue and green and red.
      First byte is blue, next is green, last is red.
      These sets of 3 bytes are ordered by left to right, top to bottom.
    So, first 3 bytes indicates top-left corner pixel, and last 3 bytes are
      right-bottom pixel. Of course, total buffer size is (width x height x 3).

Sample Program for CGifDecoder:

//Read gif file and write decoded RGB image to file.
#include <stdio.h>
#include <GifDecoder.h>

int main(int argc, char* argv[]){
	FILE* file;
	long i, j;
	unsigned char* buffer;
	CGifDecoder decoder;
	if( argc != 2 ){
		printf("Usage:\n");
		printf("  %s gif_filename\n", argv[0]);
		return -1;
	}
	if( file = fopen(argv[1], "rb") ){
		//	read gif file
		fseek(file, 0, 0x2);
		i = ftell(file);
		buffer = new unsigned char[i];
		fseek(file, 0, 0x0);
		fread(buffer, i, 1, file);
		fclose(file);
		//	decode and write to file
		j = decoder.Open(buffer, i);
		decoder.Decode();
		file = fopen("tmp.dat", "wb");
		fwrite(decoder.GetImageBuffer(), j, 1, file);
		fclose(file);
		decoder.Close();
		delete[] buffer;
	}
	else{
		printf("File open error: %s\n", argv[1]);
		exit(1);
	}
	return 0;
}