Readme.md

GIF Parser

Compile On Windows (Use Mingw64)

# create a gif picture test.gif  
make creator  
./creator.exe  

# load file to GIF structure and output a same picture from GIF structure  
make parser  
./parser.exe [FileName]  

Manuals

Using parser.exe [filename] to load a file when you have passed compile. The parser.exe will load GIF file transform it to internal data structure and output a same gif picture, it also decomposes gif to bmp in many frames (or maybe one frame). All of gif picture in assets have passed test, and their frames in folder frames (I just upload those take me a lof of problem, although their frames generated).
The GIF structure mainly consist of Link List.

typedef struct GIF {
  // File Header - 6Bytes fixed section  
	GIF_HEADER    Header;                    
  // Logical Screen Descriptor - 7Bytes fixed section  
	GIF_LOGICAL_SCREEN_DESCRIPTOR   LogicalScreenDescriptor;   
  // Global Color Table - need to calculate size  
	GIF_COLOR_TABLE   *GlobalColorTable;         
  // Application extension      - link list header  
	GIF_APP_EXT_DATA    *AppExtHeader;             
  // Comment extension          - link list header  
	GIF_COMMENT_EXT_DATA    *CommentExtHeader;         
  // Graphics Control extension - link list header  
	GIF_GRAPHICS_EXT_DATA   *GraphicsExtHeader;        
  // ImageData extension        - link list header  
	GIF_IMAGE_DATA    *ImageDataHeader;          
  // Record extension and image data order in file   
	GIF_COMPONENT_DATA    ComponentOrder;    
  // Here are how many frames in gif  
  UINTN   FramesCount;        
  // End label - 1Bytes fixed section, the value = 0x3B  ';'  
	CHAR    trailer;  								 
} GIF;

We transform its frames to bmp by different disposal method in gif, usually repainted a range or reserved a range. It depends on graphics.GIF_GRAPHICS_EXT_DATA.flag_disposal_method.
When we load gif file, we record order of extension and image data in ComponentOrder (all of them have their mark), and when we get file from gif structure, we load data via ComponentOrder.

A Bug

When program parses No.5 gif picture (5ooqq.gif), it always couldn't free data buffer in main function (frame_buffer). Maybe there memory is out of bound, and I have no energy to solve it at present.

Reference

GIF Wiki Extremely Recommend!! GIF Animation Extremely Recommend!!
GIF Overview1 - English reference "What is GIF" Extremely Recommend!!
GIF Overview2
GIF Overview3 Recommend!!
Comprehend GIF format - This paper is enough Recommend!!
How to storage image in GIF - LZW Compression
GIF Example1 Recommend!!
GIF Example2
BMP Overview

GIF Essentials

Extension block start with 0x21(ASCLL: !) and Extension Label follows it.
0xFF is Application Extension.
0xF9 is Graphics Control Extension.
0xFE is Comment Extension.
0x01 is Plain Text Extension.
All extension block terminator is 0x00.

The Image Descriptor start with 0x2C(ASCLL: ,) Local Color Table and Image Data follow it.
Image Data terminator is also 0x00, and there are data block (block size + data byte) processed by LZW algorithm maybe repeat many times.

GIF file always terminated by a byte with a value of 0x3B(ASCLL: ;).

Color index amount (Size of Color Table) must be power of 2, assume we use M colors and size of color table is N, which must $2^{(N+1)} \geq M$, if $N > M$ we set $N - M$ color(s) (RGB) occupy excess space but do not use that(those) color(s).

GIF uses index anf color table set each pixel color, color table max amount is $2^{7+1}=256$ because there are 3 bits set size of color table, and there are 3 bits set color resolution(or called it color depth), so GIF color has R(0255) G(0255) B(0~255), but it has colors max 256 because of size of color table.

BMP Essentials

BMP have file image header and color data, the image header consist of bmp file header[14Bytes] and bmp information header[40Bytes]. You can see at bmp.h.

BMP storage color data order is BGR, not RGB, this is important. The file size calculated follow (Byte):
$Size = (PixelWidth * PixelHeight * BitPerPixel) / 8$
For gif to bmp, BitPerPixel always is 24.

test.gif

it created by pragma gif_creator, and it will be parsed in pragma gif_parser

Mainly Resource