Format File GIF

            Format File GIF

            Format file GIF ( Graphics Interchange Format) merupakan hasil rancangan CompuServe Incorporated. Format ini dirancang untuk memudahkan pertukaran citra bitmap antarkomputer. GIF hanya mendukung resolusi warna sampai 256 warna (8-bit) [HPJ90]

            Format file GIF memiliki dua versi yaitu GIF 87a dan GIF89a. Versi GIF89a diperkenalkan pada bulan Juli 1989 merupakan perbaikan dari versi GIF87a. Pada GIF89a ditambahkan kemampuan untuk menampilkan citra dengan latar belakang transparan ( background transparency), penyimpanan data citra secara interlaced dan kemampuan untuk menampilkan citra animasi.

            GIF menggunakan variable-length code yang merupakan modifikasi dari algoritma LZW (Lemple-Ziv Wetch) untuk mengkompresi data citra. Teknik kompresi data ini mampu menghasilkan kompresi yang baik dan merupakan teknik kompresi yang mampu mengembalikan data sama persis dengan aslinya (lossless data comperssion).

Format lengkap file citra GIF dapat dilihat pada tabel berikut ini.

Tabel 2.4  Format File GIF

Name				Size	Description
Signature				6 bytes	‘GIF87a’ or ‘GIF89a’
Global Descriptor				7 bytes	Always present
	Width			2 bytes	Width in pixels
	Height			2 bytes	Height in pixels
	Flags			1 bytes	Global descriptor flags
			Global Color Map	Bit 7	=1 if Global ColorMap exists (should be true in almost all cases) =0 if default map is used. Or if every citra has a Local Color Map
			Color Resolution Bits	Bit 6-4	+1 = significant bits per color in Global Color Map
			Reserved	Bit 3	= 0
			Pixel Bits	Bit 2-0	+1 = Color Depth, Number of Global Colors := 2colordepth
	Background Color			1 byte	Background color number (from Global Color Map or default map)
	Aspect Ratio			1 byte	Usually =0
Global Color Map				Number of Global Colors * 3	Global color table, present only when Global Descriptor Flags Global Color Map =1
			Red	1 byte	Red intensity of color (not necessarily 8 significant bits)
			Green	1 byte	Green intensity of color (not necessarily 8 significant bits)
			Blue	1 byte	Blue intensity of color (not necessarily 8 significant bits)
	Repeated Number Of Global Colors Times
Extension Blocks				Cannot be pre calculated	Optional, may be repeated any number of times. Read first byte to check its precense
	Header			1 byte	= $21
	Function Code			1 byte	There is a list of known function codes
			Length	1 byte	>0 !
			Data	Length bytes	Interpretation of this data depends on its function code
	Repeated any number of times. Read first byte to check for terminator
	Terminator			1 byte	= 0
Local Descriptor				10 bytes	Local descriptor, always present
	Header			1 bytes	=$2C
	Pos X			2 bytes	Horizontal position of citra in global citra
	Pos Y			2 bytes	Vertical position of citra in global citra
	Width			2 bytes	Width of citra
	Height			2 bytes	Height of citra
	Flags			1 bytes	Local descriptor Flags
		Local Color Map		Bit 7	=1 if Local Color Map exists =0 if Global Color Map (or default map) is used
		Interlaced Citra		Bit 6	=1 Interlaced! =0 Non Interlaced
		Sorted		Bit 5	Usually =0
		Reserved		Bit 4-3	=0
		Pixel Bits		Bit 2-0	-1= Color Depth, Number of Local Colors := 2color depth
Local Color Map				Number of Local Color * 3	Local color table, present only when local Descriptor Flags. Local Color Map =1
		Red		1 byte	Red intensity of color
		Green		1 byte	Green intensity of color
		Blue		1 byte	Blue intensity of color
	Repeated number of Local Color times
Rasted DataBlock				Cannot be pre calculated	Always present
	Initial Code Size			1 byte	Usually = Color Depth, except for balck & white, where it is 2!
		Length		1 byte	>0
		Data		Length bytes	The pixel data bit stream. See decompression and compression schemes
	Repeated any number of times, read first byte to check for terminator
	Terminator			1 byte	=0!
Extension Blocks				Cannot be pre calculated	Optional. Read first byte to check its presence
	Format is identical to the Extension Block above
Terminator				1 byte	=$3B

 Next page . . .