Multimedia Systems. Part 4. Mahdi Vasighi

Transcription

1 Multimedia Systems Part 4 Mahdi Vasighi Department of Computer Science and Information Technology, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran

2 Image Formats GIF o Stands for Graphics Interchange Format o The first graphic file type recognized by early web browsers o Originally developed by CompuServe, to distribute images across network (1987) o Available in two versions GIF87a GIF89a

3 o LZW compression, Lempel-Ziv-Welch algorithm a loss less copyright protected algorithm. o Limited color palette A GIF image can contain 2, 4, 8, 16, 32, 64, 128 or 256 colors o Transparency in GIF89a format one of the colors in the palette can be ignored while processing the image for display purpose

4 Interlacing a mechanism that makes images appear faster onscreen and gives the reader a quick preview of the full area of the picture. a four-pass sequence

5 Interlacing First pass has pixel rows 1, 9, 17, etc. (every eighth row) Second pass has rows 5, 13, 21, etc. (every remaining fourth row) Third pass has rows 3, 7, 11, 15, etc. (every remaining odd row) Fourth pass has rows 2, 4, 6, etc. (all the even-numbered rows)

6 Animation Allows timed display of images in sequence, giving the effect of animation.

7 GIF87a Supports LZW Compression Interlacing Limited to 256 colors Multiple image storage GIF89a All the features in GIF87a Delay times, image replacement parameters Background transparency Animation

8 A sample gif file (10x10) and its corresponding bytes: A 00 0A FF FF FF FF FF F C A 00 0A C 2D A 1C DC 33 A EC 95 FA A8 DE 60 8C C B

9 All GIF files must start with a header or Signature block. The header takes up the first six bytes of the file Signature GIF87a Version 47="G", 49="I", 46="F" 38="8", 39= 7", 61="a" These bytes should all correspond to ASCII character codes

10 the Screen Descriptor immediately follows the header a seven-byte set of flags

11 the Screen Descriptor Background color 0A 00 0A width height 000A 000A 10 pix 10 pix Packed byte delimiter Size of global color table Separator Color resolution +1 (# of bits of color res) Global color table flag (2^N+1)

12 the Global Color Table a list of all the colors that can be in the image each GIF has its own color palette Not every GIF has to specify a global color table FF FF FF FF FF

13 the Image descriptor A set of attributes that belong to every image in the file Each image begins with an image descriptor block.

14 the Image descriptor 2C A 00 0A image width image height Packed Field

15 the Local Color Table looks identical to the global color table always immediately follow an image descriptor, if the local color table flag is set to 1. If no local color table is specified, the global color table is used for the following image data.

16 the Raster Area or Image Data composed of a series of output codes which tell the decoder which colors to spit out to the canvas uses the LZW compression scheme

17 the Raster Area or Image Data C 2D A 1C DC 33 A EC 95 FA A8 DE 60 8C C 01 00

18 the GIF terminator or Trailer indicates when you've hit the end of the file

19 When to use GIF? Images with areas of flat color logos, line art, icons & cartoon-like illustrations Not much suited for photographic images Suited for adding simple animation Whenever transparency in image is required