1 gfi files graphics interchange format. 2 gif history compuserve developed 1987 versions 87a, v89a...
TRANSCRIPT
1
GFI Files
Graphics Interchange Format
2
GIF History CompuServe developed
1987 Versions 87a, V89a Because there was no standard
3
GIF Characteristics Compressed
Less Space, Less loading time 8-bit color (or less) Can contain more than one image Interlacing possible – image can develop
from fuzzy to clear, rather than top to bottom Free – so became widely used
4
Problem LZW Compression Algorithm
Lempel-Ziv-Welch Believed to be open for use Developers claimed patented
PNG resulted GIF is still most widely used
5
GIF Format Header – version, screen layout, palette Control Blocks – governs display of image
Fading, timing, animation Image Block – information for 1 image
Size, palette Extension Block – comments
6
GIF Compression via LZW Lossless Eliminates repeated strings of data &
replace with token to previous data
1 1 1 2 3 2 1 2 3 19 19 1 1 1 19 19
1 1 1 2 3 2 [3,3] 19 19 [1,3] 19 19
No need to zip.
7
GIF Animation A multi-image GIF File Timed display gives appearance of motion GIF87a – Multi-image but no motion GIF89a – Multi-image with animation Some browsers are not compatible with the
animation – 1st image is shown
8
JPEG Files
Joint Photographic Experts Group
9
JPEG Files JPEG: Actually a compression algorithm
File format – JFIF .jpeg, .jpg, .jif, .jfif
Better for Photos 24-bit color Powerful Compression Engine
But lossy compression
10
JPEG Compression Basis: humans are much more aware of
small changes in brightness (luminance) than small changes in color or large changes in color or brightness
11
JPEG Compression – 3 steps1. Convert RGB to system identifying
brightness (e.g. HSL, YCbCr)2. Data Reduction through Subsampling –
brightness unchanged, eliminates ½ of other 2 scales- replace pairs of neighboring pixels with a single value – the average- reduces to 2/3 of size without noticeable loss of quality
12
JPEG Compression – 3 steps3. Direct cosine transform (DCT) & Quantization
Works on 8 x 8 blocks Color & Brightness in each block are identified
and rounded, more weight is given to small changes
E.G. 3 5, 75 100 Result: many duplicate values, compresses well
by a standard lossless method But image is NOT as accurate as original
13
Compression vs. Quality Always a tradeoff – 3 main factors
More compression less quality Aggressiveness of rounding Accuracy of jpeg viewer – speed vs. quality
Compression level – 1 to 100 (not %) E.G. 75 is sometimes default – good quality,
medium compression – not consistent E.G. Lo-Med-Hi-Max
14
Progressive JPEG Image comes into focus slowly, like with
interlaced GIF Not interlaced Starts with low quality image, then is
replaced with higher quality images – appearance of coming into focus
Same storage requirements
15
Transparent Color
JPEG does not support transparent color Technically difficult due to the other
aspects of storing the image
16
Compression Ratios - Comparison
For Photos GIF: 5:1 (1/5 the storage) JPEG: 10:1 at lowest level
200:1 at highest level Typical: 30:1 slightly noticeable reduction
in quality
17
Conclusion JPEG is superior to other methods for
photorealistic images – smooth edges & shading
GIF is best for drawings – hard edges & sharp color changes
18
JPEG: Multiple Compressions Resaving without changes in image or
compression level – quality is exactly same Each 8x8 block affected is recompressed &
lose more quality – can affect image unevenly
Cropping is most damaging Work in TIFF or PNG then save as JPEG
19
Final Comments Consider your monitor
256 colors vs. 16 M You can change by changing settings
GIF JPEG conversion Degrades Can get decent results, see other sources
20
BMP Files
Bit Map
21
BMP Files - Basics Native format for Windows & OS/2
Backgrounds, graphics, wallpaper Not good for sending via Internet Only format for Windows wallpaper Only slightly compressed, or not at all, so
large – can be zipped – load & display quickly, since no decompression
22
BMP File Format File Header
BM, number of bytes, file layout Information Header – describes image
Dimension in pixels, color depth (1, 4, 8, 24-bit), compression, number colors used, number colors considered important
If all colors can’t be displayed, replaced or dithered; palette stored in order of importance e.g. Brown bear vs. American Flag
23
Dithering
Creating the illusion of new colors and shades by varying the pattern of dots.
24
BMP Compression Only 4 & 8-bit images Run-length encoding (RLE) – lossless Replaces string of identical bytes with token
telling how many
10 11 11 11 11 15 15 15 6 7 8 8 8 8 8
10 [11,4] [15,3] 6 7 [8,5] Saves minimal space but decompresses fast
25
DIB? BMP files are referred to as
Device independent bit maps Can display on any monitor Sometimes .dib extension
Same as .bmp
26
Convert Image to Wallpaper Software can help
Microsoft Plus! Paint Shop Pro Web Browsers Paint Graphics Workshop
See handout
RLE compressed bit map can be used a wallpaper – 4-bit only
Change extension to .bmp
27
Other Graphic File Formats
28
PNG – Portable Network Graphics Developed due to legal problems with GIF
compression algorithm Lossless Compression
More than GIF, less than JPEG More changes than just compression
29
PNG: Other Changes over GIF More Color Depth
Up to 48 bits per pixel Improved gray scale
Better interlacing & transparency No Multiple images (thus no animation) Becoming common, won’t always display
Netscape, Internet Explorer AOL Browser, but not email
30
TIFF – Tagged Image File Format Early Graphics format Originally only grayscale High quality graphics
Developed for scanning images Common in scanning programs
Scanner, fax, desktop publishers Flexible format
31
TIFF Characteristics Causing its popularity Lossless compression (LZW, JPEG, others) Handles huge images Stores image in strips (tiles in latest version)
Can access one portion 2 – 96 bits per pixel
32
TIFF Disadvantages Tried to be everything to everybody Versions are not compatible due to too
many options You may not be able to display others TIFF
files New versions are improved, but not fixed
yet
33
ICO – Windows Icon (.ico) In toolbars, etc. Can store in .exe or .dll files 4-bit, 32 x 32 pixel image
Limited size & color Transparent color = screen color Image in inverse color, always shows
Can create only icons with icon editor
34
ART – from AOL Designed to download quickly
Display time same, transmission faster Trades quality for size by compression Your downloads may be converted to ART AOL includes a converter
Necessary unless display only (email, use on web site, etc.)
35
PCD – Photo CD from Kodak For storing photo images on a CD-ROM About 100 images per CD Each photo is digitized at 5 - 6 resolutions
Chose one to work with Can convert to other formats Many editors will read
Paint Shop Pro, Graphics Workshop
36
Macintosh PICT Native format for Macintosh Like BMP is to Windows Can store 1-bit to 32-bit color information 8-bit grayscale