Image Compression

1. Image Compression CSC 1040

2. Image Compression • Goal: reduce redundancy • Encode the same information using fewer bits • Originally developed for fax transmission • Send high quality documents in short calls • Image compression can lose information by approximating colors: • Lossy: can’t reconstruct original image, but looks the same • Lossless: image can reconstructed exactly CSC 1040 - Computing with Images

3. Human Vision • Human eye tends to “blend” nearby colors • Visual acuity varies markedly across features • Discontinuities easily seen, absolutes less crucial • High frame rates produce apparent motion • Smooth motion requires about 24 frames/sec CSC 1040 - Computing with Images

4. Do these colors look the same as ... CSC 1040 - Computing with Images

5. ... as these? CSC 1040 - Computing with Images

6. Not quite CSC 1040 - Computing with Images

7. Two main techniques • Run-length encoding • Group areas of similar pixels together and encode their color only once • Reduces size as if image had fewer pixels • Palette selection • Encode only the colors actually used in the picture (may be much fewer than total number of possible colors) • Reduces size by using fewer bits per pixel CSC 1040 - Computing with Images

8. Run length encoding • Opportunity: • Large regions of a single color are common • Approach: • Record # of consecutive pixels for each color • An example of lossless encoding 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Uncompressed 000000000000000000000111111111111100000000000001111111111111111111111 RLE Row 1, 21:0,13:1;13:0;22:1 LZW, etc. use algorithms in addition to RLE 01010101010101010101 CSC 1040 - Computing with Images

9. JPEG • Joint Photographic Experts Group • Opportunity: • Eye sees sharp lines better than subtle shading • Approach: • Retain detail only for the most important parts • Accomplished with Discrete Cosine Transform • Allows user-selectable fidelity • Results:Typical compression 20:1 CSC 1040 - Computing with Images

10. JPEG • Joint Photographic Experts Group • Preferred format for scanned photographic images for use over the internet or Web. Not meant for printing. • Not good for images with a lot of solid color, vector drawings, type, or line art or images with “Web-safe” colors. • JPEG compression is lossy! Save and archive the original before converting to JPEG. CSC 1040 - Computing with Images

11. In Photoshop, when you Save as… a JPEG file, you can choose the level of compression and, therefore, the size and quality of the file. CSC 1040 - Computing with Images

12. Palette selection • Opportunity: • No picture uses all 16 million colors • Human eye does not see small differences • Approach: • Select a palette of 256 colors • Indicate which palette entry to use for each pixel • Look up each color in the palette CSC 1040 - Computing with Images

13. Size of images Same as grayscale Half of grayscale CSC 1040 - Computing with Images

14. GIF • Graphics Interchange Format • Palette selection, then lossless compression • Opportunity: • Common colors are sent more often • Approach: • Use fewer bits to represent common colors. Example: • 1 Blue 75% 75x1= 75 75x2=150 • 01 White 20% 20x2= 40 20x2= 40 • 001 Red 5% 5x3= 15 5x2= 10 • 130 200 CSC 1040 - Computing with Images

15. GIF • Graphics Interchange Format • Industry standard graphic format for on-screen viewing through the Internet and Web. • Not meant to be used for printing. • The best format for all images except scanned photographic images (use JPEG for these). • GIF supports lossless LZW compression. CSC 1040 - Computing with Images

16. “Lossy” compression types: • JPEG • Genuine Fractals • Mr. SID 9:1 “averaging” Original Compressed CSC 1040 - Computing with Images

17. Tools details for this option Photoshop Save for Web settings CSC 1040 - Computing with Images

18. JPEG cautions: • Images with hard edges, high contrasts, angular areas, and text suffer from JPEG compression. • Scanned “natural” photographs do not lose much, especially at High or Maximum quality. • Only save finished images as JPEGs, every time you open and save again, even if you don’t edit, you lose quality. • Always keep the original non-JPEG version (the native .psd format). • So why use JPEG? • It is the best format for photographic images on the Web. • Its compression ability is very great. CSC 1040 - Computing with Images

19. GIF cautions: • Restricted number of colors – not suitable for photographs • Very good compression for line drawings (uniform color areas) • Only save finished images as GIFs • Always keep the original non-GIF version (the native .psd format). • So why use GIF? • It is a good format for images that have few colors and is widely available • Can do lossless compression • Its compression ability is great, especially for images with few colors • Can also be used for animation CSC 1040 - Computing with Images

20. printing web Adobe Photoshop Photoshop can save files in many file formats: .psd — Photoshop Document (“native” format) .pdf — Photoshop Portable Document Format .eps — Photoshop Encapsulated PostScript .tiff — Tagged Image File Format .gif — CompuserveGraphic Interchange Format .jpg — JPEG, Joint Photographic Experts Group .bmp — Windows Bitmap .png — Portable Network Graphic etc., etc. A summary of Graphics file formats and uses can be found in: http://en.wikipedia.org/wiki/Graphics_file_format_summary CSC 1040 - Computing with Images

21. EPS • Encapsulated PostScript • Preferred file format for importing into page layout programs such as QuarkXPress, PageMaker, InDesign, XPress, etc. for subsequent printing. • An object-oriented format. • Will only print to a postscript printer. • Uses lossy JPEG compression. • Only save your file as EPS if you need to import it into a page layout program. CSC 1040 - Computing with Images

22. TIFF • Tagged Image File Format • Widely used cross platform file format also designed for printing. • A bitmap image format. • TIFF supports lossless LZW compression which also makes it a good archive format for Photoshop documents. CSC 1040 - Computing with Images

23. PDF • Portable Document Format • Developed to transfer and read documents without having to print them—the “paperless office.” • Cross platform format that can be read with the free download Adobe Acrobat Reader. • Can represent both vector and bitmap graphics. • Can also contain electronic document search and navigation features as well as hypertext links. • Can be created from almost any application, but the user cannot edit or modify the file except with Adobe Acrobat (or other software). • Document formatting, fonts, colors, etc. are maintained and appear identical across platforms. • Excellent in the “prepress” process — can be sent to the printer, but can also be placed in other documents. CSC 1040 - Computing with Images

24. Some of the slides in this presentation were adapted from: http://www.entomology.umn.edu/museum/links/coursefiles/ENT 5051/PowerPoint presentations/Lec 7-File formats.ppt and http://www.umiacs.umd.edu/~jimmylin/LBSC690-2004-Fall/Week10.ppt CSC 1040 - Computing with Images