1. CSC 1040 - Computing with Images1 File formats for images on the web CSC 1040

2. CSC 1040 - Computing with Images2 What’s a picture? We have seen that programs represent pictures as grids of picture elements or pixels Stephanos with his eraser collection

3. CSC 1040 - Computing with Images3 Pixel encodings Bitmap 1 bit Grayscale 8 bits RGB Color 3 colors: red, green, blue 8 bits/color 24 bits

4. CSC 1040 - Computing with Images4 How much can we encode in 8 bits? Let’s walk it through. –If we have one bit, we can represent two patterns: 0 and 1. –If we have two bits, we can represent four patterns: 00, 01, 10, and 11. –If we have three bits, we can represent eight patterns: 000, 001, 010, 011, 100, 101, 110, 111

5. CSC 1040 - Computing with Images5 8 bits = 1 byte General rule: In n bits, we can have 2 n patterns –In 8 bits, we can have 2 8 patterns, or 256 –If we make one pattern 0, then the highest value we can represent is 2 8 -1, or 255

6. CSC 1040 - Computing with Images6 RGB In RGB, each color has three component colors: –Amount of redness –Amount of greenness –Amount of blueness In most computer-based models of RGB, a single byte (8 bits) is used for each –So a complete RGB color is 24 bits, 8 bits of each

7. CSC 1040 - Computing with Images7 Position: (12,9) x = 12 y = 9

8. CSC 1040 - Computing with Images8 Color:(108,86,142) Position: (12,9) x = 12 y = 9 red=108green=86blue=142

9. CSC 1040 - Computing with Images9 Encoding RGB Each component color (red, green, and blue) is encoded as a single byte Colors go from (0,0,0) to (255,255,255) –If all three components are the same, the color is in greyscale (50,50,50) at (2,2) –(0,0,0) (at position (1,2) in example) is black –(255,255,255) is white

10. CSC 1040 - Computing with Images10 Is that enough? We’re representing color in 24 (3 * 8) bits. –That’s 16,777,216 (2 24 ) possible colors –Our eye can discern millions of colors, so it’s probably pretty close Some graphics systems support 32 bits per pixel –May be more pixels for color, or an additional 8 bits to represent 256 levels of translucence

11. CSC 1040 - Computing with Images11 Size of images 320 x 240 image 640 x 480 image 1024 x 768 monitor 24 bit color1,843,200 bytes 7,372,800 bytes 18,874,368 bytes 32 bit color2,457,600 bytes 9,830,400 bytes 25,165,824 bytes

12. CSC 1040 - Computing with Images12 Compression: to lose or not to lose? Goal: reduce redundancy –Send the same information using fewer bits Originally developed for fax transmission –Send high quality documents in short calls Two basic strategies: –Lossless: can reconstruct exactly –Lossy: can’t reconstruct, but looks the same

13. CSC 1040 - Computing with Images13 Bitmap vs. grayscale http://www.nga.gov/exhibitions/2007/snapshot/snapshotinfo_fs.shtm

14. CSC 1040 - Computing with Images14 Human Vision Closely spaced dots appear solid –But irregularities in diagonal lines can stand out High frame rates produce apparent motion –Smooth motion requires about 24 frames/sec Visual acuity varies markedly across features –Discontinuities easily seen, absolutes less crucial

15. CSC 1040 - Computing with Images15 Do these colors look the same as...

16. CSC 1040 - Computing with Images16... as these?

17. CSC 1040 - Computing with Images17 Not quite

18. CSC 1040 - Computing with Images18 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 Opportunity: Large regions of a single color are common Approach: Record # of consecutive pixels for each color An example of lossless encoding Uncompressed 0000000000000000000001111 11111111100000000000001111 111111111111111111 RLE Row 1, 21:0,13:1;13:0;22:1 LZW, etc. use algorithms in addition to RLE 01010101010101010101 Run length encoding

19. CSC 1040 - Computing with Images19 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

20. CSC 1040 - Computing with Images20 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

21. CSC 1040 - Computing with Images21 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.

22. CSC 1040 - Computing with Images22 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.

23. CSC 1040 - Computing with Images23 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: 1Blue75%75x1= 7575x2=150 01White20%20x2= 4020x2= 40 001Red 5% 5x3= 15 5x2= 10 130 200

24. CSC 1040 - Computing with Images24 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.

25. CSC 1040 - Computing with Images25 Photoshop Save for Web details for this option Tools settings

26. CSC 1040 - Computing with Images26 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.

27. CSC 1040 - Computing with Images27 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

28. CSC 1040 - Computing with Images28 http://en.wikipedia.org/wiki/Graphics_file_format_summary 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 — Compuserve Graphic 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: printingweb

29. CSC 1040 - Computing with Images29 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.

30. CSC 1040 - Computing with Images30 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.

31. CSC 1040 - Computing with Images31 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.

32. CSC 1040 - Computing with Images32 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