Image Compression CSC 1040

CSC Computing with Images2 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 Computing with Images3 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 Computing with Images4 Do these colors look the same as...

CSC Computing with Images5... as these?

CSC Computing with Images6 Not quite

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 Computing with Images7

Opportunity: Large regions of a single color are common Approach: Record # of consecutive pixels for each color An example of lossless encoding Uncompressed RLE Row 1, 21:0,13:1;13:0;22:1 LZW, etc. use algorithms in addition to RLE Run length encoding

CSC Computing with Images9 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 Computing with Images10 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 Computing with Images11 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 Computing with Images12 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 Computing with Images13 Type of image Bits/pixel320 x 240 image 640 x 480 image 1024 x 768 image Bitmap 1320x240=76,800 pixels =9,600 bytes =9.375 KB 307,200 bits =38,400 =37.5 KB 786,432 bits =98,304 bytes =96 KB Grayscale8 =76,800 bytes =75 KB = 307,200 bytes =300 KB 786,432 bytes 768 KB RGB color 24 (3 bytes) 225 KB900 KB2,304 KB =2.25 MB 32 bit color (4 bytes) 32 (4 bytes) 300 KB1200 KB3,072 KB = 3 MB 256 color GIF 8 75 KB300 KB768 KB 16 color GIF KB 150 KB384 KB Size of images Same as grayscale Half of grayscale

CSC Computing with Images14 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= Red 5% 5x3= 15 5x2=

CSC Computing with Images15 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 Computing with Images16 “Lossy” compression types: JPEG Genuine Fractals Mr. SID OriginalCompressed 9:1 “averaging”

CSC Computing with Images17 Photoshop Save for Web details for this option Tools settings

CSC Computing with Images18 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 Computing with Images19 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 Computing with Images20 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

CSC Computing with Images21 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 Computing with Images22 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 Computing with Images23 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 Computing with Images24 Some of the slides in this presentation were adapted from: /PowerPoint presentations/Lec 7-File formats.ppt and