This document summarizes common image file formats, including their extensions, color capabilities, compression types, and common uses. JPEG files use lossy compression and support up to 24-bit color for photos shared on the web. GIF files use lossless compression and are limited to 8-bit color, making them suited for simple web graphics like buttons and icons. PNG files can support up to 24-bit color and use lossless compression, intended as a replacement for GIF files. TIFF files support 24-bit color and various compression types, used commonly for professional photos.

1. Image File Formats Harrow Computer Club – Wed, 1 Dec 2010 Bob Watson MA CMath MIMA MBCS

2. Start at the End Extension Colour Compression Common Uses JPG, JPEG 24-bit Lossy Photos, web pics GIF 8-bit Lossless Web graphics – buttons, icons, etc PNG up to 24-bit Lossless Web – replacement for GIF TIF, TIFF 24-bit Lossless Professional Photos etc

3. Types of Image Files Vector Images created from geometrical primitives such as points, lines, curves and other mathematically defined shapes Bitmap Images recorded as an array of pixels – typically used for the representation of photographic images

4. Vector Images Not really relevant to this talk but we need to define them so we know what we're not talking about Stored as instructions, not pixels eg: Draw Line from point A to point B with thickness T and colour C or: Draw Circle with centre at X, radius R, line thickness T, line color C, inside colour Z Essentially "drawings" or cartoons Created by specialist tools such as Adobe Illustrator or Corel Draw

5. Vector Images Files are typically quite small as they contain just simple instructions not information about every pixel Resolution Independent – nothing in the instructions need specify absolute measurements – can all be relative to the picture size Vector images can be resized (and enlarged in particular) without any loss of quality

7. Bitmap Images Images stored as a rectangular matrix of pixels Pixel = picture element = a coloured dot Used for photographs, "paint" images, etc Can capture more subtlety than vector images The colour of every separate pixel is stored, so typical file sizes much larger Pixel dimensions are fixed - cannot easily be enlarged without loss of quality

9. x 5 x 20

10. File Sizes Bitmap files typically larger than vector files For photos, need at least 8-bits for each of the three primary colours (Red, Green, Blue) Inkjet printers typically print at 300 or 600 dots per inch (dpi) Picture Size Resolution (dpi) Pixel Size Pixels File Size 6" x 4" (postcard) 300 1800 x 1200 2M 6 MB 6" x 4" (postcard) 600 3600 x 2400 8M 24 MB 10" x 8" 300 3000 x 2400 7M 21 MB 10" x 8" 600 6000 x 4800 28M 84 MB

11. Compression Lossless Reduce file size without losing image quality Not as effective as lossy compression Prioritise image quality over small file size Lossy Take advantage of limitations of human vision Discard “invisible” information Allow variable quality levels (compression) Lowest Compression Highest Compression Larger file size Smallest file size Best image quality Worst image quality

12. Original 1.5 MB High Lossy Compression 92 KB

13. Compression Lossless RLE (Run Length Encoding) – Windows bitmap files (bmp, ico) LZW (Lempel-Ziv-Welch) – GIF & TIFF files ZIP – TIFF files Lossy JPEG (Joint Photographic Experts Group) Best suited to photos and paintings of realistic scenes with smooth variations of tone and colour

14. Colour For photos, need 8-bits per primary colour 24-bits (3 bytes) per pixel 16M different colours Can reduce file size if image does not need so many distinct colours Use fewer bits per pixel – eg 8-bits (1 byte)

15. GIF Files Pictures can contain at most 256 different colours File format defines a “palette” of 24-bit colours Each pixel stored as an 8-bit index into this palette Use 8-bits (1 byte) per pixel LZW Compression – lossless Good for images with limited set of colours such as logos, web buttons etc Also support animation Supported by all web browsers Possible copyright problems

16. Animated GIF

17. Animated GIF

18. JPEG Files Full 24-bit colour – 16 million colours Compressed with JPEG algorithm Good for pictures with subtle colour variations eg: photographs Typically produced by digital cameras Supported by all web browsers

19. TIFF Files Tagged Image File Format Owned by Apple but a published spec Originally designed as a common format for scanners but now a popular professional format for colour images, photos, etc Can hold various “tags” as well associated with the image eg: photographer, copyright, subject details, ... Supports several compression formats – mostly lossless Commonest is LZW, others include ZIP and JPEG and NONE!! Many possible variations “ Thousands of Incompatible File Formats”

20. PNG Files Portable Network Graphics Designed to replace GIF files as there was a patent issue with LZW compression Also eliminates the restriction on number of colours Does not support animation Lossless compression (DEFLATE related to ZIP) Supported by most modern web browsers

22. The End Extension Colour Compression Common Uses JPG, JPEG 24-bit Lossy Photos, web pics GIF 8-bit Lossless Web graphics – buttons, icons, etc PNG up to 24-bit Lossless Web – replacement for GIF TIF, TIFF 24-bit Lossless Professional Photos etc