Image File Formats Harrow Computer Club Wed, 1 Dec 2010 Bob Watson MA CMath MIMA MBCS Start at the End ExtensionColourCompressionCommon Uses JPG, JPEG24-bitLossyPhotos, web pics GIF8-bitLosslessWeb graphics buttons, icons, etc PNGup to 24-bitLosslessWeb replacement for GIF TIF, TIFF24-bitLosslessProfessional Photos etc 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 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 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 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 x 5 x 20 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 SizeResolution (dpi) Pixel SizePixelsFile Size 6" x 4" (postcard) x 12002M6 MB 6" x 4" (postcard) x 24008M24 MB 10" x 8" x 24007M21 MB 10" x 8" x M84 MB 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 CompressionHighest Compression Larger file sizeSmallest file size Best image qualityWorst image quality Original 1.5 MB High Lossy Compression 92 KB 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 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) 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 Animated GIF 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 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 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 The End ExtensionColourCompressionCommon Uses JPG, JPEG24-bitLossyPhotos, web pics GIF8-bitLosslessWeb graphics buttons, icons, etc PNGup to 24-bitLosslessWeb replacement for GIF TIF, TIFF24-bitLosslessProfessional Photos etc