Multimedia Systems Part 5 Mahdi Vasighi www.iasbs.ac.ir/~vasighi Department of Computer Science and Information Technology, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran Image Formats PNG o Portable Network Graphics o Developed in early 1995 as an alternative to GIF o Three advantages over GIF • Alpha channel (up to 16 bits) • Gamma correction • 2D Interlacing Image Formats - PNG Interlacing o Adam7 algorithm • operates in both dimensions • uses seven passes and through each 8 × 8 block of an image Image Formats - PNG Interlacing Pass 1 Pass 2 Pass 3 Pass 4 1/64 = 1.5625% 1/32 = 3.125% 1/16 = 6.25% 1/8 = 12.5% Pass 5 Pass 6 Pass 7, 1/4 = 25% 1/2 = 50% 1/1 = 100% Image Formats - PNG Interlacing ADAM7 Pass 4 Row-wised Pass 1 1/8 = 12.5% 1/8 = 12.5% PNG GIF GIF pixels have an 8-to-1 aspect ratio, whereas PNG pixels are 2-to-1 Image Formats - PNG Interlacing ADAM7 Pass 5 Row-wised Pass 2 1/4 = 25% 1/4 = 25% PNG GIF GIF pixels have an 4-to-1 aspect ratio, whereas PNG pixels are 1-to-1 Image Formats - PNG PNG GIF interlacing interlacing Image Formats - PNG PNG • Supports up to 16 bits per pixel in each color channel • Supports lossless compression • Does not support multiple image format/ animation like GIF • embedded textual information • title • author's name • description Image Formats JPEG • Standard method of compressing photographic • Widely used image format • Joint Photographic Experts Group • Compression is lossy • allows the user to set a desired level of quality • For a Quality factor 75%, JPEG achieves image size around 6% of original size Image Formats - JPEG JPEG Preprocessing 1. partition the image into blocks of size 8 x 8 pixels 2. subtract 127 from each pixel intensity in each block Image Formats - JPEG JPEG DCT Preprocessing Discrete Cosine Transformation Transformation Image Formats - JPEG JPEG Preprocessing Transformation make decisions about values in the transformed image Quantization • elements near zero will converted to zero • All quantized values will then be rounded to nearest integers. Image Formats - JPEG JPEG Preprocessing Transformation Quantization quantization matrix for a quality of 50% Image Formats - JPEG JPEG • Huffman coding algorithm Preprocessing The original image: 1602408 = 307,200 bits Coded image: Transformation 85,143 bits ~70% less storage space than that of the Quantization original image! Encoding Original Compressed Image Formats - JPEG JPEG Issues and Problems • Decoupling • Loss of information Original Compressed Image Formats - JPEG JPEG What about colors? • convert the RGB space channels to YCbCr space • each color component as would be compressed as a separate image. Image Formats TIFF • Tagged Image File Format • Developed by Aldus Corporation in the 1980s • Support for attachment of additional info. (Tag) • was originally a lossless format • platform independent • universal to software programs • can store many different types of images • are divided into sections with a different data type Image Formats Other formats • EXIF (Exchange Image File) • WMF (Windows Meta File) • BMP (BitMaP) • PTM (Polynomial Texture Mapping).