Post on 15-May-2020
Multimedia SystemsPart 5
Mahdi Vasighiwww.iasbs.ac.ir/~vasighi
Department of Computer Science and Information Technology,Institute for Advanced Studies in Basic Sciences, Zanjan, Iran
Image Formats
PNGo Portable Network Graphicso Developed in early 1995 as an alternative to GIFo Three advantages over GIF
• Alpha channel (up to 16 bits) • Gamma correction• 2D Interlacing
Image Formats - PNG
Interlacingo Adam7 algorithm
• operates in both dimensions • uses seven passes and through each 8 × 8 block of
an image
Image Formats - PNG
Interlacing
Pass 1 1/64 = 1.5625%
Pass 2 1/32 = 3.125%
Pass 3 1/16 = 6.25%
Pass 4 1/8 = 12.5%
Pass 5 1/4 = 25%
Pass 61/2 = 50%
Pass 7, 1/1 = 100%
Image Formats - PNG
ADAM7 Pass 4 1/8 = 12.5%
PNG
Interlacing
GIF pixels have an 8-to-1 aspect ratio, whereas PNG pixels are 2-to-1
Row-wised Pass 1 1/8 = 12.5%
GIF
Image Formats - PNG
ADAM7 Pass 5 1/4 = 25%
PNG
Interlacing
GIF pixels have an 4-to-1 aspect ratio, whereas PNG pixels are 1-to-1
Row-wised Pass 2 1/4 = 25%
GIF
Image Formats - PNG
PNG GIFinterlacing interlacing
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 - PNG
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
Preprocessing
JPEG
1. partition the image into blocks of size 8 x 8 pixels
2. subtract 127 from each pixel intensity in each block
JPEG
Image Formats - JPEG
Preprocessing
Transformation
Discrete Cosine Transformation
DCT
JPEG
Image Formats - JPEG
Preprocessing
Transformation
Quantization
make decisions about values in thetransformed image
• elements near zero will converted tozero
• All quantized values will then berounded to nearest integers.
JPEG
Preprocessing
Transformation
Quantization
Image Formats - JPEG
quantization matrix for a quality of 50%
JPEG
Image Formats - JPEG
Preprocessing
Transformation
Quantization
• Huffman coding algorithm
The original image:1602408 = 307,200 bits
Coded image:85,143 bits
~70% less storage space than that of theoriginal 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)