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)