Web Design and Development Design – Standard File Formats

Learning Intention I will learn about standard file formats for audio and image files.

Compression File compression is the process of reducing the size of a file. Why is compression needed? to reduce space taken on backing storage to shorten time to send a file between two computer systems

Types of Compression Lossless (no loss) compression does not affect the quality of the file Lossy compression:- results in a file of poorer quality than the original results in smaller files than lossless compression can result in artefacts (flaws) appearing in JPEGs

Artefacts – flaws due to compression

Standard File Formats Standard File Formats are used to store files so that the files can be used in a different application package from the one which created it.

Graphics Files – GIF Graphics Interchange Format Compressed using lossless compression GIF files may be interlaced – low quality version can be shown while rest of file downloads

Graphics Files – GIF (cont) Supports animation - multiple images in one GIF file Supports one transparent (see through) colour Colour depth of 8-bit (256 colours) Good for cartoon like images

Graphics Files – PNG Portable Network Graphics Includes all the advantages of GIFs but without the limitations No animation Uses lossless compression 24 bit colour depth Allows partial transparency

Graphics Files - JPEG Joint Photographic Expert Group Lossy compression – amount of compression is adjustable – allows higher quality but takes more storage good for natural real life images (photos) Does not support transparency or animation Supports up to 16.7 million colours (24-bit colour depth)

Bitmap Graphics GIF, PNG and JPEG are all bitmap graphic formats. Each pixel in the image is stored as a set of bits representing the colour. E.g. 256 colour = 8 bits per pixel (28=256) so each pixel colour stored as a binary digit from 00000000 to 11111111

Graphics File Formats - Summary JPEG GIF PNG Compression lossy lossless Animation no yes Transparency no one colour partial transparency Colour Depth 24-bit 16,777,216 8-bit 256 colours

Graphics Files – SVG Scalable Vector Graphics Stores objects (line, rectangle, polygon, ellipse) as a list of attributes e.g. fill colour, line colour, co-ordinates No compression Does not lose quality when scaled Supports layers

Factors Affecting Graphic File Size & Quality resolution (no. of pixels) higher resolution = better quality = bigger file size colour-depth (no. of bits per pixel) higher colour depth = better quality

Audio Sound is analogue Computer changes it to digital by sampling (measuring the sound wave and storing the measurement as a digital number) The sampling rate is the number of times per second that the sound wave is measured CD quality sound is sampled at 44.1KHz = 44,100 time per second

Factors Affecting Sound File Size Sampling rate higher sampling rate = better quality = bigger file size

Audio Files - WAV WAVeform audio file format WAV is the standard format for storing sound files on Windows systems. Uncompressed, therefore large file size Used for high quality sound

Audio Files – MP3 Created by Motion Pictures Expert Group MP3 is a file format for compressing sounds CD quality sound Uses lossless compression removes sound human ears cannot hear removes quiet sounds when a loud sound happens at the same time file size reduced to 10% of original size

Success Criteria I can compare standard audio and video file formats. I can describe the factors affecting file size and quality.