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BOOT CAMP 461 (30/01/07)

Multimedia Formats and Codecs part 2

 

In part two of this short series on multimedia files we turn our attention to image and graphic formats and codecs. There are dozens of them but it’s not as bad as it sounds. Most are proprietary formats, tied in with image editing programs and specialised applications and only a small handful file types are of interest to PC and digital camera users.

 

By far the most popular image format is JPEG (extension .jpg or .jpeg) and this is the one you are most likely to come across if you own a digital camera or use pictures to illustrate documents, pdfs, web pages and so on. For the record JPEG stands for Joint Photographic Expert Group; it was set up by the International Standards Organisation (ISO) in 1982 and the technical standard it created (ISO 10918-1) was ratified by the ISO in 1990.

 

JPEG is a compressed or ‘lossy’ system that reduces the size of picture files by discarding repetitive data and information that the eye cannot perceive or doesn’t notice. It is based on the principle that we are much more sensitive to variations in brightness than colour, so it is possible to reduce the amount of colour information in an image file without significant loss of quality. JPEG compression uses 24-bit coding (8 bits for each primary colour), which basically means it is capable of representing 16.7 million colours (also known as Truecolor’) and is roughly the same number of colours that the human eye can distinguish. Extra data about the image can also be embedded in JPEG files, see this week’s Top Tip.

 

The amount of compression applied to a JPEG file or image can be varied but as it is increased so too do the number of defects or ‘artefacts’ and highly compressed images become increasingly ‘blocky’.  However, minor artefacts tend not to show up show up in photographs that contain a lot of non-uniform texture and fine detail. Image quality also suffers when JPEGs are edited and repeatedly re-saved and it is not suitable for line drawings and graphic images as processing artefacts can result in blurring and fuzzy edges.

 

Bitmap (extension .bmp) is an elderly ‘raster graphics’ file format, nowadays mostly used by the Windows operating system for, amongst other things, background desktop images, splash screens and icons. Some scanners also have it as the default output file type. Bitmaps are uncompressed so it is a ‘lossless’ format, capable of very high quality images with 2 (1-bit), 16 (4-bit), 256 (8.bit), 65,535 (16-bit) or 16.7 million (24-bit) colours but files tend to be very large. Windows Paint and most image editing programs can handle bitmap files but in the wider world it tends not to be very popular due to size of the files, which can be slow to copy and transfer.

 

GIF or the Graphics Interchange Format (extension .gif) was developed by the pioneering Internet Service Provider CompuServe back in 1987 and is the standard file format for web page graphics. It can only handle up 256 (8-bit) colours so it isn’t much use for photographs but it is very good at rendering drawings, diagrams and animated graphics and because data is highly compressed, file sizes can be very small. Problems over patents and licensing dogged the GIF format for many years. Several alternatives have been developed, the most popular being PNG or Portable Network Graphics (extension .png). In addition to being more flexible and capable of better quality PNG files sizes are even smaller than equivalent GIFs and it is rapidly growing in popularity.

 

TIFF or Tagged Image File Format or TIFF (extension .tff), developed in the early 1990s by Aldus (now owned by Adobe) is mostly used for storing and processing images in high-end picture editing and publishing applications like PhotoShop and Quark. It’s also used on scanners and some digital cameras. It’s a flexible and lossless raster graphics format with file sizes falling somewhere between bitmaps and JPEGs but the key benefits are that TIFFs contain embedded technical data or ‘tags’ about the image and files can be repeatedly edited and saved without any loss of quality.

 

Some mid-range and most high-end digital cameras have the option to save or export images in ‘RAW’ format. In fact this isn’t a format as such, or at least not a single format but a way of storing or copying picture files without any compression, to ensure maximum quality. However, there is no set standard and each camera manufacturer has its own way of processing the data so specialist software is needed to view the image and convert it to one of the mainstream image formats.

 

Next Week -- Multimedia Formats part 3

 

JARGON FILTER

 

 

CODEC

Compression/decompression (or coder - decoder) file system, software utility or program add-on used to process digital data

 

PDF

Portable Document Format - web-friendly, cross-platform format for encoding documents and graphics, normally viewed using free Adobe Acrobat Reader program

 

RASTER GRAPHICS

A means of representing an image using digital data, by describing the colour of each individual picture element or ‘pixel’/

 

 

TOP TIP

Most digital cameras attach a small data file, called Exif (Exchangeable Image File), to JPEG and RAW image files when they are saved or exported to a PC. These are similar to the data ‘tags’ in TIFFs. It is up to the manufacturer to decide how to use the  ‘tags’ but normally they include the make and model of the camera, the time and date the photograph was shot plus a variety of camera settings, such as aperture, focal length, film speed, shutter speed, whether or not flash was used, AE mode a thumbnail or preview image plus titles and copyright details. Cameras equipped with GPS receivers can use Exif tags to record information about where the picture was taken.

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 © R. Maybury 2006, 2401

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