Where do all the unelected pixels go? 02 March 2011

Looking at all the election poster that have been smiling down at us for the last number of weeks my thought turned to images and image processing, this technology is used by anyone who sends images by e-mail or deals with a web site. Image technology has in my life time moved from where a picture was either black and white or colour, and knowledge of the inner details of photography was confined to enthusiasts and professionals in dark rooms. to a time where pictures are described in various formats; jpeg, png, gif; sizes and resolutions.

Most people now have access to or own a digital camera, although it is easy to take a picture, taking a good picture requires understanding of the technology behind digital cameras, which in turn has its own terminology.

Pixel which is short for picture element is the basic structure of a digital image. Resolution which describes the number of pixels per inch or centimetre. Take for example a hypothetical picture which as a digital image could be described as being 1800 pixels across by 1200 down with a resolution of 300 pixels per inch, this would give us an image of  6” x 4 “ in size.  The amount of disk space that this image would take up is calculated by multiplying the pixels i.e. 1800 x 1200 giving 2160000 bytes or 2.16 Mbytes.

When providing images for websites they are often required to be a certain size.   Twitter accepts images up to 700K, the aforementioned image needs to be reduced if it is to be accepted by Twitter, the same goes for any image which is posted to a web site. How is an image of 2.16 Mbytes reduced in size while maintaining its recognisability as an image? Answer: Pixel elimination

The process is commonly called optimizing for the web but effectivly it is image compression, a successful image compression will reduce the size of the image without reducing its quality.  To do so, some pixels are eliminated and some are re-elected and placed into the new smaller image.  There are a number of methods or more precisly algorithms used to calculate what is the destination of each pixel after compression.

So as the posters are taken down spare a thought for the pixels that made a brief appearance in large posters, but are unlikely to be seen again.