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Digital images are made up by a grid of, potentially, millions of tiny coloured dots, called pixels. The pixels can be represented by coloured inks when printing on paper or by coloured light when displaying on a screen. Pictures with very many pixels are described as high resolution, whilst those with few pixels are described as low resolution. To store a picture digitally on the disk, a number is used to represent each and every pixel in the picture. You can see that the higher the resolution (more pixels) the larger the amount of numbers (data) required to represent the picture.
Modern digital cameras take pictures at incredibly high resolution. Because of the very high resolution, you can enlarge the pictures to very big sizes and still the pixels are so small that you see a very high quality picture. When viewing on a computer screen, only a relatively small number of dots can be displayed, perhaps only one-tenth of those produced by the camera. For example, a computer screen might display 1024 pixels horizontally by 768 pixels vertically, ie. 786432 pixels in all. That's less than one-tenth of the number produced by an 8MPix (ie. 8192000 pixel) camera. So, there's no point in storing all of those pixels (on our web-server) if so few of them actually get displayed.
When you upload a picture to the website, all of the data representing the picture gets stored on a piece of a server for which we effectively pay rental. We pay for a certain amount of disk space. More disk space costs more money. When someone views your picture, they download all of the data that represents the picture. The higher the resolution of the picture, the more data there is, the longer it takes to download and the more disk space required to store it.
Resizing allows most of those pixels to be removed, without affecting the quality of the picture displayed at the size viewed on a typical computer screen. Doing so ensures that our disk space is sufficient to store very many more pictures than would otherwise be the case and that the delay for users to view a webpage with images is kept to an acceptably short time.
How to easily resize your pictures for gallery and forum postings
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