It is important to know which image format to use on your website. Some image file formats can be large and therefore take a long time to download to your browser. Other formats trade quality for file size by reducing the number of pixels and colour. Pixels are the tiny dots that make up the image. These types of files are known as bitmaps.
Different file formats use different methods to achieve the trade-off between quality and size but most use a method of file compression. When quality is lost, the term used for compression is 'lossy'; when no quality has been lost during compression the term used is 'lossless'.
Some image formats use a method of gradually downloading the image in chunks, for example, the first rows of pixels then the next and so on. This method is called raster non-interlaced. Raster interlaced is a more efficient method, which sends every other line of pixels and then returns to download the lines in between. This method fools your eye into believing the image has appeared more quickly and is fading into view.
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