Raster images (or bitmap images) are the images that we see
on a digital screen and is the most common type of image file. The millions of
pixels need to store a lot of information therefore the file sizes for raster images
need to be large. The most popular types of file formats are JPEG (joint photographic
expert group) and GIF (graphic interchange format). JPEG is the file extension of JPG which is regularly used for
digital camera memory cards. Other file formats of raster images include BMP (bitmap
image), PNG (portable network graphics) and TIFF (tagged image file format).
Compressing data to save disk space is called ‘raster compression’,
whereas ‘data compression’ is slightly different; it is the process of
compressing blocks of data before storing it. Lossy compression is irreversible
and aims to dismiss unnecessary information to make the file size smaller,
although they do slightly lose quality. JPEG and GIF files are lossy file formats. Another
form of compression to make file sizes smaller is lossless compression, whereby
the data/cells are not changed or lost, but rewritten to be more efficient.
Lossless file formats include BMPs and PNGs.
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