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Bitmap Resolution

When bitmaps are going to be used in a website they should have the correct image resolution applied before they are added to the webpage(s) eg 72 dpi. When producing bitmaps that are going to be printed, you will be working with higher resolutions of 300 dpi+. You will however, be working with this on monitors that have a lower dpi, eg 72, 96, 130 dpi. Bitmaps are resolution-dependent, meaning that if you scale a bitmap up, the pixels will become more visible and the quality of the graphic compromised. The following graphic illustrates this.

Example of Bitmap Resolution

Interpolation has been applied to the scaled picture. This process adds pixels to the bitmap, whereby the values of the new pixels are estimated based on the original pixels, on either side of the new pixels. For example if one pixel is a light red shade and the next pixel is a dark red, then the new pixel will be somewhere in-between the two colours .

"Insert or introduce between other things. Interpolation" (Oxford Dictionary, Oxford University Press, England, 1980)

Digital capture devices such as cameras and scanners have Optical resolutions. This is the actual amount of dpi that they support. Scanners also have interpolated resolution, which is a much higher resolution, eg the optical resolution may be 1200 dpi, but interpolated resolution is 9600 dpi. Image editing packages such as Adobe Photoshop can be used to change the dpi of a bitmap and apply interpolation. If you want to test how a bitmap would look at different screen resolutions, you can change the settings of your monitor. In Windows this is done via the 'Control Panel>Appearance and Themes'. Bear in mind when you are doing this the display will also look different on different sized monitors, eg 21" compared to a 15" monitor.

Next: Anti-Aliasing