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The resolution of a file is the number of squares per inch it contains, measured in dots per inch (dpi).
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Bitmap files are made up of grids of squares, but they have to represent shapes with curved edges. If you look at the two circles above, the one on the left has a high resolution, the one on the right a low resolution; in the latter circle the outer curve has broken down into a set of steps, This stepping is known as bitmapping.
This means that you cannot enlarge a bitmap image and retain image quality, since enlarging the image enlarges the grid of pixels it's made of.
The optimum resolution for images to viewed on a monitor screen is 72dpi, since that is the resolution of the screen itself. For printed images, the resolution is much higher; 300dpi for colour images and black and white photographs, 600dpi for black & white line drawings.
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| Close up of a 600dpi file, showing hard, bitmapped edges. | Close up of a lower resolution 300dpi file, with edges softened by anti-aliasing. |
Why is the resolution lower for colour images? Because a black
& white line drawing is made only of black dots printed onto
white paper. At 600 dots to the inch, the dots become too small
for the human eye to differentiate, and curved lines appear smooth.
But in a colour image, there are shades of grey available as well
as pure black. Though a 300dpi image would look a little bitmapped
when examined closely, by putting a thin rim of grey pixels either
side of the line, the eye can be fooled into seeing the image
as smoother than it actually is. This effect is called anti-aliasing,
and it is also used to make 72dpi screen graphics look smooth.
But the effect must be carefully controlled, or the image will
start to look out of focus.
The other effect of resolution is on file size. You want as high a resolution as possible to ensure a sharp image, but the higher the resolution, the more information has to be stored, so the bigger the file. This is one reason why files for print take up so much memory.
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