Generally when working with digital images, if it becomes necessary to work with coordinates or dimensions, pixels are used as units. However this is not always convenient. Sometimes it would be preferable to work with physical units such as inches, centimeters, etc.
Let's assume that you need to generate a business card, which is sized at 3.5x2 inches. How many pixels will it take? How do you specify how the image should be scaled during printing? To answer these questions, images have a parameter which is called resolution - a number of pixels per some physical unit. Normally resolution is measured in DPI (dots per inch) - the amount of pixels per one inch. Generally horizontal and vertical resolutions are equal, but sometimes they may differ.
The higher the resolution, the better the quality (i.e. the more detailed the image, the less pixelization will result) but the high-resolution image is noticeably larger. As usual screen resolution is 72 or 96 DPI, scanned documents have 300 DPI. Note, printers have limited resolution (some printers cannot print more than 600 DPI, although some professional models have capacity for up to several thousand DPI). So it does not make sense to choose a resolution which cannot be reproduced by your printing device.
For instance let's use resolution 300 DPI. This means that the 3.5x2 inch image will have a width of 3.5*300 = 1050 pixels and a height of 2*300 = 600 pixels. If we had selected 96 DPI, the width would be 3.5*96 = 336 pixels and height would be 2*96 = 192 pixels.
Not only the image's size can be measured in inches (or other device-independent units). You can also measure all its coordinates and spatial parameters. All of them will depend on the resolution.
To specify the resolution for the image (or read it from the file along with other data) use the DpiX and DpiY properties of the Bitmap class. In Graphics Mill image size and coordinates are measured in pixels, and resolution - in DPI. However if you need to convert pixels to device-independent units or vice versa, you can use a utility class UnitConverter. This class doesn't affect image data or attributes, it just converts spatial parameters to different units. For example, you can use it to calculate image size in centimeters to preview ready-to-print photos.
Inches are not the only device-independent units that can be used. Graphics Mill provides the Unit enumeration for supported units: