Basics of Digital Images

A digital image is any image that exists in the form of binary code (ones and zeros). Digital images can be created from analog images (by scanning photographs, drawings, prints, etc.) or can be created directly on a computer or other device (using drawing programs like Paint or Photoshop, or digital still or video cameras). Digital images can only be viewed on computer or television monitors; once an image is printed on paper, it isn't digital anymore.

Digital images are stored using the same kind of code that other digital information uses, a series of ones and zeros. Depending on the color depth of an image (how many different colors it has), it can take from one to thirty-two different ones or zeros (1 to 32 bits) to store the information for one pixel. For example, a black-and-white image only requires 1 bit of memory per pixel in the image; each pixel is colored either black (0) or white (1). A greyscale image (with varying shades of grey as well as black and white, also known as a monochrome image) requires 8 bits of memory per pixel, because each pixel can be one of 256 different shades between black and white (all of the binary-code numbers between 00000000 and 11111111).

You (or anyone else with access to the image) can change an image from color to greyscale, or color to black-and-white, but you can't go from black-and-white to greyscale or color, because the computer only keeps as much information as it needs (1 bit worth of information can't be turned into 8 bits' worth; it's easy to get rid of information, but hard to get it back, and impossible to pull out of nowhere). The same principle applies to things like resizing, recoloring, and some methods of compression.

For more information about digital images, especially as they relate to photography, go to Digital Image Basics by Jonathan Sachs.

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