Definition of Bitmap and Raster Image

Isometric pixel character

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A bitmap (or raster) image is one of the two major graphic types (the other being vector). Bitmap-based images are comprised of pixels in a grid. Each pixel or "bit" in the image contains information about the color to be displayed. Bitmap images have a fixed resolution and cannot be resized without losing image quality. 

All About Pixels

Each pixel on your screen is, in very simple terms, a "bit" of color information used to display the image on a screen. That screen could be as small as the one on an Apple Watch or as large as a Pixel Board found in Times Square.

Along with needing to know the three colors (red, green, blue) applied to the pixel another "bit" of information is where, exactly, that pixel is located in the image. These pixels are created when the image is captured. Thus if your camera captures an image at 1280 pixels across and 720 pixels down there are 921,600 individual pixels in the image and each pixel's color and location must be remembered and rendered.

If you double the size of the image, all that happens is the pixels get larger and the file size increases because the same number of pixels are now in a larger area. No pixels are added. If you reduce the size of the image the same number of pixels are in a smaller area and, as such, the file size reduces.

How Resolution Factors In

One other factor that affects bitmaps is the resolution. The resolution is fixed when the image is created. Many of today's modern digital cameras, for example, capture images with a 300 dpi resolution. All that means is there are 300 pixels in every linear inch of the image. This explains why digital camera images can be rather huge. There are a ton more pixels to be mapped and colored than commonly found on a normal computer display.

Typical Bitmap-Based Formats

Common bitmap-based formats are JPEG, GIF, TIFF, PNG, PICT, and BMP. Most bitmap images can be converted to other bitmap-based formats very easily. Bitmap images tend to have much large file sizes than vector graphics and they are often compressed to reduce their size. Although many graphics formats are bitmap-based, bitmap (BMP) is also a graphic format although its use today is very rare.