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Raster graphics, also called bitmap graphics, are digital images that are composed of tiny rectangular pixels, or picture elements, that are arranged in a grid or raster of x and y coordinates (includes a z coordinate in case of 3D) in such a way that it forms an image. It is also referred to as bitmap since it has information that is mapped directly to the display’s grid.
The file size of a raster image depends also on the size of the image, which is determined by the number of pixels being used in the image. This means that an image with a 1280x720 resolution will contain 921,600 pixels while a full HD 1920x1080 image will have 2,073,600 pixels, which will obviously give it a bigger file size when compared to the former.
The word raster was borrowed from the term 'raster scan,' which was how old CRT monitors displayed images, by magnetically steering a concentrated electron beam line by line to form an image.
The main disadvantage of raster graphics is that it is dependent on resolution. It can be scaled down with no changes in quality, but when the resolution is scaled up, quality loss is unavoidable. The image will look blocky and pixelated. Vector graphics, on the other hand, is able to scale up to any resolution because of it uses geometry and mathematical equations to define images rather than directly mapping pixels in a grid. Vector graphics are better suited for typesetting and graphic design.
Since raster graphics store much information, they require large file sizes and they can be a bit of a hassle to work with. Fortunately, there are already image compression techniques and algorithms that have been made to address that problem. BMP, TIFF, GIF and JPEG are some of the raster image file formats available.