A raster graphic is a type of graphic composed of pixels of various colors that form an image. They are generated with pixel based programs, a camera, or a scanner. Basically, pixels are painted using raster programs to blend colors to soften the transition from one color to another. Some processes cannot use raster formats, though, and depending on the image, conversion to vector formats can be time-consuming. Raster means bitmapped.
They are saved in the file extensions of .BMP, .TIF, .GIF, and .JPG. Another element to consider is raster images often occupy more space, which is dependent on image quality. Large dimensions and detailed images require a larger file size.
Raster graphics are ideal for non-line art images because they typically have subtle chromatic gradations, undefined lines and shapes, and complex composition. Photo editing is better to do with pixel-based images. A smooth color blend is made where this wouldn’t be possible with vector-based images by arranging pixels and slowly changing the color or shade of those adjacent to them.
However, image degradation is an issue that comes with raster graphics. Since the raster images are pixel-based, a raster image gets jagged, rough, and blurry when blown up or scaled down. You cannot enlarge the pixel dimension and resolution without losing quality. When you use Photoshop to increase a photo’s resolution manually, it randomly adds pixels, creating poor image quality.
To print a raster graphic and maintain its quality depends on the pixel dimension of the image and the pixel resolution. For paper printing, 300 PPI is the minimum, and shirt printers are 240 PPI. Large format printers vary due to the distance the sign will be viewed and can range from 20 to over 200 PPI.