Raster images, also known as raster graphics or bitmap images, consist of pixels. Pixels are tiny colored squares or dots that together form larger graphics, they are the building blocks of images. Pixels are individually editable and are expressed in either dots per inch (ppi) or dots per inch (dpi). With an image editing program such as Photoshop, you can zoom in on individual pixels, view and even edit them. Once you zoom out again, the ppi or dpi controls how high the resolution of the final image is, the resolution simply refers to the maximum number of pixels per image. For example, a six-inch square image with a resolution of 400 dpi or ppi is expressed by 5,760,000 pixels. That’s 400 inches for 160,000 per square inch, times 36 for the entire piece. Normally, online images are 72ppi, while print images are about 300ppi. 600ppi is considered a very high-resolution image where it is impossible to enlarge raster images without losing information. This happens when you enlarge existing images and make them look “pixelated” or blurred. On the other hand, very large, high-resolution images can be difficult to edit. You can complicate the work in a drawing program with many layers and slow down the computer. However, freehand drawing with raster graphics has a more natural character than vector graphics and is more similar to drawing on paper: digital photos are raster images, as are images from most freehand drawing programs. Some of the most common types of raster image files are gif, jpg, png, psd and tiff. Among the programs you can use to create raster images are Gimp, Photoshop or Sketchbook.
How raster images have evolved.
The first computer screens were developed in the middle of the 19th century. From a technical point of view, they functioned similarly to television sets that scanned electron beams from left to right and from top to bottom to produce moving images.