Edited by Pat Bitton
November 25, 2019
Defining the Raster Image, File, and Best Applications
In the age of the internet, the website, and the webpage the raster image has entered its glory days. So, what is a raster image? We would define raster as a technique for reducing a photograph or other image to a precisely reproducible format by using mapped dots of color. In this form, the photograph or image can be resized at will – although not without loss of sharpness (low resolution) in some instances.
The raster method, initially used in photography and printing, where it was known as contones ("continuous tones"), proved perfect for computers and for the digital world of the internet. In computer graphics, a "raster graphics" or "bitmap" image is a dot matrix structure – a rectangular grid of pixels or tiny points of color. Each pixel's color is digitally specified by a certain number of bits. You then have a "bitmap" that can be stored in video memory or as a "raster" (bitmap) file specific to a storage device.
Raster files have varying formats with different features and each file format has its fans. A rasterized image is technically characterized by the width and height of the image in pixels and by the number of bits per pixel, which translates into the image's "color depth," or how many colors can be represented.
Raster Definition and Resolution
So, what raster graphics are made up of? Most of the emojis that we see are very small, but you probably have seen one blown-up, resized, for effect. The effect is a fuzzy-edged, grainy, low-resolution image. The reason is that each raster image is composed of a fixed number of pixels. When the image is greatly enlarged, there is no increase in the number of pixels. Instead, the existing pixels are "stretched" to fill the space.
If the enlargement is great enough, we see individual pixels, a very rough texture. And if the enlargement is still greater, we can see that each pixel is structured from values of red, blue, and green – each color value represented by a different number of bits in the pixel.
The meaning of the word "raster," by the way, has its origins in Latin, "rastrum," a rake, and that in turn is derived from "radere," to scrape. That doesn't seem to relate immediately to any of what we have discussed. Perhaps that is because the use of the word "raster" in English first referred to the raster scan of cathode ray tube video monitors. That process created the image line by line by magnetically steering a focused electron beam across a screen.
For more perspective on the distinctive nature of the raster format, raster graphic, and raster file, the best comparison is with a vector file. If you want to have your logo printed in a book or magazine advertisement or on the side of the corporate jet, you want it at that (and any) size to look very consistent with all other reproductions of the logo. Its precise look is part of the brand of your enterprise. For these purposes, you almost certainly will use the most common, widespread alternative to raster imagery: vector imagery.
Vector images are created by using lines instead of pixels and those lines are anchored in location by mathematical algorithms. The lines may be straight or curved and wrap around each other in the image, but their relative placement is mathematically precise. That means that the entire structure can be resized with perfect reproduction, no loss of resolution, because the mathematical relationships are precisely the same, just at a different scale.
Applications of Raster Images
Raster is most at home in computers and on the web, as contrasted with printing, say. Most modern computers employ bitmapped displays. On the computer screen, each pixel corresponds to a small number of bits in that pixel's memory. Therefore, the screen is refreshed just by scanning through the pixels and coloring them in obedience to the instructions in each pixel's set of bits. No one wants to wait for the screen to refresh, so the process is greatly accelerated by using dedicated circuitry, usually within a graphics processing unit. One of the earliest such displays using raster computer graphics was in the late 1960s at Bell Labs. At that time, however, the right to patent computer software was being fought in the courts, so no patent resulted for Bell Labs.
Today's computer monitors characteristically use about 72 to 130 pixels per inch (PPI), while the best consumer printers may go to 2400 dots per inch (DPI) or higher for superb resolution of your images. There are limits, however, and the appropriate image resolution for the printer is a matter to be calculated, since printed output may have a greater level of detail than any viewer can discern on a monitor. A resolution of 150 to 300 PPI has turned out to be generally right for 4-color process printing.
Most images used in your computer are stored in raster graphics formats or some compressed variation of them such as JPEG and GIF, all of which are staples of the World Wide Web. And three-dimensional voxel raster graphics are employed in video games and have important uses in medical imaging such as Magnetic Resonance Imaging (MRI) scanners.
Editing the Raster-Based Image
The editing of raster images is by software that edits pixels, just as editing vector-based images is by editing lines and shapes (vectors). Although a raster-based image is composed of millions of pixels, the raster image editor works by manipulating each individual one.
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When used for the right job, raster files have advantages that have shaped the entire web experience. For other uses, especially printed logos, with or without text, printed photographs, and graphics that must be resized with absolute veracity, the choice obviously is one of the vector file formats.
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