Basic Concepts of "Resolution" in the Field of Computer Graphics

While I wrote this article a few years ago for a class I was teaching, I have found people still refer to it. Thus, I am including it and updating it.

The term Resolution in the field of computer graphics has various meanings depending upon the context in which it is used. When an image is scanned into a computer, this image is called the input image. The technical term for the number of pixels per linear inch of this input image is ppi (pixels per inch) or spi (samples per inch). However, the output, especially when referring to an inkjet, laser, or dye sub type of printer, is in terms of dpi (dots per inch). Unfortunately, the terms are often used synonymously even though they have technical differences. Most scanner manuals refer to dpi and do not even discuss ppi. Some books also talk about monitor resolution in terms of ppi.

When referring to bitmapped  or raster images, the higher the ppi or spi, the larger the file size. For example, if one created two TIFF files of a 6" diameter circle, one at 72 dpi and one at 300 dpi, the 72 dpi size would be 596KB while the 300 dpi size would be 10MB or almost 20 times larger in terms of bytes. At this point, one might say Awhy is the author talking about ppi, spi, and then using dpi for her example?@ This probably seems to be confusing; but it need not be.

When printer resolution is discussed, it is in terms of dpi (dots per inch). However, a printer, that has a maximum resolution of 720 x 720  is commonly stated as having a dpi of 720 (however, usually it is not as high as stated although it is not done to mislead the user). That dpi has a different meaning than the dpi of an image. For example, let's take the 6" in diameter circle that has an output resolution of 300 dpi. I can print that on an inkjet printer, for example, at 180 x 180, 360 x 360, 720 x 720, or 720 x 1440. What the above numbers are theoretically saying is that the output of the printer will create an image where the dots are spaced vertically at 180 dots per inch and horizontally at 180 dots per inch. However, these are physical Ablobs@ of ink on the paper. Moreover, for better quality they are randomly spaced; thus, the term 180 x 180 or 720 x 1440 do not refer to actual "blobs" of ink. But people will speak of printing at a dpi of 720 or 360. Thus printer dpi is different from image output dpi. The best way to understand this is to try experiments on your own. Create an image at 72 dpi, 200 dpi, and 300 dpi. Print these same images at various settings on your printer. Then, compare the results.

One last term to dissect is lines per inch or lpi. One will sometimes see this term used when discussing  scanners in terms of output resolution. This term refer to screen frequencies and only screen frequencies. These screen frequencies refer to screens used to produce halftone images. When one has had the unfortunate experience of scanning in a beautiful scene from a magazine only to have horrid intersecting lines (called moiré patterns) appear on it, one is capturing the imprints of the halftone dots used to produce that image on an image setter using halftone screens. A Halftone image is produced by various size dots. A halftone screen is used in this process. A process called rasterization produces tiny dots, spots or pixels (imagesetter pixels are different from the regular definition of pixels). These are contained in cells per linear inch.

The home user does not have to concern himself with lines per inch other than to recognize that the common relationship of ppi to lpi is 2:1. With the input size of the image equaling the output size. If the ppi (input resolution) is 300, the lpi will be 150. Some scanning software designates output using the screen frequency terms of lpi. This can appear to be very confusing. However, if the input linear size of the image, say the 5x7 photo is to equal the output linear size, (5x7) and the final image resolution in Photoshop, Painter, PhotoPaint, Fireworks, etc. is to be 300, the image can be scanned in using an image input of 300 ppi (although often stated incorrectly as dpi) or an output of 300 dpi or 150 lpi. If at this point, one says "how will I ever understand this?" I recommend that you go to your computer, scanner, printer, etc. and see how these numbers work.

When scanning in an image, I always scan it in at a ppi or dpi (used incorrectly by many scanner manufacturers to be the same as ppi) of 300. I do this so that if I ever want to have the work printed on an imagesetter, it will contain enough information. However, according to various printer manufacturers, the dpi of an image can be as low as 180 or 200. I usually do not go below 240.

Many digital cameras advertise themselves as having a certain number of Megapixels; This does not really give the purchaser any knowledge about how large (linearly) a picture can be printed and still maintain high quality. If one digs hard enough, the actual pixel dimensions can usually be found.

The following chart will show the linear size of digital pictures taken at certain resolutions as well as their linear and pixel file sizes working in RGB, assuming a dpi of 240, and a video card set at 24 bit depth or millions of colors:





640 x 480

2.667 x 2.00


1024 x 768

4.267 x 3.2

2.25 MB

1152 x 864

4.8 x 3.6

2.85 MB

1280 x 960

5.33 x 4

3.52 MB

1280 x 1024

5.33 x 4.267

3.75 MB