The Confusion about Image Resolution

What is screen resolution?
Why can’t you print an image off the internet and keep its quality?
Why is it not a good idea to use a photo in your logo?
Why does your image blur over time?

Resolution

An image on a computer is displayed by tiny pixels, and a printed image is printed with tiny dots. The sharpness of your image or its resolution, is measured by how many pixels or dots it’s represented by in 1 inch square: pixels per inch (ppi) or dots per inch (dpi). The image below is displayed in different resolutions, from higher to lower.

A printed image and a computer screen image require different resolutions to display well. Print requires a higher resolution; standard print resolution is 300 dpi. A computer screen displays 72 ppi . This is why a low resolution screen image doesn’t print well on paper.

The basic machine language used by a computer is represented by binary code of bits, (you know all those 0’s and 1’s.) A computer image is therefore referred to as a bitmap. An alternative name for a computer image is raster image.

Vector Graphics

Besides bitmap/raster images, there is a second type of image format called vector graphics. Although vector images are flash-rendered to be represented by pixels on your computer screen, they are configured differently than bitmap images. Vector graphics are not resolution dependent but can be scaled freely. Vector graphics are configured with mathematical algorithms based on the relationships between geometrical primitives such as points, curves, lines and polygons. Vector files can represent simple, geometric images in much smaller file sizes. Because of the clean, simple and illustrative aspects of vector graphic along with its resolution independency and small file sizes, it is ideal for company logos. Vector files may be saved into bitmap file formats for varied purposes. Vector graphics are edited with special vector graphics editors.

File formats and compression

File formats also affect the quality of your images. There are two main groups of file formats: lossy and lossless. Lossless image files use algorithms that reduce the image information without losing any quality of your image and are therefore larger in file size. Lossy image files compress your image information and lose some of the quality of your image to make the file size smaller.

Although you may not notice any reduction in quality, your image will degrade more with time if you keep re-saving the same image over and over with a lossy compression. It is a good idea to keep your original images and save copies as smaller file sizes to use on the internet where file size matters. Typical file formats which uses compression to minimize file size to be used on the web are JPG, PNG and GIF. JPG is the standard choice for photographs, while PNG and GIF both support transparency but are more ideal for simple graphics with less colors and simple animations.

Many social internet sites and blogs automatically re-size your images for you. However, automation doesn’t always create perfect results. It’s always a good practice to size your own images for better control of the results.

By Jessica Lang, Multimedia Team Specialist, Travel Planners International