Core_Computer Studies : Resolution is important!
Resolution is important! This one subject can mean the difference between an image printing correctly and printing as a blurry banded pixilated mess. One of the reasons that people find resolution so confusing is that you will encounter 4 different but similar descriptions of resolution during the process of working through a project.
Samples-per-inch (spi): The number of pixels acquired during the scanning process
Pixels-per-inch (ppi): The number of pixels that an image file has recorded or that a monitor uses to display images.
Lines-per-inch (lpi): The number of halftone dots in a printed image.
Dots-per-inch (dpi): The size a printers smallest printable dot.
Lets take this one step at a time:
At an extremely simple level there are two different levels of resolution that a visual artist might use; screen or web resolution and printing resolution.
When we speak of image resolution we are referring to the number of pixels per inch in the digital image. This is usually spoken of in pixels per inch (ppi), or samples per inch (spi). Both terms more or less mean a similar thing. For instance with samples-per-inch if you scan an image at 150 spi, the scanner takes a sampling of the image every 150th of an inch. The higher the scanning resolution, the greater the information about that image so the greater the amount of detail in the image. An image that has 72ppi contains less information than one that is 300ppi.
Monitor and Web resolution (72dpi) is different from printing resolution. Now pixels-per-inch (ppi) as you can see from the list above refers to monitor pixels-per-inch. High-resolution images are needed in professionally published materials but are not necessary for images that are to be viewed only on screen for instance, web pages.
For web projects or on-screen presentations, it's important to understand what is necessary for the image's best appearance and what is redundant. Sometimes, people make the mistake of putting image files, of the same resolution that they use in printed materials, on the Web. As a result, the images take a long time to download.
Think of resolution as referring to how well an image can be rendered. In the case of an image on-screen your computer monitor displays at 72 dpi so its pointless having a higher resolution as it can't be rendered any better. Any more resolution than that is wasted because an image at 72dpi will appear just as clearly and as smoothly as one that is 600dpi. The images will look the same but the one that is 600 dpi will have a much bigger file size. This larger the file the longer it will take to download on the web.
A question I often get from people is about the quality of their home printer. They puzzle at the fact their image looks fine on screen but when printed it looks poor. There is nothing wrong with their printer the problem is with the resolution of their file. When an image is too pixilated it means that it is at too low a resolution.
People confuse pixels-per-inch (ppi) with dots per inch (dpi) which is actually a measurement of the number of ink dots a printer outputs. To clarify it in your own mind think of it in terms of being a physical measurement (the number of dots a printer prints per unit of measurement) the other is a digital measurement (the number of samples or pixels in an image per unit of measurement).
What all this means is that when you are working on an image ask yourself what is the final use of the image? Is the image for a web based project or a project that you need to print.
Vector images however, because they are created using PostScript principles, are resolution independent. They are mathematical files and are treated as objects. This is one of the advantages of vector images.
This leads us to last abbreviation on our list Lines-per-inch (lpi) or linescreen. The linescreen refers to the number lines (of dots) per inch (lpi) that will be used to simulate tones of color and gray in the printing process. A typical linescreen for a laser printer is 85 lpi. When scanning the trick is to remember that the resolution should be twice the linescreen. That is to say that, in the case of a laser printer, an image would require 170dpi for the best appearance on a printer using 85 lpi. Less will cause an image to deteriorate and pixelate.
These notes are written for students who undertake the Core Computer course at School of Art, Australian National University. Students of the course should feel free to contact any member of staff with comments questions and general suggestions..
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