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Computer Images
by By Hu Filleul—Big Blue Cousins Newsletter - Web Edition” The Newsletter of the Greater Victoria PC Users’ Association - Web Edition Volume 15, Number 7, September 1998 - March 19, 1999 at 18:20:55:
Graphic images form an important part of many documents. They serve to add both interest and information to the basic text. There is a lot for us to learn about making the best use of images in our work. This article touches on some basic information about bitmap image size and resolution. Most of the data were taken from the Adobe PhotoShop 5.0 User Guide which is an excellent resource for understanding how to make the best use of computer imaging capability.Graphic Categories—Computer graphics fall into two main categories—bitmap images and vector graphics. Bitmap images are also called raster images and use a grid of small squares called pixels to represent the image. Each pixel is assigned a specific location and color value. When editing bitmap images, you edit pixels rather than objects or shapes. A bitmap image depends on resolution in that it contains a fixed number of pixels to represent the image data. As a result, a bitmap image can lose detail and appear jagged if viewed at a high magnification or printed at too low a resolution. Bitmap images are best for representing subtle gradations of shades and colour such as in photographs.
Vector graphics are made up of lines and curves defined by mathematical objects called vectors. Vector graphics are resolution independent and can be scaled to any size and printed at any resolution without losing clarity. Vector graphics are best for type and graphics that must retain crisp lines when scaled to various sizes. As computer monitors represent images by displaying them on a grid, both vector and bitmap images are displayed as pixels on-screen.
Bitmap Images—To produce high quality bitmap images, it is important to understand how the pixel data are measured and displayed. The display size of an image on screen is determined by the pixel dimensions of the image plus the size and setting of the monitor. A typical 14 inch (Canadian) monitor displays 640 pixels horizontally and 480 vertically. An image with pixel dimensions of 640 by 480 would fill this screen. On a larger monitor with a 640 by 480 display, the same image would fill the screen but each pixel would be larger. The number of pixels displayed per unit of printed length in an image is usually measured in pixels per inch (PPI). An image with a high resolution contains more and therefore smaller pixels than an image of the same size with a lower resolution. For example a 1-inch by 1-inch image with a resolution of 72 ppi contains a total of 5148 pixels, 72 X 72 = 5184. The same 1-inch by 1-inch image with a resolution of 300 ppi would contain 90,000 pixels. While higher resolution images usually produce more detail and subtler colour transition they can also become very large. It is, therefore, important to use the optimum resolution for the output device you plan to use to display your final product.
If you are producing an image for on-line display, the image resolution only needs to match the typical monitor resolution (mostly 72, but occasionally 96 ppi). Using too low a resolution for printed output, however, usually results in pixelation-output with large, coarse-looking pixels. In setting the image resolution for printed output, it is necessary to consider the output resolution of the printer or image setter. For best results use an image resolution that is proportional to but not the same as the printer resolution. Most laser or inkjet printers have output resolutions of 300 to 720 dpi and can produce good results with images from 72 to 240 ppi.
If you are going to photocopy your work or send it to a commercial printer, it is necessary to consider the screen frequency of the image. (See the article “The Dot is the Thing” in our http://www.bbc.org/bbc/oct97nl/dot.htm” October, 1997 issue.) To print masters for a grayscale image on a 300-dpi printer, a coarse screen of around 60 lines per inch is generally used, while you can get good results on a 600-dpi printer at around 80 lines per inch. Some ink jet printers do not allow you to set a screen frequency but use coarse or fine dither settings to produce similar results.
Scanning—When scanning an image it is best to scan at the image size and resolution best suited to your intended output. You can determine the settings for your scan using the original and final image dimensions and the screen frequency of your output device to calculate a scan resolution to enter in the scanner driver settings. To estimate scan resolution: Multiply the screen frequency by 2 (the typical ratio of image resolution to screen frequency needed to produce a high-quality image). Multiply the result from step one by the ratio of final dimensions to original dimensions to determine an estimated scan resolution.
For example, suppose you are scanning a 2-inch by 3-inch image and want to produce a final 6-inch by 9-inch image. You are using a screen frequency of 85 lpi. First multiply 85 (the screen frequency) by 2 to get 170. Then multiply 170 by 3 (the ratio of final image dimensions to original ones) to get a scan resolution of 510 dpi.
I found that this system works well, but that software drivers for both scanners and printers vary considerably. For example, the TWAIN interface I use allows you to set the image scan up to double or half the original image size. I always use this setting to get the scanned image as close as possible to the output size I want. I nearly always scan the image as a TIF file and do all necessary sizing and editing in this format. If I am going to output to a printer I use the .TIF format. If, however, I am going to use the image on a Website or strictly for viewing on a monitor, I convert the TIF file to a JPG file at 72 dpi, with about a medium JPG quality setting. This process nearly always produces an acceptable screen image at an acceptable file size. If obtaining a very low file size is necessary, it is often possible to downgrade the quality of the JPG file even further. I have managed to get a 1.4-Mb .TIF file down to 12K for satisfactory web display.
If you are concerned with high productivity, it is, of course, possible to shorten or even automate the image producing process to give good results with a minimum of time and effort.
Most of the Adobe Classroom in a Book series of learning sessions to be given this coming fall will involve the handling of images in various types of digital documents. Sign up and join in the fun and learning.
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