|lecture notes for Media Tech|
I do not want to take up class time for lecture unless necessary. Thus, I may be sending emails from time to time to impart information you will need to master whatever application we are working on.
This discussion of graphics formats applies primarily to photoshop, but is relevant to all desktop and web publishing.
Resolution and file size
Graphics for print must be a good deal higher than 72 dpi. A mimimum is 144 dpi, or twice the online. If you want to use an internet graphic in print, you must find a picture that is twice the width AND twice the height of the size you want it to be in printout. If, for example, you want a one-inch printout, you must find a 2x2 inch internet graphic. The math: four square inches is 4 times the size of one square inch. Strictly speaking, then, you would reduce the 2x2 graphic to 25% of its original size to get a 1x1 graphic. Photoshop, however, gives you the percentages for height and width separately. So you would reduce the width by 50% and the height by 50%, yielding a total reduction of 25%.
Scanning and descreening
In the best of all worlds, you would measure the existing screen (or dpi) of what you plan to scan. This takes a special tool (which I have and you can find). Then you would command the scanner to descreen, or remove the effects of the existing dot pattern. If you do not do this, you can get a moire pattern, or a crosshatching on halftone material. Unfortunately, only a few scanners offer this feature. The workaround is to scan fat and then reduce the resolution.
File formats overview
The gif file is for graphics or line art. Its colors are crisp, and not lossy, so it will not start throwing away color information when it is reduced. It does not need a background color, and works well where transparency is required. The gif is a proprietary format, created by Compuserve. Some years back, America Online bought Compuserve; so now AOL owns the gif format. It is possible, if unlikely, that AOL could start asking a nickel for every gif on the internet. Because of this, Adobe created the png.
Adobe created a non-proprietary file format, the png. The png file does an adequate job with photos, though the jpg does better. It does well on illustrations, and can be saved as a transparency -- without a background color. Photoshop and illustrator offer the png-8 and the png-24. Some photoshop effects do not work on png-24s or on any other 24-bit graphic. So save as the png-8. You won't be able to tell the difference.
The jpg file is also commonly used in print for photos.
Text renders poorly in photoshop, becoming fuzzy when it prints. A good rule is to do all your photographic work in photoshop, and everything else in illustrator or indesign.
Vector vs. Raster formats
Raster images are what you get in photoshop: rows of pixels, each having its own color. Photo files are large because every part of the document MUST have pixels in it. Vector images are mathematical constructs. In a vector image, if you see nothing there, like white space, there is in fact nothing there. A circle, for example, is an imaginary dot with a color, say, eight pixels away from itself, and then another pixel eight pixels away, and another and another on around the center point. Thus, the only pixels are the circle itself -- not the white space inside, not the background upon which it rests. In general, vector images are smaller and cleaner-looking than raster images.