Scanning Images for Web Use - Getting the Best Image
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Photographs are a common item to find on your scanner. If the image is fairly clean and needs little adjustment, scan the image at millions of colors at 72 dpi. This is the straightest path to getting an image on the Web. Save the file as a JPEG at image quality 3 or medium. Done!
Are you scanning images from a brochure or catalog? Color printing usually consists of dots of different colors placed closely to each other to give the illusion of continuous color. If you have a magazine or other printed piece handy look closely at the images. You may not see these dots at first glance. In fact, you're not supposed to. The whole point is to create the illusion of continuous tone. Even if you don't see these dots, you're scanner will. The resulting patterns are called moiré patterns. Most scanner software has a "de-screening" function available. If yours does, select the setting that most closely matches the type of print you are scanning. If you're scanning newsprint, select newsprint. If scanning a magazine, select magazine, and so on. By using the de-screening function you may get a perfectly usable image at 72 dpi. If after using the de-screening filter you still have unwanted moiré artifacts or if you don't have a de-screening function try using these suggestions.
Even though you're only going to need a 72-dpi image, you may find it necessary to scan the original at a higher resolution and then down sample it to get acceptable results. Choose a resolution that isn't a multiple of the line frequency of the printed material you are scanning. Not sure what the lpi is for the piece you're scanning? Here is some number to give you an approximate idea; these numbers are by no means exact.
News Print - 65 lpi
Magazine - 133 lpi
Art Magazine (fine prints) - 175 lpi
I usually use 147-dpi. It's a fairly odd number (in context) and will usually help to break up any unwanted patterns. Be sure to still use the de screening filter on your scanner even if you employ this over sampling technique (assuming that you have the de screening filter). If the result is satisfactory, or very close, down sample the image to 72-dpi and save it out as a JPEG.
Still not useable? Try blowing the darned thing up! Not with explosives, enlarge the image in your image-editing program. 300% should do the trick. Then resize the image back down to 72-dpi. Making the image larger forces the program to "fake" the additional pixels leaving the image slightly blurry. When you resample the image back to 72-dpi, the program has to throw away a lot of the pixels making it "average out" the results. You can also use odd multiples when resizing an image to further reduce unwanted artifacts. Resizing an image this way "jumbles" pixels together minimizing artifacts.
Another trick you can try is rotating the image a few degrees. Sometimes this just isn't acceptable, but if so, it can help you break up visible and unwanted patterns in your image. Of course, you can employ any or all of the above techniques. Just keep in mind that all of these are "destructive" actions (except for scanning, of course.) A destructive action is one that destroys original pixels and replaces them with pixels of a different value. For example, cropping an image isn't a destructive action but resizing it is. Cropping an image doesn't change the pixels that are left; it removes those that are no longer wanted. If you resize an image, you change every pixel left in the image to a different value.
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