Number 259 - December 2004

Scanning Basics
by Clarke Bird September 2000
If you were going to scan a photograph to print on a 600 dots per inch (dpi) printer, at what resolution should you scan the image for the best output?    I'll give you the answer in a moment but first - some basics of scanning. The first several times I scanned a photo, the resulting file size was frighteningly bloated. When I brought the scanned image into Photoshop and clicked on image size, I discovered my photo was 26 inches wide. Whoa! Whatsup? I think the only scanning options I knew to select were choosing between B&W, grayscale and color for the image to be scanned. There were some other options but I had absolutely no idea of what they were or how they worked. I did have the option of scanning from 50 dpi all the way up to 4500 dpi. The default was set at 600 dpi and I think I left it there for about six months before figuring out that I had better change it to a lower dpi selection. Based upon questions that occasionally arise from our reader audience, I'm not the only neophyte scanning enthusiast in town.    The most commonly used device to acquire line art or photos into a computer is a scanner. A scanner is a device that captures an image and converts it into a digital pixel map for computer processing. The most common and versatile scanner is a flat bed scanner whereupon you place a the object to be scanned on a flat glass sheet, close the lid and initiate the scanning process (either a button on the scanner itself or via the scanning software. Three dimension objects can be scanned with a flat bed scanner but the quality of the scanned results will vary. Some multipurpose printers also have the ability to scan pages and send or receive faxes but are limited to scanning flat sheets only since the sheet travels through the scanner. Some digital cameras allow images to be transferred directly into your PC but the most common technique remains scanning.    Most scanners provide a TWAIN interface that allows photo enhancement programs such as MGI PhotoSuite, Adobe Photoshop or Corel Photo-Paint to interface directly with the scanner. What does TWAIN stand for? No one seem to know - but TWAIN is a specification agreed to by scanner manufacturers to eliminate the necessity of using a separate scanning software program to acquire the image before bringing it into your favorite photo enhancement software program. In other words, TWAIN allows your photo enhancement software to talk to the scanner directly through a common interface.    Back to the question posed above: at what resolution should you scan an image if you want to later print it at 600 dpi? If you answered scan at 600 dpi, not only are you wrong, Jocko, you are going to have a image bigger than a house. Why? Keep reading.    It seems logical that the resolution of a scan should be the same, or nearly the same, as that of a printer. The problem is scanning resolution is different that than of a printer. Printers print dots (which are round) and are measured as dots per square inch (dpi). Each dot made by a 600-dpi printer is 1/600 of an inch in diameter. To produce the 256 shades of gray that exist between black	and white on a printer, these tiny dots are grouped together to form halftone cells, and put enough of these halftone cells will eventually form a halftone image.    Scanners scan pixels (which are square). Even though the resolution of a scanner is measured in dpi, this is misleading because the scanner resolution is more accurately described in samples per inch. Each sample represents a pixel. Each sample of a scan at 600 dpi is 1/600 of an inch square. In other words, a scanned pixel is much more like a printer halftone cell than the original printer dot.    So how do you determine dots per square inch to scan? Any resolution from 100-150 dpi woul be a correct scan, giving you an image size that you can manage without having to bring it into a photo enhancement program and reduce the image size. Not all images are created equal    Text and line drawings are truly black and white images. Adobe refers to them as bitmap images. With these type images, either a white dot or a black dot is scanned and then printed. Unlike continuous-tone images - like photographs - which have smooth transitions, changes in line art are abrupt, producing sharp edges. It is because of these sharp edges you should use the following rule:    Scan line art and text with a resolution equal to the maximum resolution (in dpi) of the final output device (printer), up to but not more than the scanner's maximum optical resolution, and apply sharpening with the scanner (if possible).    Color text and drawings present a problem for scanning because they require high-resolution scanning with the addition of the overhead associated with color. In these cases, file sizes become as large as your house in a big hurry. Here is a work-around: If your scanner supports it, scan the image at 256 colors. If your scanner can only do 24-bit color, scan the image and bring the scanned image into a photo enhancement program and convert it to 256 colors. Converting the file to 256 colors reduces its file size by 66 percent.    Photographs or continuous tone images can be color or black and white. This type of images is less detailed and requires lower resolution than line art. The rule for this is simple: Scan photographs at 100-200 dpi. Some variations to this rule: For 300-dpi laser printers, scanning at 100 dpi is sufficient. For 600-dpi laser printers, scan at 150 dpi.    Some believe that scanning at a higher than necessary resolution somehow gives their image extra detail, making it look sharper. In fact, doing this rarely improves image quality and produces very large file sizes.    Remember that each time scanning resolution is doubled, the file size quadruples    Copyright (c) 1996-2004 Alamo PC Organization, Inc. San Antonio, TX USA
Number 259 - December 2004