Notes on Scanning and Image Resolution

optical resolution of a scanner (also called the hardware resolution of the scanner) � the resolution at which a scanner captures an image without interpolation 24-bit RGB color (3 channels of color � red, green and blue � 8 bits per channel to indicate the shade of each), used for computer display 32-bit CMYK color (3 channels of color as with RGB, plus one channel for a �key� color, usually pure black to give better contrast and shadow detail), used for professional printing calibration � calibration helps to standardize the capture response of a scanner by comparing swatches of known color value with the result of a scan, and then adjusting for error. A pixel is a picture element, either in its abstract representation in an image file, or in its physical manifestation on a computer monitor. A 300 dpi laser printer can print dots that are 1/300 inch in diameter, i.e., it can print 300 dots horizontally and 300 dots vertically in each square inch. Stochastic dots can vary in density (how far apart they are) across a printed line, but they are all the same size for a given printer. Line art intended for printing should be scanned at a resolution corresponding to the dpi of the printer on which it will be printed. Contone images (i.e., continuous color) are scanned at a resolution that corresponds to how the image file will be used. , then it should be scanned at a resolution that is between 1.5 and 2 times the lpi of the printer on which it will be printed (assuming that you want the image to remain in its original size). If you want to change the size of a color image that you intend to print, and you can do the rescaling at scan-time, use this formula: If you put an image file into another program like Word or Powerpoint and then make it smaller, the size of the image file remains the same (so it�s better to make it smaller An advantage to vector images is that they are resolution independent. That is, a vector image always prints out at the resolution of the printer. Even if you enlarge it, you won�t get jagged edges because the same mathematical equations are used to created the printed copy, and at the same resolution. The disadvantages are that (1) it takes time to learn vector drawing programs, especially to master how curves are done and (2) you can�t get the continuous colors (one color blending smoothly to the next) that you get with photographic images saves as contone files. Vector images are ideal for logos, maps, and other smooth-edged images where the change from one color to another needs to be sharp and definite. GIF � a pixel-based image format used on the Web, originally created by CompuServe for transferring images through phone lines. This type of image file format is used for grayscale and index color images (<,= 8 bits per pixel), typically at 72 ppi. To reduce the file size of a GIF file, you can cut it down to just the colors that are needed (rather than requiring all 256). JPEG (joint photographic experts group)� a compression format commonly used with continuous tone (24-bit) or grayscale (8-bit) images to be displayed on the Web. JPEG compression is a lossy compression technique � that is, some information is lost in the compression, so the resulting image may not be as true to the original as it was before compression. TIFF was originally created for scanning, so when you scan photographs and images you may often want to save them as TIFF files. vector-based EPS � programs such as Macromedia FreeHand, Adobe Illustrator, and CorelDraw allow you to save vector images as EPS-vector files. (portable document format) � a compression scheme that embeds, right in the file, all the necessary information to view a single document or an entire publication : Source.


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Last Modified: April 24, 2016 @ 3:08 am