Book Scanning & Digital Image Production The VRC Guide to Imaging By Kate Stepp Why doesn’t the scanned image look exactly like the print image? Why does the image need software to manipulate it? Print Dot Matrix Versus Pixel Maps and CMYK Versus RGB Above is a CMYK print process matrix (left), and an RGB overlay color pixel map (right). Print matrix images rely on the blending of overlapped spot (circular) colors and the white spaces of the paper. This is what is used to create color images in books and magazines. Pixel maps rely on overlapped individually colored pixels (square) with no spaces. This is what is used to create color images in computers. Book Images are Created with CMYK dot matrix overlays Scans of a Printed Image must be Transformed from a Dot Matrix into a Pixel Map When scanned, we use a descreen filter to turn these overlapping color dots into individually colored pixels – which blurs the image but creates a uniform color map with no white spots or visible matrix pattern. Here are close-ups of both Dot Matrix and Pixel Maps. Detail ---> Detail ---> Scans of Printed Images must be transformed from CMYK to RGB. With the CMYK process, to print any color image, Cyan, Magenta, Yellow and Black inks are overlapped to achieve the full color spectrum. Color monitors for desktop microcomputers are based on cathode ray tubes (CRTs) or back-lighted flat- screen technologies. Because monitors transmit light, displays use the red-green- blue (RGB) additive color model. Technical Note: What resolution do we use? - Part II There are two types of resolution: Display resolution and Document/Printing resolution. DISPLAY RESOLUTION The display resolution of projectors/computers is simply the physical number of columns and rows of pixels creating the display. Note that the use of the word resolution here is misleading. The term "display resolution" is usually used to mean pixel dimensions (e.g., 1024 X 768), which does not tell you anything about the resolution of the display on which the image is actually formed (dpi/ppi). Tulane’s Smartroom Projectors display a maximum display resolution of 1024 Pixels Wide X 768 Pixels High. How many pixels per inch is determined by the physical space of the display. There are only 1024 X 768 pixels - whether they take up 10 X 7 inches of physical space or 10 feet X 7 feet of physical space. Technical Note: The smartroom digital projectors have a display resolution of: XGA (1024 x 768) native SXGA (1365 x 1024 widescreen or 1280 x 1024 regular) maximum Overall, the scaling engines have gotten to be very accurate with video these days. Quite often a scaled video image looks just about as clear and crisp as it would if displayed in its own native format. However, this is not as true of computer data signals (digital images). So let's focus on data projection. It is common for a native XGA resolution projector (1024x768) to have, say, SXGA (1280x1024) listed as its maximum resolution. All that means is that you can feed a 1280x1024 computer signal into the projector. However, when the projector compresses this signal into its native 1024x768 display, the picture will be somewhat fuzzier (sometimes a lot fuzzier) than it would be if the signal was native XGA to begin with. There are projectors being built with SXGA (1365 X 1024) and SXGA plus (1400x1050) native resolutions. More on that to come… 72 ppi became known as “Web Standard” resolution. This actually refers to the physical space/size of monitors, not document or printing resolution. Most screens are configured at 768 X 1024 pixels. At 72 Pixels Per Inch this equals a visible space of 10.66 Inches High X 14.22 Inches Wide, about the physical size of most computer monitors. Thus the true but often times misleading saying “You can’t see more than 72 ppi on a computer.” What is dpi or ppi? DPI stands for Dots Per Inch. PPI stands for Pixel Per Inch. It has nothing to do with how your images will display on computers or through a digital projector. Combined with the document size, it has everything to do with how your image prints. The Dpi/Ppi of digital images is stored information for printing on paper. It tells the printer how to process the image, how many dots per inch. As digital images are created with pixels (squares) not dots, it helps translate pixel information into dot information for the printer. Thus we have stored Dpi/Ppi resolutions in digital images. The higher the Dpi/Ppi is, the better quality of print you get. Again - Dpi/Ppi has nothing to do with how your images will display on computers or through a digital projector. That is based entirely on pixel dimensions. What resolutions do we use? The VRC uses these settings for images: 1000 Pixels Wide (Display Resolution) 750 Pixels High (Display Resolution) 10 Inches Wide (Document Resolution) 7.5 Inches High (Document Resolution) 100 Pixels Per Inch (Document Resolution) Why don’t we use larger display resolutions? We are balancing out display resolutions, printing quality (dpi/ppi) and printing sizes as well as the built in size defaults of Powerpoint and Keynote (7.5 X 10 inch presentation screens). 1000 X 750 is 97.65% of 1024 X 768 Pixel Dimensions Matter for Projection. If your image has MORE pixels than the maximum your computer or projector can display - then your image will not look good. If your image has LESS pixels than the maximum your computer or projector can display and you try to enlarge your image to full screen - then your image will not look good. Scaling digital images is in no way similar to scaling slides or film (which you can scale simply by adjusting the lens throw on a film projector). For Example: Say your maximum screen display is 6 Pixels, and your image is 12 Pixels. You can not “shrink” your pixels down to squeeze in the smaller pixel area. You can only discard pixels. Those 6 discarded pixels still exist, however, in the image data. The data is not changed (your image is still 12 pixels wide) but only 6 of your pixels will be displayed. This often leads to unwanted visual affects: blurring, pixelation, color changes, etc. An example of 50% scaling. Reducing an image by 50% throws away approx 75% of the information. This is important to remember when doing side by side comparisons - you are shrinking the image (in general) and may want to also present the image full screen to examine details and have better clarity.
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