CHAPTER 3
Not all images require high dot densities. The need for high dot density decreases as the viewing distance increases. For example, a large roadside sign may require separations screened at only four
Pixelization, or the ability to see individual pixels, typically becomes a factor when a digital image undergoes magnification. At 300 dpi, a dot measures 0.00333... inches (0.081666... mm). Individual pixels in such an image remain unobserved to the naked eye. Observable pixels do not show up until a magnification reduces the resolution well below 200 dpi. Notably, monitors display images at 72 dpi.
An original at 72 dpi is at a threshold. Any magnification of pixels of a 72 dpi original for a screen image occurs by repeating pixels taken from the original. Higher resolution originals can sustain greater magnifications before the need for repeats begins. In other words, an original bitmap can only be magnified so much before the need for duplicated pixels occurs and pixelation begins to become an issue.
In raster systems, where line scans take data from a video memory, graphic objects comprised of lines typically undergo antialiasing. Without this, lines created at other than zero, 45, 90 degrees and their complements would have a jagged appearance. Line Generators can only put data in available dot positions in a video memory. In effect, each
Office Printers offer resolutions typically from 300 to 600 dpi. A degree of resolution reduction (down sampling) can be tolerated in order to reduce the amount of data required. When a file size needs to be kept in check by not exceeding a viewer's needs, the printed image should typically tolerate a resolution reduction to about 180 dpi. Typically, the
Because a monitor and a printer produce color through different methods, users can expect somewhat different results. A monitor uses an additive process, meaning a particular color derives from intensity control. For example, a color moves toward the green by intensifying excitation of green CRT phosphors. Printed images, on the other hand, use a subtractive process. These images display their color through reflected light. To generate a particular printed color, the process must subtract (that is, filter out and not reflect) parts of the source illumination, which then lets other parts dominate. For example, a reduction in cyan (greater cyan filtering) would allow yellow and magenta components to become more dominate.
The light reflected off of the surface of a white card passes through any colored dyes deposited on the surface of the card, both going and coming. The dyes used to form printed images serve as filters of light that would otherwise reflect off of what is typically a white print media. In printed images, complete filtration (or what serves as the maximum subtraction capability) results in black. Conversly, the absence of filtration results in the
