Color Printing Issues

The following provides hints and tips for obtaining optimal quality when printing. This assumes you are using a color printer, but it is important to note that the user may print grayscale images with a black and white printer if desired. This would of course be best implemented by creating grayscale colormaps to eliminate ambiguities associated with different colors that have the same gray-scale representation.

Optimal printing of a raster image requires taking several factors into consideration. First, you must know the characteristics of the printer and the intended size of the printed image. Printers vary considerably and no single recommendation can be appropriate. Color printers fall into three primary categories, inkjet, color laser, and dye sublimation. EVS, for example, produces raster images which are continuous tones with 256 shades each of red, green and blue for a total of 16.7 million possible colors (256 * 256 * 256). Color printers either produce continuous tones or approximate them using a pattern of primary colored pixels in an n-by-n grid.

Among these three printer categories there is considerable variation. Inkjet printers are generally capable of producing one of only eight primary colors for each printer pixel (or dot). These colors are white, black, cyan, magenta, yellow, red, green and blue. Inkjets must therefore use a grid of primary colored pixels to approximate continuous tones. The larger the grid (4 by 4 vs. 2 by 2) the better the color approximation. However, larger grids tend to create artifacts called jaggies that are visually undesirable. The challenge is to balance the need for smoother color rendition with the desire to have higher resolutions.

Dye sublimation printers are at the other end of the spectrum. Their ability to reproduce continuous tones makes the task of choosing a resolution easy. A typical dye-sub printer has a resolution of 300 dots per inch (dpi). If the intended size of the final printed image is 10 inches wide by 7 inches tall, then the optimal image size is 10*300 by 7*300 or 3000 x 2100 pixels. If quicker image creation and print times are desired, a compromise resolution would be exactly half or 1500 wide by 1050 high.

It is best to have an integer number of printer pixels for each "source" image pixel. When the image size is half of the printer pixel resolution, each source pixel gets a 2-by-2 grid. The n-by-n grid concept applies to all types of printers. This "rule" is actually a guideline for best results. Other resolutions (non-integer ratios) create banding artifacts that are usually objectionable.

For inkjet printers you should always allow for at least a 2x2 grid and usually 3x3 to 5x5 gives the best results. For an EPSON printer with 720x1440-dpi resolution you should use the smaller resolution number (720) for your calculations. The printer uses the additional resolution to better approximate the colors.

Example: For a printer with 720 dpi, to print an image 9 by 7.5 inches (landscape) we recommend that you start at a 4x4 grid which gives an effective printed resolution of 180 dpi. Your image width and height would therefore be:

Width = 9.0 * 180 = 9.0 * (720/4) = 1620

Height = 7.5 * 180 = 9.5 * (720/4) = 1350

Finally, color laser printers vary in their abilities to approximate continuous tones. This means that the rules to apply will be somewhere between dye-sub and inkjet properties.