THR - Image Matters - Printing, after Scanning

Printer considerations with scanned images-

Color printers must print several of the printer's dots for each image pixel. Inkjets have only 3 or 4 colors of ink (a few have 6 colors), and this is all they can print. They CANNOT print any one of 16 million colors Typical small inkjet printer

on any one dot. So to represent each image pixel in various colors, shades, and intensities, the image is ''dithered'', meaning the printer uses a pattern of several of its dots to simulate the color of each pixel in the image.

For example, to print one "pink" pixel on an inkjet, it must mix some red and some white. There is however no white ink, white is (usually) the paper color, no ink at all. To make red, the printer only has the CMYK (Cyan, Magenta, Yellow, and Black) ink colors, and so must use a few magenta and yellow ink dots, not necessarily equal numbers of each, to achieve a certain shade of red. To make lighter shades of red, blank white space is used in the right amount. Black ink dots are used to darken some colors.

The average visual effect of all these individual dots of magenta, yellow, white paper, and perhaps black ink too, looks pink to us. But all of these multiple ink dots represent or simulate the color of only ONE pink image pixel. So it is clear that we don't get anywhere near 600 or 720 dpi of "image" resolution from our printers in Color mode. This requirement for multiple printer dots for one image pixel greatly reduces the printer's real image resolution capability to a fraction of the printer's advertised dpi.

Printer specifications are real and accurate and meaningful, but are NOT to be confused with image resolution. Printer ink dots and image pixels are simply very different things, and one color image pixel requires many printer ink dots. This is why we need a 600 or 720 dpi printer to print an image at 150 or 200 dpi. And like B&W printers, attempting higher resolutions on color printers simply limits the pixel size area, allowing fewer ink dots, which then limits to even fewer possible color tones. This is important to remember.

We need the several ink dots in that space to simulate the correct color of the pink image pixel. If you attempt to print at higher values than the printer is designed for, say 360 or 480 dpi, then you are only allowing space on the paper for a very few ink dots per pixel. The color accuracy is reduced (fewer possible values of color tones in that space). Our home color inkjet printers do not require the four CMYK halftone screens (Cyan, Magenta, Yellow, and blacK) from an imagesetter like commercial printing presses use ....... instead, our home printers expect (and get) RGB images, and the drivers use dithering (error diffusion), to produce the color combinations from 3 ink colors.

Basically, dithering is the use of scattered dots, somewhat randomized instead of ordered halftone grids, and it looks smoother. The printers limited combinations of 3 ink colors can rarely make the exact color for an image pixel. There is an error, the difference in the desired color in the image, and what the printer can do. Error diffusion means that the color error difference is carried over to four adjacent image pixels, to the right and below the pixel in error. Those next pixels are intentionally overcompensated in the opposite amount. If the one pixel is not pink enough, the next neighboring pixels are made overly pink, so to speak. Then their own error term is carried over to their neighbors in turn, etc. As this process moves across the image, compensating the color error, it all balances out and we see the right color.

Scanned images obviously look much better on the monitor than on the printer. And yet we think the printer requires greatly MORE resolution than the screen? Their images are often larger (in pixel dimensions), but printers are still rather crude devices. The big difference is that the monitor screen shows continuous tone RGB images, but the printer must simulate each pixel with many printed dots, each dot of a single CMYK color.

Making the distiction then -- Printer "dpi" is a different thing from image dpi (except in Line art mode). For inkjet or laser printers, here are good Scaling Guidelines for Color or Gray Scale photographs --

300 or 360 dpi printer -
Use 60 to 120 dpi image.

600 or 720 dpi printer -
use 120 to 240 dpi image.

This statement is made for Grayscale and Color modes. Note there is an exception for line art mode, which is 1-bit 2-color (B or W, like ClipArt or fax), which contains no gray, no halftones, no dithering. In line art mode, the printer CAN use its full resolution, making use of the full resolution of the scanner. Because in line art mode, every image pixel is either Black or White, so there is no dithering necessary to simulate intermediate tones. Line art is the one mode that the printer can use an image resolution equal to the full resolution of the printer.

If you have a 600 dpi printer, then 600 dpi line art can be printed advantageously to reduce the visible jaggies. But for Color or Gray Scale modes, the dpi guide above is appropriate. Generally the middle of the range above is about right, but modern Color dithering may use a few dots less per pixel than B&W halftones, so Color may be best towards the high end. I've exaggerated the high side, and I'd bet on the midpoints instead in most cases.

Actual printer dpi divided by 3 or 4 is the correct range. I'd suggest scaling to 100 or 120 dpi for older 300 dpi inkjets, 120 to 170 dpi for 600 dpi laser printers, 180 to 300 dpi for today's better color ink jets on the best paper. Scaling to more printed resolution than this won't often be very helpful. A little less than this is often just as good. Paper varies. images vary too, some are sharp and some are not. The low end of the range is often fine, experiment to see if you can tell the difference. Giving the printer huge images can be pretty slow, it takes the printer time to receive and discard all of those pixels.

Note that if you are printing the printed image larger than the original photo, then the scanning resolution should be increased accordingly, in the same ratio as the image size increase, to achieve these same numbers (see the previous section). This means you might scan at 300 dpi and scale to 150 dpi to print at 2X size. Inkjet printers have come a long way in the last couple of years, and the once impossible photo quality is taken for granted today.

Epson is suggesting printing images at 240 dpi if maximum possible detail is desired on the best glossy paper. 240 is 720 / 3, to account for the several dots required to dither the 24 bit RGB colors of each image pixel, and corresponds to lpi (lines per inch) x 2. Experiment with 240 dpi on your sharpest images, but also experiment to see if 180 dpi will give the same results (180 dpi corresponds to lpi x 1.5 and also to 720 / 4). Many of our snapshots really don't have maximum sharpness, and don't need maximum resolution, and frankly, 150 dpi is often plenty. But some certainly can use more. The 240 dpi is a maximum limit of usefulness, and not necessarily a "gotta have" goal in every case.

HP inkjets are 600 dpi, but this is not comparable, because they use a unique REt technology that blends colors by printing several ink drops on any one dot. A pixel's color value is dithered on the same one dot instead of by a group of dots. Some of these possible combinations are murky, but each dot can still make many useful colors (instead of one color) and error diffusion dithering still corrects any error. It is not continuous tone, but it's rather close.

Printer dots and image pixels can be matched one for one, like on a monitor but with higher color error. Printing images at 300 dpi is indeed realistic now but the more pixels you give the printer, the more it will discard, and in fact, the excess can be detrimental to the printed image, depending on how skillfully the printer discards the excess.