Photoshop Method 2 for Calculating Number of Print Colors

Ink and toner are different

In terms of rendering halftone images, printers and laser printers are two different physical ways. Halftone ink dots on paper will penetrate into the media, while laser toner dots will only reside on the surface of the paper. Experienced commercial printers tell customers that they should avoid appearing nearly pure white and dark shadows in delivering their work to be printed. The reason is that although the laser printer prints a halftone screen that can aggregate 100% density and place black areas on the paper, the printer ink will penetrate the page and expand. Depending on the paper, ink, printer, and wire web used, digital half-tone images that contain conditions that indicate brightness extremes may not be "persistent." For example, areas that are webbed at 90% or 95% density may promote full saturation of the area printed using ink. When the hanging net cannot be “held” on the press, the darkest area becomes a mixed area of ​​black and bleeding areas, forming potholes and eventually producing an incorrect representation of the original image after drying.

The wrong representation of the image may also appear in bright and white areas. On laser replicas delivered to commercial printers, uncovered areas sometimes lead to image areas with "hotspots," due to the lack of halftone ink dots in the image. Density of 0% and 1% in the image, if expressed in ink halftone dots, creates unwanted borders in the image—known as banding. Readers should take some time to think about it. A 1% outlet must start somewhere. Isn't it? The solution here is to use a halftone dot with a density of at least 1% until the white area in the original image is completely covered.

Reduce contrast in laser replicas

To deal with the problem of hue extremes of an image, it is necessary to go back to the original digital image and make a copy for modification, and then reduce the contrast of the image in the copy so that there is no absolute black or white. Although the modified image appears flat on the display, the result will be a new look when the plate is made from the laser replica and the image is printed with the plate. It is emphasized that if the final output is on paper, the "correct" result of the rendered image will be presented on paper instead of on the display.

Take this Toaster image as an example. Assume that after talking with a business printer, the customer learned that the printer cannot handle halftones with brightness less than 12%. At this point, the solution the customer can use is to change the tone distribution in the image and redistribute the darker values ​​to the top 12% (very bright area). In doing so, the tonal range of the image can be shifted into the range of printable tones. Readers should not think of this as reducing the quality of the work; instead, it should be seen as an optimization of the image so that it can be reproduced on other media better.

The first thing to do is to figure out which shades in the image are at the top 12%. Since the brightness range of 0 to 255 does not directly correspond to the density percentage range of 0% to 100%, we need to use the following formula: 256-[halftone density (%) x 2.56] = brightness value, which will be printed now The machine's 12% minimum density value is substituted into this equation: 12(%)*2.56=30.72, 256-30.72=255.28. According to this solution, it can be known that the output luminance level of the image should be at most 255.

Similarly, if the business printer tells the customer that 90% black is the densest halftone dot the press can provide, the same rule should be used as follows: 90 (%) x 2.56 = 230.40, 256-230.40 = 25.60. In this example, the customer should enter 26 in the box to the left of the OutputLevels area in the Levels command dialog. The following steps show how to reduce the number of tones in the Toaster.tif image. After optimization, it can be accurately reproduced on the press into a beautiful image.

Reduce the contrast of the image to be printed

1. Open the Toaster image on an earlier hard disk.

2. Press Ctrl(æ)+L to display the Levels command.

3. In the OutputLevels area, enter 225 in the right box.

4. Enter 26 in the left box of the Output Levels area, as shown in Figure 18.12.

5. Click OK to make the tone changes take effect, press Ctrl (æ) + Shift + S (File, SaveAs), and then use Output.tif as the file name to save the file to the hard disk.

Note: If the business press only specifies a high or low density threshold, do not use the Levels command to simultaneously redistribute the upper and lower tone extremes in the target image. When you change a particular area of ​​brightness, Photoshop recalculates and redistributes the pixels in the image. Photoshop can redistribute the hue values ​​scheme to make them appear smooth, but when the user specifies a OutputLevel not specified by a business printer, this function may also cause the user to lose the opportunity to obtain a good image.

If you proceed with the steps outlined in the previous section, converting Grayscale mode images to RGB and then examining the underlying Indexed palette for the image, you will find that the Output.tif image now contains 172 different shades. In the case of 85 lpi, it is already within the hue range of the 1200 dpi printer, so we can successfully print out this image. In this case, it can be used as an optimized and beautiful print result for business printing.

After printing is complete, you should then delete the Toaster image copy or any other image that has been optimized for printing so that you can free hard disk space. After reducing the color tone and reducing the image quality, the pixel information that has been simplified to meet the printing requirements can no longer be retrieved. Therefore, when the user prints according to a certain copy of the image, the size of the digital image should always be specified. And resolution.