The Role of Photoshop Spot Channels in Prepress Inspection

The printing color of the topographic map or thematic map is generally black, brown, blue, green or more, which is different from the four-color printing of yellow, blue, green and black. Due to the special nature of its spot color, in general pre-press processing, some common digital map publishing software first divides it into monochromatic versions of EPS or PS Postscript files.

The version check is an important job before printing. The general inspection method is to inspect and print monochrome proofs. The disadvantage of this method is that it is not able to carry out inspection and fitting, element relations, capping, and color proofing. The root of this situation is that the layers of the EPS vector file (or the layer in which the EPS is converted to a grayscale image) cannot be transparent, retain the cover, and other reasons, making it difficult for the color separation files to be combined to form the correct color graphics. Color inkjet printing is difficult and error prone, so it is usually monochrome inkjet inspection.

In the work, the author used the Photoshop color channel to convert monochrome EPS files of different colors into grayscale images, then merged them into a color PSD file, implemented color inkjet printing, and realized the effect of fast analog printing and color proofing (such as Figure 1).

First, convert the EPS file to a PNG grayscale image

1. Convert EPS (PS) to PNG Grayscale Image by GSView
GSView is an English version of commercial software, mainly used to browse PS, EPS, EPI, PDF and other Postscript language files, and can easily convert between these formats. The author is using GSView 2.4 version. The following is a brief introduction of the following specific operations.
(1) Unit settings. In Option/Units, select mm units.
(2) Height and width settings of the frame. Generally 1:50000 frame width can be set to 1600 × 1200, other size frame can be set according to specific needs. In menu "Media/UserDefined", enter the height and width: 1600 x 1200. The formula for the conversion of the exact value is: The required number = the actual size of the frame in millimeters ÷ 0.35278.
(3) Display sharpness settings. In the menu "Media/DisplaySettings", the recommended values ​​are as follows.
1Resolution: 30dpi
2Zoom Resolution: 300dpi
3Depth:Default
4TextAlpha:1
5Graphics: 4
(4) Print to file. After opening the file, select menu "File/Print/Printsetup", "Device" select Pnggray; "Resolution" can be set to 500 (dpi); select "Print to file"; print PNG grayscale file. The "Resolution" value can be set as needed. Each version file needs to be opened one by one and printed as a PNG file.

2. Convert EPS (PS) to PNG Grayscale Image by Acrobat Distiller
Acrobat Professional's Distiller module can batch convert EPS files to PDF files, and then Acrobat can open PDF files and save them as PNG files. The resolution can be set as needed during the conversion process.

Second, Photoshop spot color channel customization and spot color image input adapter

In Photoshop, you can create a new spot color channel and customize the ink color of the channel. We place the monochrome version of the document in different spot color channels to achieve overlapping of the channels, because the channels are transparent and can achieve the effect of color combination, and easy inspection and fitting, mutual relations of elements, and inspection of the glands And other operations. The simple operation is as follows.

1. Create a new file in Photoshop. The size is the same as the PNG grayscale file generated in the previous GSView, and the resolution is the same as that set in "Resolution".
2. Establish a spot color channel. Select the window "Channel", click on the small triangle in the upper right corner, select "New spot color channel", you can create each color channel and define its name, and adjust channel ink characteristics according to the color of the sub-EPS (custom ink CMYK content) to achieve the effect of analog printing. For the sake of identification, the channel name can be defined as the name of the partition color. The sequence of channels is in accordance with the order of publication presses (as shown in Figure 2).
3. Copy the monochrome PNG file to the spot color channel. Open the monochrome PNG file, select it, copy the content, and paste (or drag) it into the corresponding spot channel created by the previous step. You will see that the data has been displayed according to the channel's color settings, and that the effects of the capping, overprinting, etc. simulate the printing results.
If a large number of frames are inspected, the created spot color channel can be saved as a template, and each version of the splicing process can be saved as Photoshop's action. In this way, the subsequent actions can be applied when the stencil is used, and the splicing can be easily achieved. Improve efficiency.

Three, prepress inspection

Digital proof output check or screen check can be used.

With the drawing printer directly printing paper drawings, you can easily check the content of the sub-version, you can also easily make mistakes on the paper map.

When there are not many errors, you can directly check the screen and find that the error directly modifies the data. If you want to mark an error, you can continue to apply the channel principle, establish a channel in the top layer with a striking color (this color should be clearly distinguished from the color in the figure), generally available in red. Use Photoshop's tools to easily plot errors and add text captions on this channel.

Fourth, the conclusion

This method has been widely used in the actual work of our company and has facilitated the rapid realization of the color version of the EPS version. This has provided a powerful technical guarantee for the verification and verification work, saving a lot of inspection time and improving work. Efficiency and picture quality have eliminated many mistakes that were previously difficult to find, such as capping and overprinting. Its main advantages are the following five points.

1. Simulated print proofing and digital proofing.

We used a computer with inkjet printers. The process completely simulated the laser photocopying of single-color EPS files. After printing on the film, the ink was printed using various colors.

2. Save time and improve efficiency.

The method used by this process to convert EPS to PNG images is basically the most efficient and effective method among various methods. The speed is 10 times faster than using Photoshop to directly convert; the PNG format used as the image format is most In this case, it is more than five times smaller than the equivalent JPEG file.

3. Avoiding errors that could not be found with previous monochrome checks.

This process avoids the consequences of white area error capping caused by combining various versions of EPS files in Illustrator and other vector software. Instead, it converts the EPS to a grayscale image and puts it into the channel, completely simulating the ink distribution in printing. The result is that it provides convenience for checking images and avoids errors such as glands, overprints, and the like that could not be found in previous monochrome inspections.

In addition to map preflight inspection, other similar EPS (PS) files can be used to implement prepress inspections. However, for different color designs, we have to customize different spot channels.

Source: Ke Yin Network