How to perform image quality control

Founder strongly introduced products of color management technology. The signing ceremony of the cooperation between Beijing University Founder and Kodak in the application of color management technology was held in Beijing. The signing of this agreement marks a new step for Peking University Founder and Kodak in technical cooperation. At the signing ceremony, there were two newly developed color output products developed by Peking Founder: one is a digital proofing system for printing and publishing, and the other is an ad production output system for large-scale advertising production.

Founder Digital Proofing System, Founder Century RIP, Kodak color desktop operating system consists of a number of hardware devices and application software, such as scanners, work hosts, monitors, inkjet printers, imagesetters, scanning software, P-HOTOSHOP, PAGEMAKER and so on. Any of these hardware and software will have an effect on the color quality of the image. For example, if the monitor is too bright, you will feel that the printed image is brighter and the tone level is very rich. However, when you see the proof, You were surprised to find that the original image was black and did not show! If there were some deviations in each process, the system would be confusing, and the standardized operation and quality control would be impossible to talk about. The author felt that in actual work, There are some units and individuals who do not have a clear understanding of how to make good use of the prepress system and how to control the system quality. The author has been engaged in desktop color work for many years and has accumulated some experience. He hopes to help the readers, and also hopes that everyone will work together. Improve the quality of prints and beautify our lives. Discuss below from image scanning, color separation correction and output correction.

First, scan:
Scanning is the gateway to the desktop system. Analog images (such as photos, reversal films, etc.) must be scanned into digital images before they can enter the prepress system. The scanning equipment mainly includes flatbed scanners, desktop roller scanners and electricity. Extension. Although the performance of different image input devices is not the same, but from the point of view of the operation of the standardization, process, etc. are not much difference. Here we take the most used flat-bed scanner as an example, the scanner correction Instructions.

The principle of calibrating the scanner is to calibrate the scanner to be able to faithfully copy the gradation information, color change, and gray balance of the original. The specific method is to use a special reflective or transmissive color scale to adjust the highlights and dark values ​​in the scanning software. Etc. If necessary, adjust the red, green, and blue single-channel values ​​so that the electronic image's tone, color, and color scale are consistent. The specific operation is as follows:

1. After the scanner has been turned on for 10 minutes, place the standard color in the scan area, start the scan software, and scan with the system's default scan parameters (take RGB as an example).

2. After the scan is completed, use the probe to measure the color data of the grayscale in the PHOTOSHOP. Adjust the highlight value and the darkness value in the scan software as required so that the first step of the gray ladder is between 250 and 255. The dark tone 22 is between 0 and 5. For example, after the scan, the dark tone value is 50, which means that the dark tone value is not enough. The dark tone value can be increased from the original 0 to 15 so that the black level after scanning is 22 Ladder values ​​can be around 5. Brightness adjustment methods are similar. For originals with rich darkness levels, it is recommended to scan using a roller scanner.

3. Adjust the mid-level Gramma value so that the value of the gray scale 11 is about 125. Some 11-level value is too small, you can increase the Gramma value. At the same time pay attention to modify the brightness and dark tone to ensure that all three can reach one To request.

4. According to the relationship between the magnitude of RGB values ​​for each level of gray scale, adjust the values ​​of the three channels in a single channel so that the values ​​of red, green and blue of each ladder are approximately equal, so as to ensure that the scanned image and the separated image are not biased. blue.

If you can complete the above steps according to the required data, then the scanner has been basically calibrated, and the adjusted scanning parameters are the valid parameters after the calibration. For most manuscripts, scanning with this parameter is sufficient. It can be said that the scanning After the electronic image is basically able to restore the original gray component information. For higher saturation, darker honey degree manuscript, the reader can also make adjustments based on this parameter. For example, use the pre-sweep tone Distribution histogram adjustment and so on.

Now readers may find that the level of the 19th to 22th level of the grey ladder ruler is not much different, as large as about 10 to 5, the dark field level is not pulled apart, and the tone is compressed by the gradation which is caused by the scanner Its performance is determined by itself.

Second, the color tone is positive

The scanned RGB image needs to be converted to CMYK after separation. The color correction is mainly performed in PHOTOSHOP. PHOTOSHOP software has a strong color separation function and many adjustment parameters. Here are two main issues: Separation parameter setting and color separation adjustment. Separation parameter settings mainly include two items, namely printing ink setting and color separation setting.

1. Printing ink settings:

A: If you use coated paper printing, Ink colors can use Toyo Inks (Coated) or SWOP Inks (Co-ated), other parameters need to be provided by the proofing company, readers can also refer to themselves.

B: Dot Gain refers to the enlargement of halftone dots in the middle tone. If printed on coated paper, it is recommended to take 20% to 25%, offset paper is 30%, and newsprint is 35% to 40%. The larger the value is set, the smaller the CMYK data after color separation.

C: Gray Balance means that the printing proofing company has its own ink gray balance value, and the reader must remember it before proceeding with the image processing.

D: Use Dot Gai-n for Grayscale Image. Click this item to change the grayscale image according to dot gain compensation.

The larger the dot expansion value, the darker the display. This option will also affect the conversion data of other modes to grayscale mode.

Color correction: After the separation of the image often requires a certain amount of correction to meet the requirements, PHOTSHOP software more correction, usually the following:

A: Use the Curves curve to correct the high and dark tone values ​​of the image, taking into account the tone and gray balance of the entire image

B: Use selec Colors to select the color correction tool to adjust the local color block of the image.

C: Use sharpening tools to improve the sharpness of the image.

There are 4 kinds of sharpening tools under Filter/Sharpengh, among them unsharp mask function is the strongest.

AMOUNT represents the amount of sharpening, RADIUS represents the number of pixels that participate in sharpening, and THRESHOLD represents the starting point of sharpening. The reader can set these three parameters, but first of all to ensure that the monitor focuses on the normal color image to achieve light and dark in the normal range, the middle to respond to the human visual needs, faithfully restore the gray balance. By doing this, you can say that the map is done.

2: Separation setting: Select the Separat-ion Setup item under File/Preference:

A: Select the color separation type. There are two color separation types, GCR and UCR. You can choose one color at a time. The GCR is an ash component replacement and the UCR is a ground color removal. GCR is recommended.

B: Determining the Black Generation of the Blackboard The grading distribution of the blackboard has a very important influence on the image. The reader must modify the essence of the image.

C: Black Ink Limit. Different types of manuscripts should have different values, from 70% to 90%, usually 85%, to ensure that the black field of the printed matter is sufficiently dark and the blackboard has enough contrast.

D: Total ink limit. It represents the maximum value of the sum of four color dots. For offset offset printing, the value may be 340% to 380%, and newsprint may be taken as less than 260%.

E: UCA Amount. The color is mainly to increase the value of cyanine in the dark tone. For darker manuscripts with rich color levels, the value can be larger, such as 40%. At this point, the setting of color separation parameters is basically completed.

Third, the output correction

The output correction mainly refers to the correction of photosetters or color printers. The adjustment of the image-setter is mainly to look at the linearization of the image-setter.

There are many factors that affect the linearization, such as camera performance, Photographics machine ground voltage, film, processing solution and temperature, speed and so on. We can output a 0 to 100% dot scale to see if the number of dots on the film corresponds to the number of dots on the electronic image. If there is a basic correspondence, the linearization of the image-setter is better under this condition; otherwise, the above-mentioned influencing factors will be changed, and even the linearized data in the photo-taking software will be changed until the data on the electronic image is reached.

After we control the quality of the scanning, separation, and output, the quality of the prepress system is guaranteed. Operational norms are the prerequisite for quality control. I hope everyone will have a comprehensive understanding of the prepress system and carefully control each step as required to achieve a high level of image quality.

Source: Shaanxi Ancient City Trading Company