Desktop System Image Color Processing and Control

The color desktop publishing system consists of hardware devices and application software. The hardware part mainly includes: A. Image input devices, such as scanners, digitizers, electronic color separation machines, video cameras, etc.; B. Image processing equipment, refers to the computer; C. Image output devices, commonly used inkjet printers, image machines. The software part includes: A. Scanning software, such as Photoshop, Twin; B. Image processing software, such as PhotoShop; C. Graphics processing software, such as Coreldraw; D. Typesetting software, such as Freehand, PageMaker, Fit, etc.

Since the image is processed by hardware and software in the process, the software and hardware factors have an effect on the copy quality of the image color. If the monitor is too bright, the image on the screen is bright and the tone level is rich. However, when the proof level does not know when the level disappears, the whole picture is dark and it is the influence of the computer monitor on the image. If you use PhotoShop again, the color separation curve is set differently. When converting from RGB to CMYK, the image information of each color version is very different. In order to ensure the reproduction quality of the color image, we must strictly control the image input, image processing, and image output quality in the desktop system so that the color of the image is faithful to the original.

First, the quality control of image input

Scanning with a scanner is the main method for the image to enter the desktop system. Original images (such as photos, reversal films) are scanned and converted into digital images to enter the prepress system. At present, the scanners in the desktop system are divided into three types according to their working principle: platform type, drum type and electric extension. Although there are many brands of each type of scanner, the performance is slightly different, but from the perspective of quality control, the difference is not significant. The following describes the scanning quality control using a common flatbed scanner as an example.

The scanning quality of an image includes two levels of image input and color input, in which the level of image copying is the most critical. Because an image is unclear at the time of input, even if the color is very bright after processing, it can only give people a blunt feeling; on the contrary, a layer of rich images can be processed after processing by image processing software. Bright and faithful to the original. The visible level is the frame of the image and is an important factor in controlling the quality of the input.

The key to controlling image input quality lies in the determination of scanning parameters.

The flatbed scanner consists of a scanning light source, a mirror, a CD (charge coupler), and is driven by a scanner driver. During scanning, the quality of images scanned by different scanning parameters is different.

The so-called scan parameter refers to a set of values ​​set in the scan software that faithfully reproduce the tone levels, color changes, and gray balance in the original. The setting process is as follows:

1. Ten minutes after the scanner is turned on, place the standard color patch in the scan area, start the scan software, and scan with the system's default scan parameters.

2. After the scan is completed, use the probe to measure the color data of the gray scale in PhotoShop, and adjust the highlight value, dark tone value, and gamma value as required so that the first level of the gray ladder is between 250-255. The number is around 125, the 22nd level is between O-5, and the three channels R, G, and B are close to each other. For example, if the shadow tone value is 50, it means that the dark tone is not black enough, and the dark tone value can be increased, so that the value of the scanned 22-step ash ladder is about 5 (the gray grading scale 19 to 22 that the general flatbed scanner scans out). The extreme values ​​are not much different, only about 10-5, so flat-bed scanners have less expressiveness on dark originals than drum scanners.) Adjust the midtone Gamma so that the gray level of the 11th level is about 125. If the level 11 is too small, increase the Gamma value. At the same time pay attention to modify the brightness and dark tone to ensure that all three can meet the requirements. According to the size of the R, G, and B values ​​of each level of the gray ladder, the three channels are adjusted so that the red, green and blue values ​​of each ladder are approximately equal. For example, if the intermediate tone 11 is R=118, G=B=124, we can increase the gamma value of the red channel, increase the intensity of the red channel, make the 11th level value around 125, and the green and blue channels basically. Flat, to ensure that the color separation of the image is not cyan.

Adjust the high light and dark tone values ​​in the scanning software and the midtone Gamma value. If necessary, adjust the red, green and blue (or yellow, magenta, cyan, and black) single channel values ​​to make the tone, color, and gray balance of the image. The color scale is the same. The bright and dark data and Gamma values ​​of the above channels are the scanning parameter values. After setting, most of the originals can be basically restored by scanning with this parameter. As for some originals with special requirements, they can be adjusted slightly based on this parameter to meet the requirements.

Second, image processing quality control

1. Separation quality control

Most of the image files scanned by the flatbed scanner are in the RGB mode, and they need to be converted into CMYK after color separation before proofing. Therefore, the first step in the image processing quality control is the setting of the color separation parameters (ie, the values ​​of the CMYK mode for converting from the RGB mode to the four-color color separation unit). PhotoShop software has a strong separation function, so the color separation parameters are set mainly in PhotoShop.

The color separation parameter setting mainly includes four items: the display setting, the printing ink setting, the color separation setting, and the color separation table setting.

A. Monitor Setup

In order to make the colors on the screen as close as possible to the colors on the printing paper, we must first calibrate the display and change the display properties of the display by adjusting the Y value, white point, and black point. The so-called Y value is the relationship between the input picture and the color value of the output picture on the display screen.

B. Printing Inks Setup

In the Photoshop 4.O version of the File Preference, the Printing Inks Setup dialog box is used to set the type of back-end printing ink, gray balance data, and dot gain in printing.

1 ink type settings. There are two kinds of ink standards commonly used in China, one is Japan's Toyo Inks (Coated) standard, and the other is the US's SWOP Inks (Coated) standard. One of them can be selected according to the specific printing ink.

2 Dot Gain refers to the enlargement of outlets at 50% in the middle tone. Coated paper generally takes 20-25%, offset paper is 30%, and newspapers are 35-40%. The larger the value is set, the smaller the CMYK data will be.

3Gray Balance (Gray Balance) refers to the ink proof balance value of printing proofing. The default value of the system is C:1.00; M: 1. 00; Y: 1.00; K: 1.00. Since each proofing company has its own set of gray balance data, these data must be known before image processing. If the CMYK data after color separation of the gray scale deviates from the desired value, these settings need to be adjusted. For example, after the color separation, the 11th level ash ladder is C: 55; M: 53; Y: 46; K: 7; and the data provided by the proofing company is C: 55; M: 46 Y: 46 K: 7 After the image has a reddish magenta, M needs to be lowered and can be reduced from 1.0 to 0.95. Dot Gain for Grayscale Images are used for the grayscale image. When this item is clicked, the display of the grayscale image will be changed according to the gain of the dot gain. The larger the Dot Gain value, the darker the display. This option will also affect the conversion of other modes to grayscale conversions. For example, when converting from RGB to grayscale, selecting this software will automatically invoke the dot gain value to compensate the grayscale value. The larger the Dot Gain value, the smaller the grayscale data.

C. Separation Setup

Select the Separation Setup item under File/Preference and call up the icon dialog.

1Select Separation Type

There are two types of separations available in PhotoShop, namely GCR and UCR can be selected one at a time. GCR replaces the gray component and UCR removes the ground color. The background color removal of the desktop platemaking system is different from the previous color removal on the electric extension, so the GCR method is often used.

2 Determine the black version of the Black Curve (Black Generation)

The tone distribution of the black version of each image has a very important influence on the image. The operator must modify the essence of the image. Specifically, when there are not many gray components in the image, such as the daily shooting landscape images, news portraits, etc., the black version is usually set to be in the black version of the tone. When the image is a high-saturation, high-contrast art camera, we can set the black version as a short-cut skeleton black version. When there are many gray components in the picture, such as some ancient paintings, in order to achieve a better gray balance, we can use long tone black version, or even full tone black copy.

3 Black Ink Limit

Different types of manuscripts should have different values, from 70%-90%, usually 85%, to ensure that the black field of the printed matter is sufficiently dark, that is, the black version has enough contrast.

4 Total Ink Limit

Indicates the maximum value of the sum of four-color dots. The offset value for coated paper may be 34O-380%, and it is better for the newspaper to take 260% or less.

5 UCA Amount

This value is mainly to increase the value of cyanine in dark tone, and for darker manuscripts with rich levels, the value can be larger, such as 40%.

2. Color processing quality control

A. Common tool

The separated images need processing to meet the requirements. PhotoShop has many processing tools. We often use:

1 Curves curve can be used to correct the highlights and shadows of the image, taking into account the tone and gray balance of the entire image. The Curvse curve is powerful and has little loss of tone in the image, making it a versatile tool.

2 Use Selectivecolor to select the color correction tool to adjust the local color patches in the figure.

For example, we want to increase the saturation of the blue block in the graph. You can select the color block and then add or subtract to change the saturation of the color.

3 Sharpen tools to improve the clarity of the image.

There are four sharpening methods in Filter/Sharpen, where Unsharp Mask is the most powerful. Amount represents the amount of sharpening (intensity); Radius is the number of pixels that participate in sharpening; Threshold represents the starting point of sharpening. The operator can modify the three parameters according to the screen display to ensure that the image is clear, natural, and granular.

The bright and dark tone of the image should be within the normal range, and the middle tone should meet the visual needs of the human eye, faithfully restore the gray balance.

B. Gray balance control in color images

The key to color reproduction is the correct reproduction of the color of the original manuscript. This is the basis for the realization of tone, gradation, and color reproduction. In theory, the three colors C, M, and Y are superimposed to produce different shades of gray. However, due to the nature of the ink itself, equal amounts of C, M, and Y do not form a true gray color, and only a warm gray color is formed, resulting in a color cast of the replica. Therefore, color reproduction must adjust the ratio of the CMY tri-color version according to the printing suitability in order to truly reproduce the original color.

The gray balance control is mainly achieved by adjusting the ratio of C, M, Y, and K dots in the three regions of the image highlight point, middle adjustment point, and dark adjustment.

1High light point gray balance adjustment control

When setting the highlight, place the mouse in the brightest white part of the image and record the C, M, Y, and K values ​​here. The principle of adjustment is that the percentage of the C version of the high gloss area should be greater than about 2-3% of the M and Y version, and care should be taken to ensure that the minimum number of dots of the minimum version is equal. Because of the limitation of printing conditions, the minimum dot printable at present is about 3%, so the minimum spot height spot is usually set at 3%. High-light areas generally do not put K version.

2 Midfield gray balance control

Usually midtone refers to the area between 40%-60% in the image. In principle, the percentage of outlets of M and Y is adjusted based on the C version. The commonly used ratio is that the percentage of C edition outlets is greater than the percentage of M and Y outlets of 10-15%. In order to ensure the stability of the grey balance in the intermediate tone, the application of the K version should be considered. When the C version is greater than 55%, it is necessary to ensure that the K version has a 2-5% network.

3 dark area gray balance control