The first step before printing: scanning color separation

1. Scan color separation

The actual scanners currently in use mainly include flatbed scanners and roller scanners. The core component of the flatbed scanner is the CCD, the photoelectric coupler; the core component of the roller scanner is the PMT, the photomultiplier tube. The scanner relies on its core components to convert the scanned image light signal into an electrical signal; the electrical signal is then converted to a digital signal by an analog-to-digital converter and transmitted to the computer. Obviously, the scanner's core components have a great influence on the results of the scanning separation. For this reason, the quality of the scanning light source, the mirror, and the analog/digital converter will also have an effect on the electronic image. Therefore, the first step in controlling the quality of the prepress system is to calibrate the scanner.

The scanner's calibration principle is to adjust the scanner to be able to faithfully copy the original's tone level information, color change, and gray balance. The specific method is to use a special reflection or transmission color scale to adjust the gamma value of the highlight and shadow values ​​in the scanning software and the median value. If necessary, adjust the R/G/B or C/M/Y/K single channel values ​​so that the tone, color and gray balance of the electronic image are consistent with the color scale.

2. The key technology for scanning color separation

The quality of the image scanning determines the final output quality of the image. How to obtain the best quality image is the long-term common goal of the industry. The quality of the scan has a great deal to do with the scanner, scanning software, and the actual experience of the operator. Control quality is nothing more than manuscripts, scanning equipment, scanning parameter settings and scanning techniques.

2.1 Original Types and Original Analysis

The faithful reproduction of manuscripts is the eternal pursuit of the industry. Therefore, manuscripts are the prerequisite and basis for ensuring the quality of color reproduction.

2.1.1 Manuscript Requirements for Color Copying

1 high definition. It is the basis for realizing the price, color, texture, three-dimensionality, perspective, etc. of recorded scenes;

2 Density contrast. The density contrast of the original must be normal. Generally speaking, the contrast density of the manuscript is between 1.8 and 4.0. The manuscripts of this type of manuscript are rich in brightness, intermediate tone, and dark tone, and the coordination level is normal. The color saturation is good, and the texture is vivid (if there are rich dark tone levels; there are few middle tone levels, you must open the dark tone level during scanning and compress the middle tone level);

3-color saturation. Under the standard light source (50O0K) check the original, color saturation is the real response to the scene being photographed, only the original with high color saturation can be a normal original with hue;

4 image graininess. The granularity of the original affects the color, level, and texture. Fine-grained originals are conducive to the reproduction of levels and tones (especially bright textures) and true representation of colors.

2.1.2 The manuscript is usually divided into three types: transmissive manuscripts, reflection manuscripts

1 secondary manuscript: ie, scouring goods; the density range is small, the level is not abundant, generally only the original large scan, not suitable for amplification processing. Usually the quality of a good secondary manuscript has a density range of only about 0 to 1.6. Far less than the scanner's density recognition capability, scanning this type of document can only be done with a normal flatbed scanner. Secondary manuscripts often have problems with color shifts and require proper color correction during scanning. It needs to be reminded that the scanning of the original manuscript must be done to the network processing.

2 reflection draft:

a. Photo. The range of high-quality photos has a density range of only O to 2.2, which basically meets the pre-press processing requirements and print suitability; it is the most common manuscript in our daily work.

b. kind.

3 Transparency:

a. Negative film. Normally, the negative film is not scanned directly. It is best to rinse the negative film into a photo and scan again. This is because, first, the negative film has no effect. It is not easy to restore the color; second, the film base of the negative film has a colored film, which can cause the negative film to have no white field and it is difficult to calibrate.

b. Positive film (slide). It is a negative film, such as 135 slides. Its film base is not as good as the reversal film, the density range is small (0 ~ 2.8), and the particles are rough, the scanned image is also dirty, and its magnification can only be controlled at 4-7 times, and then the image will be larger. distortion.

c. Reversal film, also known as positive film, the film material and process it uses are different from negative film and positive film, the quality is the best in the original, the density range is between 0 and 3.8, and it can be enlarged by more than ten times when scanning. Is the best manuscript.

2.1.3 Analysis of the manuscript

The operator should have certain experience and manuscript analysis capabilities to be able to set the scanning parameters according to the actual conditions of the manuscript and obtain the best scanning quality. In practice, special attention should be paid to avoiding automatic adjustment of any parameters. The quality of the picture is flat, and even the printing requirements cannot be met. For color cast pictures, if there is rich experience in color correction, it is better to correct the channel when scanning, and it is better to perform correction in PhotoShop after the scan is completed.

2.2 Scan preparation

To get the ideal scanned image, you should do the following:

1 warm up. The scanner requires a few minutes of warm-up. The scanner should be turned on 30 minutes before the start of the scan.

2 Regularly clean and maintain the scanner. Scanners should be regularly cleaned and maintained in accordance with the requirements of the scanner instructions. When the scanner is not in use, apply a dust cap and do not allow the scanner to drench the water.

3 Clean the handles before scanning to avoid contamination of the scanned images by fingerprints. Do not use your mouth to blow dirt and dust on the scanning platform glass.

4 Clean the surface of the object or image used for scanning. It can be blown with a balloon, and the surface can be cleaned with water depending on the object being scanned, but damage to the object should be avoided, especially care should be taken not to leave scratches on the surface of the photo and film. Care should be taken when cleaning some of the original artwork.

5 Make the original flat. If you use software to repair the traces of the original curl. It will reduce the clarity and quality of the image. Although there are many functions in Photoshop (such as blurring and sharpening) to process the image, this is not the best way to solve the defects in the scanned image.

6 approved. Use calibration software to adjust the image and improve scan quality. Most scanners come with a variety of approval options to check light sources and stepper motors. Running the calibration option once a day will keep the scanner operating stably.

2.3 scanner input indicators and parameter settings

The difference in the quality of the scanned image largely depends on the input indicators (such as bit depth, resolution, etc.) of the different scanners and the setting of each input indicator. Practice has proved that only for the different types of originals, the correct setting of the scanner's scanning parameters, can get the best image color, level and clarity, to ensure high-quality color reproduction. Among the several input indicators of the scanner, bit depth, black/white field calibration, Gamma and other parameters mainly affect the image level, and tools such as resolution and filters mainly affect the fine level of the image. Although most scanners have different menus, the basic control parameters are the same, and there are constant black and white fields and gamma values, tone adjustments, and analysis of manuscripts. This is a key factor in the quality of the scan.

2.3.1 Resolution

The resolution reflects the richness of the details recorded by the scanner and can be divided into optical resolution (physical resolution) and interpolation resolution (maximum resolution). The optical resolution is the actual resolution of the scanner. It is the true resolution (vertical and horizontal values) obtained when the scanner hardware scans. It is a key factor in determining the clarity of the scanned image. Interpolation resolution is a resolution that is enhanced by software operations to enhance the complement. It does not add new information to the image, but it is useful for images that have specific requirements or scan line drawings. In order to obtain proper image quality, the scanning resolution must match the number of screen lines. For the original size copy, the optimal scanning resolution is generally determined by multiplying the number of screen lines (lpi) by a quality factor k (1.5 to 2.0). . For example, when the number of screen lines is 175 lpi; if k=2, the scanning resolution is 350 dpi.

2.3.2 bit depth

Describes the bit depth of each color pixel information in the scanned input image, also known as the sampling depth. It is generally 24 bits, that is, R, G, and B each occupy 8 bits, each with 256 gray scales; it can represent 2(8+8+8)=1678 million colors as true color, and the professional scanner should at least There are 36 depths. At present, the use of cultivable Photoshop can only deal with 24-bit images. The images obtained with a scanner with a bit depth of more than 24 bits need to be converted into 24-bit images in order to be processed in Photoshop.

2.3.3 Choosing the right black and white field

1 The brightest point in the picture is the white field. The white field directly affects the tone levels of the bright tone and the middle tone. The human eye is extremely sensitive to changes in the bright tone. The setting of the high light point will also affect the neutral gray balance of the color. When the white field is set too bright, it will cause the loss of some of the highlights; it will be too dark and the overall level of the picture will be dark. For a general picture, when the white field is set to neutral gray, the CMYK value of the white field may be 25%, 0%, 0%, 0%. For a situation where the white field is a color cast, it must be determined according to the specific circumstances, and the ability to judge can be improved through constant experience accumulation.

The darkest point on the map is the black field. The setting principle of the black field is basically the same as that of the white field. The difference is that the highlight point can be determined through the eyes, and the black field of the image requires the operator to have a certain degree of judgment. Some scanning software can automatically identify the black field, and some also provide certain tools (such as the adjustment of the histogram) to determine the black field, and the black field is too dark, which will result in the loss of dark tone levels and cause the phenomenon of “merging”. . The black court is too bright, it will cause the dark tone too bright. For most normal pictures, the K content of CMYK in the black field must not be less than 70%, and generally not more than 80%.

In general, when comparing the black and dark tone levels, the differences between the white field and the bright tone levels, the difference between the white field and the bright tone levels is judged by comparing the values ​​shown by the dropper. Do not trust the color effect of the screen display.

2.3.4 Adjusting Gamma Values

1 tone: In general, the picture is divided into bright tone, midtone and dark tone. Set the black and white field, the entire image of the tone range is determined, but how to locate the middle tone in this range, that is how to determine the level of distribution is determined by the Gamma value. In general, the tonality of a picture should be as far as possible in the middle tone, because people are more accustomed to looking at pictures with rich midtones.

2 Gamma values: For most scanners, there are default Gamma experience values ​​for most images. It must be noted that different scanners have different Gamma experience values. When adjusting the gamma curve, it has a great influence on the tone of the entire picture. Only those with a certain scanning experience can use the curve to make beneficial adjustments to the specific picture. Otherwise, it will be self-defeating, but it is better to choose its default linear gamma curve.

2.3.5 trial and error

Do not worry about making test items while scanning. If you are not satisfied, scan again, and experience is the basis for a successful scan. Repeated trials are unavoidable and scan settings are made