Pocket Journal Notebook The first step before digital printing: scan color separation

Pocket Journal Notebook The first step before digital printing: scan color separation

 

1. Scanning color separation

At present, there are two types of scanners in practical application: flatbed scanner and drum scanner. The core component of flatbed scanner is CCD, that is, photoelectric coupler; The core component of the drum scanner is the PMT, or photomultiplier tube. The scanner relies on its core components to convert the optical signal of the scanned image into electrical signal; Then the electrical signal is converted into a digital signal through an analog/digital converter and transmitted to the computer. Obviously, the core components of the scanner have a great impact on the results of scanning color separation. For this reason, the quality of the scanning light source, mirror, and analog/digital converter will also have an impact on the electronic image. Therefore, the first step in the quality control of the prepress system is to calibrate the scanner.

 

The calibration principle of the scanner is to adjust the scanner to faithfully reproduce the original tone-level information, color changes and gray balance. The specific method is to use a special reflection or transmission color scale to adjust the Gamma value of the highlight, dark and intermediate values in the scanning software. Adjust R/G/B or C/M/Y/K single-channel values, if necessary, so that the electronic image's tone, color and gray balance are consistent with the color scale.

 

2. The key technology of scanning color separation

Picture scanning quality determines the final output quality of the picture, how to get the best quality of the picture, is the long-term common goal of the industry. The quality of scanning has a great relationship with the scanner, scanning software and the actual experience of the operator. The quality of control is nothing more than the original manuscript, scanning equipment, scanning parameter Settings and scanning techniques.

 

2.1 Types and analysis of manuscripts

Faithful reproduction of the original is the eternal pursuit of the industry, therefore, the original is a prerequisite and basis to ensure the quality of color reproduction.

 

2.1.1 Requirements for color reproduction of the original manuscript

① High definition. It is the basis of recording scene price adjustment, color, texture, three-dimensional sense, perspective sense, etc.

 

② Density contrast. The density contrast of the manuscript must be normal. It is generally required that the contrast density of the original is between 1.8 and 4.0, and the level of brightness, middle tone and dark tone of the original screen is relatively rich, and the distribution of coordination levels is normal. Good color saturation, realistic texture (if the dark layer is rich; The middle tone level is less, so the dark tone level must be opened and the middle tone level compressed during scanning.

 

③ Color saturation. Check the manuscript under the standard light source (50O0K), the color saturation is the true response to the subject, and only the manuscript with high color saturation can be the original with normal tone;

 

④ image granularity. The graininess of the manuscript affects the color, level, and texture. The fine granularity of the original manuscript facilitates the reproduction of layers and tones (especially the light tone texture) and the true expression of colors.

 

2.1.2 Manuscripts are usually divided into transmission manuscripts, reflection manuscripts and secondary manuscripts

① The second manuscript: that is, the scouring product; The density range is small, the level is not rich, generally only the original large scan, should not be amplified. Usually good quality secondary manuscript density range is only about 0 to 1.6. It is far less than the density recognition ability of the scanner, and the scanning of such manuscripts can only be done by ordinary flatbed scanners. The secondary original often has the problem of color deviation, which needs to be properly corrected during scanning. Need to be reminded that scanning the second manuscript must be done to the net processing.

 

② Reflection draft:

a. Photos. The density range of high-quality photos is only O ~ 2.2, which basically meets the requirements of prepress processing and printability; It is the most common manuscript in our daily work.

 

b. Physical objects.

③ Transmission draft:

a. Negative film. Usually do not scan the negative directly, it is best to develop the negative into a photo and then scan. That's because, number one, negative films don't work. Not easy to restore color; Second, the negative film base has a layer of colored light film, which will cause the negative film to have no white field and is not easy to calibrate.

 

b. Positive film (slide). It is produced by turning negative films, such as Slide 135. Its film base is not as good as the reverse film, the density range is small (0 ~ 2.8), and the particles are rough, the scanned image is also relatively dirty, its magnification can generally only be controlled at 4-7 times, and then large, the picture will be distorted.

 

c. reverse film, also known as positive negatives, the negative material used in it and the development process are different from negative and positive films, the quality is the best in the original, the density range is between 0 and 3.8, and the scanning can be amplified more than ten times, which is the most ideal original.

 

2.1.3 Analyze the manuscript

The operator should have a certain amount of experience and the ability to analyze the manuscript, in order to set the scanning parameters according to the actual situation of the manuscript, to obtain the best scanning quality, in the actual operation to pay special attention to avoid any parameters to choose automatic adjustment, which can only make the picture quality flat, and even can not meet the printing requirements. For biased pictures, if you have rich experience in color correction, it is best to correct them in different channels when scanning, which is better than correcting them in PhotoShop after scanning

 

2.2 Scanning Preparations

To get the ideal scan image, you should do the following preparations:

① Preheat. The scanner needs a few minutes to warm up. Power on the scanner 30 minutes before you start scanning.

② Clean and maintain the scanner regularly. Clean and maintain your scanner regularly according to the scanner's instructions. When the scanner is not in use, use a dust seal to cover it, and do not make the scanner doused with water.

 

③ Wash your hands before scanning, so as to avoid the contamination of the fingerprint on the scanned image. Do not use your mouth to blow dirt and dust on the glass of the scanning platform.

 

④ Clean the surface of the object or image used for scanning. Air bags can be used to blow, depending on the object to be scanned can also be cleaned with water on its surface, but to avoid damage to the object, especially be careful not to leave scratches on the surface of the photo and film. Some original works of art should be protected when cleaning.

 

⑤ Make the original smooth. If the software is used to correct the marks caused by the curl of the manuscript. It will reduce the sharpness and quality of the image. Although there are many features in Photoshop (such as blurring and sharpening methods) to process images, this is not the best way to solve the defects in scanned images.

 

⑥ Approved. Using calibration software can adjust the image and improve the 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 keeps the scanner stable.

 

2.3 Scanner input indicators and parameter Settings

The difference in the quality of the scanned image largely depends on the input indicators of different scanners (such as bit depth, resolution, etc.) and the setting of each input indicator. Practice has proved that only by correctly setting the scanning parameters of the scanner for different manuscript types, can we get the best color, level and clarity of the image and ensure high quality color reproduction. Among several input indicators of the scanner, bit depth, black/white field calibration, Gamma and other parameters mainly affect the image level, and resolution, filter and other tools mainly affect the subtle level of the image. Although the menus of most scanners are different in form, the basic control parameters are the same, there are fixed black and white field and Gamma value, tone, analysis manuscript, which is a key factor in the quality of the scan.

 

2.3.1 Resolution

Resolution reflects the richness of detail in the image recorded by the scanner and can be divided into optical resolution (physical resolution) and interpolation resolution (maximum resolution). Optical resolution is the actual resolution of the scanner, which is the real resolution (vertical and horizontal values) obtained when the scanner hardware scans, and it is a key factor in determining the clarity of the scanned image. Interpolation resolution is the resolution enhanced by software operations to strengthen the complement, which does not add new information to the image, but is very useful for images with specific requirements or when scanning line drawings. In order to obtain the appropriate image quality, the scanning resolution needs to match the number of additional cables, for full-scale reproduction, the number of additional cables (lpi) is generally multiplied by a quality factor k(1.5 ~ 2.0) to determine the best scanning resolution. For example, when the number of network cables is 175lpi; If k = 2, the scan resolution is 350dpi.

2.3.2 Bit depth

 

Description The sense number of color information of each pixel in the scanned input image is bit depth, also known as sampling depth. It is generally 24 bits, that is, R, G and B each account for 8 bits, and each has 256 gray levels; It can indicate that 2(8+8+8) = 16.78 million colors are true colors, and professional scanners should have a depth of at least 36 bits. While the current use of Photoshop can only process 24-bit images, images obtained with a scanner deeper than 24 bits need to be converted into 24-bit images before they can be processed in Photoshop.

 

2.3.3 Select the right black and white field

(1) The brightest point in the figure is the white field, which directly affects the tone level of the light tone and the middle tone, and the human eye is extremely sensitive to the change of the light tone, the setting of the highlight 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 levels of the light tone; Set too dark, the overall level of the picture is dark. For a general picture, when the white field fixed point is neutral gray, the CMYK value of the white field can be 25%, 0%, 0%, 0%. For the case that the white field is partial color, it must depend on the specific situation, and the ability to improve the judgment can be improved through continuous experience accumulation.

 

The darkest point in the picture is the black field. The setting principle of black field is basically the same as that of white field. The difference is that the highlight point can be determined through the eye, and the black field of the image requires the operator to have a certain judgment. Some scanning software can automatically identify the black field, and some also provide certain tools (such as histogram adjustment) to determine the black field, the black field is too dark, will lead to the loss of the dark level, resulting in the "union" phenomenon. And if the black field is set too bright, it will cause the dark tone to be too bright. For most normal pictures, the K content of CMYK in the black field should not be less than 70%, and generally not more than 80%.

 

In general, compare the difference between the black field and the dark level, the white field and the light level when scanning, and judge the scanning effect by comparing the values shown in the dropper, and do not easily believe the color effect displayed on the screen.

 

2.3.4 Adjusting the Gamma value

① Level tone: under normal circumstances, the picture is divided into light tone, middle and dark tone. Given the black and white field, the scale range of the whole picture is determined, but how to locate the middle tone within this range, that is, how to distribute the levels is determined by the Gamma value. In general, the tone of a picture should be distributed in the middle tone as much as possible, because people are more accustomed to looking at pictures with rich layers of middle tones.

 

Gamma value: For most scanners, there are their own default scanning Gamma experience for most images, it must be noted that different scanners have different Gamma experience values. When adjusting the Gamma curve, it has a very large impact on the overall tone of the picture, only people with a certain scanning experience can use the curve to make beneficial adjustments to the specific picture, otherwise it will be self-defeating, but rather choose its default linear Gamma curve.

 

2.3.5 Trial and error

When scanning, don't worry about making a test subject. If you are not satisfied, scan again, experience is the basis for successful scanning. Trial and error is inevitable, making subtle adjustments to the scan Settings, and then comparing the scan image results, when the ideal combination of Settings is found, it is written down on paper or saved in the software, not only to have accurate data, but also to cultivate the observation and judgment of the scan operation. [next]

 

3. Scan devices

The hardware device that must be used for scanning color separation is the scanner. Scanner is a kind of optical, mechanical and electrical integration of high-tech products, is an important tool to input various forms of image information into the computer. At present, color scan input instruments based on point-type photocell technology can provide high precision levels with resolutions in excess of 8,000 dpi, while reading a wide range of level values in the relative density range of 0.0 to 3.8 of the original manuscript. These techniques enable the original to maintain the ideal effect of natural authenticity in the process of input and reproduction.

 

Generally speaking, the scanner includes four parts, such as scanning head, motherboard, mechanical mechanism, and accessories. The scanning head (planar scanner) is composed of CCD photocoupler, and its accuracy directly affects the fidelity of the scanning image.

 

Scanner motherboard includes central processor, analog-to-digital converter, interface and other parts, mainly to complete the image data conversion, computer interface and other functions, it controls the whole working process of the scanner. The scanner has two working modes of reflection and transmission, which are used for scanning color photos and transmission originals respectively.

At present, the scanner market is dominated by several brands: Epson, HP, Canon, Zhongjing, Hanyi and so on. Since ordinary color scanners are common and are used in daily office work, they will not be elaborated here. The following will focus on film scanners.

 

3.1 Understanding film scanners

Film scanners are different from the reflective flatbed scanners we use every day to scan paper documents and photos, although they are also based on CCD sensors like flatbed scanners, but its scanning principle is the way it projects, it uses more sensitive sensors, and has higher resolution. Therefore, it is more ideal to digitize transmission originals of small size. The film scanner has a wide enough dynamic range to enable it to capture the full tonal range of a typical transmission copy. The image on the film is true and fully restored.

 

As it stands, film scanners can be divided into two broad categories: professional pure film scanners and flatbed scanners that can perform film scanning by installing a transmission adapter (TMA).

 

Professional pure film scanner is through the optical components of the projection imaging principle to work, with small size, light weight characteristics, the price from 5, 6 thousand yuan to tens of thousands of yuan. Because the price is more expensive, it is generally used in publishing, advertising production, professional photography and other fields.

 

Flat panel scanners with a transmission adapter (TMA) for film scanning: These scanners are originally imaging by reflection, but by installing a transmission adapter to change the direction of light reflection, so that they can complete the scanning of film. However, due to the impact of its own resolution, the film scanning quality of this type of scanner is not high. As much as possible as a home user some quality requirements are not high scanning.

 

3.2. Film scanner purchase

The purchase of pure film scanners is different from that of ordinary flatbed scanners.

 

1. Higher resolution requirements

At present, the resolution of 2400dpi is a very high professional level for ordinary flatbed scanners, but for film scanners, this is not the starting point. The resolution of film scanners can be basically divided into two grades: 2700dpi and 4000dpi. 2700dpi is the entry requirement for professional pure film scanners, and the current resolution of the vast majority of professional pure film scanners is 4000dpi. This is an important reason why film scanning with flat panel scanners with a transmission adapter (TMA) is not effective.

 

2. Learn about the supported scan format types

Unlike flatbed scanners, which have scanning format labels, a pure film scanner should be aware of the different film models it can support. Although 135 film is the most commonly used film, there are many other types of film. Therefore, it is convenient to use as many film models as possible.

 

3. The software function plays a key role in the scanning effect

Film is more prone to wear and dust than photographs, and the high resolution of 4000dpi can show them off. In this case, if the user to manually eliminate these defects is very time-consuming and laborious. Therefore, the pure film scanner must have the function of automatic scar removal to ensure the quality of the scan.

 

The above three points are the most critical to the purchase of pure film scanner clock, and other aspects of the user can choose through their actual application. [next]

 

4. Several file formats commonly used in digital printing

Many file formats exist for image storage, but only some are widely used as actual standards. In digital prepress technology, there are three commonly used data formats, TIFF, EPS and JPEG, and EPS and TIFF are the two basic formats that desktop publishers are most interested in. JPEG, on the other hand, affects people who spend most of their time working on the Internet or multimedia. Using other formats such as PICT, GIF, BMP, WMF, etc., is usually converted to the above three commonly used file formats before use.

 

4.1 TIFF File format

TIFF stands for Tagged Image File Format, a file format developed by Aldus and Microsoft for scanner and desktop computer publishing software to store black and white, grayscale, and color images. Now it has become an important file format in publishing multimedia CD-ROM. Although TIFF has a longer history than other file formats, it is still the most widely used industry standard bitmap file format, mainly due to the fact that TIFF's specifications have been improved many times. TIFF bitmaps can be of any size and resolution. In theory, it can be infinitely deep, i.e. 1-8 bits, 24 bits, 32 bits (CMYK mode) or 48 bits (RGB mode) per sample point. The TIFF format can encode grayscale, CMYK, indexed color, or RGB modes. It can be saved in compressed and uncompressed formats. Almost any application that works with bitmaps can handle the TIFF file format - whether it's embedding, printing, trimming, or editing the bitmap.

 

The TIFF specification allows the use of two color modes, CMYK and RGB, to split the image into four overprint colors, and save the image before the separation as TIFF format. When TIFF files are placed into a page layout design or similar program, no further color separation is required. When printing cyan plates, the program simply pulls the cyan channel; When printing red plates, simply hold the magenta channel; And so on. The TIFF format can also save indexed color bitmaps, but few people do this. For indexed color images, it is more often the choice to use the GIF format.

 

The TIFF format can contain compressed and uncompressed pixel data. The compression method (LZW) is non-lossless (the data of the image is not reduced, that is, the information is not lost during processing) and can produce a compression ratio of about 2:1, which can reduce the original file to about half.

 

The current version of the TIFF format supports high-resolution color, which divides different parts of an image into chunks, or chunks of data. For each block part, a logo is saved which provides information about what the block looks like. The advantage of blocky is that packages that support TIFF only need to save the portion of the image currently displayed on the screen. The parts of the image that are not displayed on the screen are kept on the hard drive until they are loaded into memory when needed. This is important when editing a very large, high-resolution image.

 

In TIFF files, none of the tools contain screen processing instructions. Screen processing is controlled by a program that prints TIFF files. If you want to save the screen processing instructions along with the bitmap, you must use the EPS file format. But the TIFF format is able to handle the clip path, whether it's QuarkXPress or PageMaker, to read the clip path and subtract the background correctly.

 

4.2 EPS File Format

Encapsulated PostScript (Encapsulated PostScript) format. PostScript is a page description language designed by Adobe to print files to any printer that supports PostScript. It is like Basic, C, or any other programming language, except that it is optimized for printing text and images on paper. When you work on a PostScript printer and tell a word processor (or any other application) to print a page, the computer writes a program describing the page in PostScript and sends that program to the printer. The printer actually has a fully functional computer and a PostScript language interpreter in it to execute the program, draw the graphics on virtual paper in memory, and then print them to the paper.

 

An EPS file is a PostScript file that contains header information that enables other applications to embed the file within the document. EPS files also have some limitations that do not apply to standard PostScript files. These restrictions are mainly rules to ensure that EPS files can be inserted into different files without damaging the file. For example, in Microsoft Word, you can embed an ESP file in a Word, or in a Word document. The most popular use of EPS files is to embed them in desktop publishing files, especially those created by PageMaker or QuarkXPress. Desktop color separation (DCS) was developed by Quark for Process color processing. DCS images are EPS cells or images that consist of 5 parts: a low-resolution screen preview, plus layers of cyan, magenta, yellow, and black. DCS2.0 files can include more than 4 sets of colors, and can also include a certain number of spot colors or high-fidelity color separations.

 

The EPS file format can be used to encode pixel images, text, and vector graphics. If EPS is only used for pixel images (such as selecting an Adobe Photoshop program as the output), the hanging information and the hue copy transfer curve can be retained in the file, whereas TIFF does not allow such information to be included in the image file.

 

Because an EPS file is actually a collection of PostScript language code, it can be printed in a variety of ways on a PostScript printer. Software that creates or edits EPS files can define capacity, resolution, fonts, and other formatting and printing information. This information is embedded in the EPS file, which is then read in and processed by the printer. There are hundreds of printers that support PostScript, including all the image typesetting systems used in the desktop publishing industry. Therefore, the EPS format is the file format used by the professional publishing and printing industry.

 

The EPS format is a format used for printing. The PostScript language code embedded in the EPS file provides important print definitions, but this increases the size of the file. In addition, the value and memory overhead required to build a PostScript engine into the software is also high. As a result, most Web browsers do not support EPS files, and most image viewing shareware and free software do not support EPS files. For this reason, the EPS format cannot be used for image display on Web sites.

 

4.3 PS Files

PS, short for PostScript, is a flexible and device-independent page description language that has become a standard printing technique for producing high-quality output. Since the development of PostScript by Adobe in 1985, after continuous improvement and improvement, it has launched PS2 and PS3, solving many problems such as double-byte and color prepress processing, and becoming an indispensable general programming language in today's printing and publishing industry, which can be output on any PS function print output device. Adobe has made a number of improvements to the PS3 in recent years, including smoothing out subtle shades of light and shade; Improved color gradient quality; The gradients generated in the PS2 application can be recognized with the PS3 and better quality can be obtained automatically using the PS3; Improved font technology, which can handle double-byte fonts such as Chinese characters; Expanding the color separation to a space greater than 4, not only can separate C, M, Y, K four colors, but also can separate other spot colors.

 

4.4 JPEG file format

JPEG file format - Best known as the Joint Photo graphic Experts Group standard for extremely dark images. Now, it has risen to become the main format for compressed files for print and Internet distribution.

 

An image saved in the JPEG file format is actually a mixture of two different formats: the JPEG format specification itself, which defines the compression method for the image, and is wrapped in an image data format that defines resolution and color patterns. Photoshop, and virtually every other application that can read and write to the JPEG File Format, saves image data in the JFIF file format (JPEG File Interchange Format) or some other format that closely resembles the JFIF format. The JFIF file format is just a simple way to compress an image cell or surround JPEG, and they don't do much more.

 

The original JFIF file format specification allowed 8-bit grayscale images and 24-bit RGB images; However, Adobe has modified this format so that it can also handle data in 32-bit CMYK mode. However, most layout applications can't actually separate CMYK mode JPEG images, so this change by Adobe doesn't make much sense. The JPEG file format allows the use of variable compression methods to save 8 -, 24 -, 32-bit depth of the image. For example, when saving a Photoshop image in JPEG format, Photoshop gives a variety of saving options: low compression, medium compression, high compression and the best resolution. Experiments have shown that when printed or viewed on a display, JPEG can generally compress an image to one-tenth of its original size without noticeable differences. The image is broken down into small squares of 8×8 pixel image units. This JPEG distortion is sometimes found in news images that are greatly compressed before electronic transmission and subsequently printed at high magnification.

 

JPEG uses a lossy compression format, which makes it an ideal format for displaying images quickly and saving good resolutions. It is precisely because the JPEG format can greatly compress scanned or natural images, which is conducive to storage or transmission by modem, so it has been widely used on the Internet.

 

There is a special variant of the JPEG format called Progressive JPEG. When you create a Progressive JPEG file, the data is arranged in such a way that when the image is loaded, only a fuzzy image is displayed at first, and as the data is loaded, the image gradually becomes clearer.

 

The main drawback of the JPEG format is also its greatest advantage. That is, the lossy compression algorithm limits JPEG to the display format only, and some data is lost every time you save an image in JPEG format. Therefore, it is usually only necessary to save an image once in JPEG format during the final stage of creation.

 

4.5 PDF file format

PDF is the abbreviation of Portable Document Format, developed by Adobe, can be text, font, format, color and device and resolution independent graphics, images, etc., encapsulated in a folder. The file format can also contain electronic information such as hypertext links, sound and moving images. Support for special files, high integration and security. Because PDF files can not rely on the language and font of the operating system and display devices, they can realistically show the original appearance of the file to every reader. PDF has now become a de facto standard for electronic document distribution and digital information dissemination. This is because: from the beginning, PDF has outstanding use advantages. Its files can be used on Macs, PCS, and UNIX computers. Because PDF can be linked to color management systems to solve production problems, it is also accepted by production experts. PDF can be displayed directly, so preview and proofreading can be done quickly and securely on the screen. To this end, PDF filters out a large amount of data that is not essential to the final product, making the data extremely efficient and compressed. Therefore, PDF file data transfer is quite ideal.

 

The correct PDF file should include all data like for display, for printer output, and for phototypesetter and CTP output. Each PDF page is independent, so PDF files can easily divide a file into different independent pages, which is very important for the spelling of hand plates. PDF features also make it possible to make last minute changes before output. In most cases, the output speed of PDF files is much higher than that of PostScript files. The function of Distiller, as its name suggests, is to "distillate" important data from PostScript files while also removing unnecessary instructions from PostScript files. As a result, the PostScript file output from Acrobat is smaller than the original PostScript file, and RIP is faster because PDF has been pre-interpreted. All data in PDF files, such as color images, continuous tone images, monochrome images, and text and vector images, can be compressed using different compression methods, which also means that PDF files are usually smaller than the original format files and image files.

 

Due to the application of PDF, it is possible to standardize the delivery of documents. The PDF data format is developing into a standard for page data exchange in the field of print production. PDF is the basis for using modern workflows and automatic output devices. The American Standardization Committee and the Printing Technology Standardization Committee have recommended PDF as the national standard for page delivery in the printing industry, and the International Organization for Standardization is also developing an international standard based on the PDF file format.