Discussion on Standard Color Image of Digital Information before Printing

For today ’s prepress digital information, standard digital image files are stored or opened using many different specifications and can be transferred from one software to another, or from one platform to another It is already a necessary trend.

These images can provide objective evaluation methods in terms of image processing time, system performance, and image quality when faced with color output methods such as printing, proofing, fax, etc .; they can also encode images and data Evaluation of compression ratio, or data transmission efficiency; in addition, for the printed matter through the traditional printing process or digital output directly, it can be characterized (characterization). There are two methods for judging the quality of the color reproduction system. One is to make a subjective evaluation of the final output image (favorable color); the other is to make an objective evaluation based on the measurement results of the color measuring instrument.

Standard color images can be roughly divided into two types: natural images and natural images. Natural images include high-resolution images such as skin tones (fleshtones), high-brightness colors, dark colors, achromatic colors (neutralcolor), difficult-to-copy tree colors, memory colors (memorycolor), complex geometric shapes, and so on. Unnatural images include resolution charts, colorcharts, arabesques drawn by primary and secondary colors, and CMYK four primary colors printed images.

All standard color images are divided into two digital encoding methods. In the first encoding method, the resolution of the image is 16 pixels per millimeter. The encoded value is 28 for 0% of the printed value (black), and 228 for 100% of the printed value (white). In the second encoding method, the resolution of the image is 12 pixels per millimeter. The encoded value is 0 for 0% of the printed value (black), and 255 for 100% of the printed value (white).

After the above encoding process, the resulting digital information can be accessed in the CD-ROM, the file format is TIFF / IT? N (ISO12639).

Features of standard color images

For the use of standard color images, there are the following characteristics:

(1) Confirmable data volume

The minimum composition unit of a digital image is bits. By adding up the number of bits, you can confirm whether an image is completely converted or restored before and after image processing.

(2) Color sequence

Any color point in the image, whether it is to be printed out or stored in a data file, can be controlled.

(3) Color value

That is, each pixel in the image is assigned a specific position and color value, which is expressed using the quantization (Numeric) method. The usual setting is 8 bits per unit pixel, which is 256 colors. For the quantified value, the range can be defined.

(4) Outlet coverage ratio

The ratio of the area covered by the dots in the image, ranging from 0% to 100%. It is worth noting that the dot coverage of the brightest part of the image is the smallest, close to or equal to 0%; and the dot coverage of the darkest part of the image is the largest, close to or equal to 100%.

(5) Color changes of the whole image

The relationship between the change of the color value of the selected area in the image and the color value of the rest of the image. That is to say, the digital image can be modified separately for the color of a specific area in the image.

(6) Gray balance

In color reproduction, if the ink volume of the three primary colors reaches the balance, neutral gray can be achieved. Neutral gray produces a color cast, which is easy to detect because the human eye feels particularly sensitive to gray. After the image is processed, if the neutral color and the neutral color value of the copy are the same, the hue balance of the color copy can be regarded as correct. [next]

(7) Color scanner (source of color signal)

Color scanners can read photos into electronic digital signals through reflection or transmission of light on the photos. These digital signals are related to the spatial area of ​​the overall image.

(8) Reading direction

When reading the content of a standard color image, it is a right-reading method.

(9) Pixel

Pixel (Pixel) is composed of two letters Picture and Element, is a unit used to calculate digital images. Like photographs, digital images also have continuous shades; if you magnify the image several times, you will find that these continuous tones are actually composed of many small square points with similar colors. These small square points constitute the image The smallest unit is "pixel".

The image resolution is expressed in terms of pixels per inch, pixels perinch, or ppi). When the image is printed or output, the number and resolution of the image will determine the height and width that appear on the printed file. Therefore, for images with the same dot size, the higher the resolution, the smaller the printed image.

(10) The order of the composition of the cyan, magenta, yellow and black values ​​in the pixel

In the image, the combination of the cyan, magenta, yellow, and black inks of each pixel is arranged in the same order as the next adjacent pixel.

(11) Threshold

The threshold of the gray scale of the image is to group the image according to its gray scale. The general gray scale divides the image into two gray scale values.

Any image that requires text recognition or streak recognition can use this method to first simplify a complex picture. Therefore, how to select an appropriate and correct thresholding value (thresholding value) is an important task.

Description and definition of standard color images

The image information is divided into two types of digital information encoding analysis methods:

The first and main encoding method has a range of information values ​​from 28 to 228, corresponding to 0% and 100% network coverage. The sampling of information is in the image of 128mmX160mm, with 16pixels / mm as the sampling frequency (406pixels / in).

The second is also a secondary encoding method. Its information value range is from 0 to 255, which corresponds to 0% and 100% network coverage. The sampling of information is in the 128mmX160mm image, with 12pixels / mm as the frequency sampling (305pixels / in).

Through the above two standard color image coding methods, the digital images generated by the calculator each have a natural image (phto-graphed) and an unnatural image.

ISO12640 uses the first image encoding method to produce eight natural images, whose codenames are N1 ~ N8; and the second image encoding method generates natural images, adding "A" to the N1 ~ N8 codenames. That is N1A ~ N8A.

Unnatural images include resolution test charts and swatches. The unnatural images produced by the first image coding method are coded S1 ~ S10; the unnatural images produced by the second image coding method are added "A letter" in the code name, namely S1A ~ S10A. [next]

Natural images include the following characteristics:

(1) Image size

The first encoding method: 2560pixels (length) X2048pixels (width)

The second encoding method: 1920pixels (length) X1536pixels (width)

(2) Pixel color composition

In the image, the combination of the cyan, magenta, yellow, and black inks of each pixel is arranged in the same order as the next adjacent pixel.

(3) Color sequence

Cyan, Magenta, Yellow, and Black.

(4) Color value

The first encoding method: 8-bit binary signal is used to linearly match the dot coverage percentage. When the digital signal is 28, the percentage of network coverage is 0%. When the digital signal is 228, the percentage of network coverage is 100%.

The second encoding method: 8-bit binary signal is used to linearly match the dot coverage percentage. When the digital signal is 0, the percentage of network coverage is 0%. When the digital signal is 255, the percentage of network coverage is 100%.

(5) Image reading direction

When the image signal is read, the reading direction starts from the upper left corner of the image and ends at the lower right corner.

Unnatural images include the following characteristics

In unnatural images, the resolution test chart (resolutionchart) is used to evaluate the resolution capability, netting phenomenon, and various color effects that the image output device can achieve. Colorchart is used to compare and correct colors.

Standard color images are stored in CD-ROM format (Electronicdata)

CD-ROM data format

The data stored in the CD-ROM contains 36 image file formats. The file name is based on the image name. Table 2 shows the file name, size, length and width, and related image names.

The TIFF (TaggedImageFileFormat) file format is jointly developed and determined by Microsoft and some other companies. It can currently be said to be the standard of the digital image format in the industry. More and more manufacturers related to image processing adopt TIFF file format, and more and more related tools and procedures. However, the TIF file format is not without its shortcomings. Due to its inclusive nature, its complexity is very high, and it is more difficult to manage or write programs than other file formats.


After continuous improvement of the prepress operating system, the use of video and color images has increased substantially. Coupled with the use of electronic and digital file exchanges, the development of image and color management systems has grown at an alarming rate, and it has created a trend for post-press processes. Therefore, it is even more important to understand the specifications of digital images.

The pre-press personnel must evaluate the quality of the digital image, and can compare the original picture as the measurement basis. By changing the pixel distribution, the quality of the scanned image can often be improved or even exceed the quality of the original. The quality of digital images is the result of consideration of many factors. The influencing factors include: the quality of the original scanned image, the resolution setting of the scan, the hardware equipment scanned, the skill of the operator, the resolution of the monitor, the gray balance, Dot area ratio, resolution, tone, natural color performance and other factors. Therefore, suitable image quality should take into account factors such as user needs, system requirements, network infrastructure, and price.

The evolution of digital information will significantly change the professional training of prepress workers. How to do a good job in the management and storage of digital image files, open color management systems, and network transmission applications, etc., all of which must be transformed along the trend to be able to respond to the future.

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