About standard drawings in print production
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The more common one is the ISO standard chart, so what role do these standard charts play in the printing process? Why use these figures as a standard instead of using my dog's photos as a standard picture?
In the whole process of printing, due to the actual production needs, many of our images need to be transferred or copied between many different devices, such as printers, computers, faxes, printing machines, etc., due to different equipment, then There are certain differences in the replicas we get from different devices. In the face of such differences, we need to have a way to evaluate, so we use standard images. In a nutshell, the standard chart has the following effects:
1. Provide an objective evaluation method in terms of image processing time, system performance, and image quality;
2. Evaluate the image encoding, data compression ratio, or data transmission efficiency;
3. Characterized descriptions of prints that have been printed through traditional printing processes or digitally.
Standard color images can be roughly in the form of both natural and unnatural images. Natural influences include high-resolution images such as skin color, high light color, dark color, grayscale color, hard-to-replicate tree color, memory color, and complex collective images. Unnatural images include resolution test charts, color scales, arabesques drawn from primary and secondary colors, and CMYK four-color printed images.
All standard color images have a total of two digital numbering methods. In the first encoding method, the resolution of the image is 16 pixels per millimeter (equivalent to 410 pixels per inch), the encoded value is 28 for 0% of the printed value (black), and 228 is 100% of the printed value (white). The second encoding method, the resolution of the image is 12 pixels per mm, the encoding 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 file can be placed in the CD-ROM, the file format is TIFF/IT (ISO12639)
Standard color image features and parameters
For the use of standard color images, there are several features:
(1) The size of the file can be confirmed
The smallest component of the digital influence is the bit (BIT). By borrowing the set of the number of bits, we can get an image that is completely converted or restored before and after the image processing.
(2) Color sequence
Any color point in the image, whether it is to be printed or stored in a file, can be controlled.
(3) Color values
That is, each pixel in the image is assigned a specific position and color value, which is represented by a quantization method. The usual setting is eight bits per unit pixel, that is, 256 colors. For quantized values, you can define their range.
(4) Network coverage
The ratio of the area covered by the dots in the image, ranging from 0% to 100%. It must be known that the brightest part of the image has the smallest coverage, close to or equal to 0%; the darkest part of the image has the largest coverage, close to or equal to 100%.
(5) The overall color change of the image
The relationship between the color of the selected area of the image and the color value of the rest of the image. That is, the digital image can be modified separately for the color of the particular area affected.
(6) Gray balance
In color reproduction, the three primary colors can reach neutral gray if the amount of overprinted ink reaches equilibrium. Neutral gray produces a color cast that is easily noticeable because the human eye is particularly sensitive to gray. When the image is processed, if the neutral color of the neutral color and the replica are the same, then the balance of color reproduction can be considered correct.
(7) Color scanning
A color scanner that reads light into a digital file by reflection or transmission of light on a photo. These digital files are related to the color space of the overall image.
(8) reading direction
The content of the standard color image is a right reading method when reading.
(9) Pixels
Pixels are made up of two words, Picture and Element, which are a unit for calculating digital images. Like the photos of photography, digital images also have a continuous tone of light; if you zoom in on the image several times, you will find that these continuous tones are actually composed of many small squares with similar colors. These small squares are the images that make up the image. Minimum unit: pixel.
Image resolution is expressed in pixels per inch. When an image is printed or output, the number of dots and resolution of the image determine the height and width that appear on the printed document. Therefore, for images of the same size, the higher the resolution, the smaller the printed image will be.
(10) In the pixel, the order of CMYK four-color values
The order of the combination of the CMYK four-color inks of each pixel in the image is the same as that of the adjacent pixels.
(11) Threshold value
The grayscale segmentation threshold of the image is processed separately according to his grayscale. The general grayscale segmentation divides the image into two grayscale values.
Any image that requires text recognition or stripe recognition can be used to simplify a complex image, so how to choose the correct threshold is also an important task.
Description and definition of standard color images
Image information is divided into two types of digital information encoding:
The first type: is also the main encoding method, the information value range from 28 to 228, corresponding to 0% to 100% of the dot coverage. The information is sampled in a 128mm × 160mm image, sampling at 16pixels / mm (406pixels / in).
The second, also secondary coding method, has an information range from 0 to 255, corresponding to a dot coverage of 0% to 100%. The information is sampled in a 128mm x 160mm image at a frequency of 12pixels/mm (305 pixels/in).
After the above two standard color image coding methods, the digital images produced by the computer each have natural images and unnatural images.
ISO 12640 uses the first image encoding method to generate eight natural images, the code number of which is N1-N8; and the second image encoding method produces natural images, then add "letter A" to the N1-N8 code. , that is, N1A-N8A.
Natural images include the following features:
(1) Image size
The first coding method: 2560 pixels × 2048 pixels
The second encoding method: 1920 pixels × 1536 pixels
(2) pixel color composition
In the image, the arrangement order of the CMYK four-color ink of each pixel is the same as that of the next adjacent pixel.
(3) color sequence
Cyan, magenta, yellow, black
(4) Color values
The first coding method: 8-bit binary signal, line-shaped with dot coverage percentage. When the digital signal is 28, the coverage of the network is 0%. When the digital signal is 228, the dot coverage percentage is 100%.
(5) Image reading direction
When the image signal is read, its reading direction starts from the upper left corner of the image and ends in the lower right corner.
Unnatural images contain the following features
In the unnatural image, the resolution test chart is used to evaluate the resolution, cyber phenomenon, and various color effects that can be achieved with the image output device. Color scales are applications that compare and correct colors.
Standard color images are stored on CD-ROM format
CD-ROM data format
The data stored on the CD-ROM contains 36 image file formats. The file name is taken according to the image name. Table 2 gives the file name, size, length and width, and related image names.
The TIFF (Tagged Image File Format) file format was jointly developed by Microsoft and some other companies. It can be said that it is a standard for digital image formats in the industry. It has flexibility, scalability, reliability and complementability. The more image-related vendors are adopting the TIFF file format, the more relevant tools are available. However, the TIFF file format is not without its drawbacks. Due to its compatibility and coexistence, its complexity is very high, and it is more troublesome in management or equation writing than other file formats.
Prepress personnel should evaluate the quality of digital images, and can compare the original images as the basis for measurement, thereby changing the distribution of pixels, which can often improve the quality of scanned images even beyond the quality of originals. The quality of digital images is the result of a number of factors. Image factors include: scanned image quality, scanned resolution, scanned hardware, operator skill, display resolution, gray balance, dot area ratio, resolution , tone, natural color performance and so on. Therefore, suitable image quality should take into account factors such as user requirements, system requirements, network infrastructure, and price.