When the designer completes a manuscript, he or she passes the file to the output center or the printing house, and the file is output on different materials, such as color proof, film or plate. The input resolution of the image will directly affect the quality of the output. To do well, you first need to understand the quality and resolution of each output device.
Pixel & Bit depth
Pixels are the basic elements of a bitmap image. The shape of the pixel is square. You only need to open a file in some image software such as PhotoShop, and enlarge the image until you see a lot of small squares. This is the pixel.
Each pixel contains the value of the color in bits. The amount of bit information that this value has (1, 4, 8 or more bits) is called the bit depth. ), the larger the bit depth value, that is, the more data data, the more detailed the image will be, the richer the color or gray scale.
A bit is a binary value, which is the smallest unit of digital information, and the content of each bit is '1' or '0'. In a meta-image, each pixel contains a bit of information, which can represent one of two states, 0 or 1. It means that the pixel is 'white' or 'black'. The binary image can represent one of four possible values, and 00, 01, 10, and 11 represent the four kinds of light and dark, 'black', 'dark gray', 'light gray', and 'white'. status. Pixels of up to octet images can represent 256 shades. As the amount of information contained in a pixel is larger, the image becomes more detailed. In general, black and white grayscale images have 256 tone changes and color has 24 bits (2 to the 24th power, which is equal to 167,000 colors) to achieve the photo richness.
The 24-bit RGB (8-bit color channel component) color image has 256 levels of red (R) components, 256 levels of green (G) components, and 256 levels of blue (B) components. As a result of the integration, the color value of each pixel in the image has a total of 16,777,216 possible values, and thus can represent more than 6,000 kinds of tones or colors. CMYK's color image is 32 bits, and each plate color occupies 8 bits of information. These 4 colors are: Cyan, Magenta, Yellow, and Black.
Image resolution
Resolution is the most confusing term in the field of digital imaging. It can be summarized into the following three categories: image input pixel number (ppi), print output points (dpi), and printed dot line number (lpi). ).
Number of lines for printing dots: lpi (1ines perinch)
This is the number of lines per inch of printing dots and is an indication of printing accuracy. In general, black and white newspapers are printed on 85 lines, while color newspapers are printed on 100 to 120 lines. Most color magazines, books, flyers and brochures will be printed on 133-175 screens. Some high-quality magazines and books will use 200 lines or higher, but note that the higher the number of screens, the higher the print quality. Because different papers have different printing capabilities or surface smoothness, for example, printing on newsprint with 150 lines is like writing on straw paper with a pen, which only leads to a significant increase in halftone dots. As a result, the subtleties are significantly lost, so the optimal number of screens for each print service should be understood. The following are some reference settings.
The number of points of the output device: dpi (dots per inch)
That is, the "number of dots per inch" is the representation of the resolution used by most people. Regardless of the resolution of the image or the output quality of the output device, we generally use dpi as the quantity. In fact, the original meaning of dpi is to describe the resolution unit of the output device. Because all output devices are made into small dots on the material to make an image, it is logical that the dpi becomes the quantity of the output device. For example, the film output machine has a resolution of 2540 dpi, which means that it can print 2,540 print points per inch of length. The following are common output machine resolutions and working settings.
Pixel unit for image input: ppi(pixels per inch)
That is, the number of pixels per inch, ppi mainly describes the resolution of input devices, such as scanners, digital cameras, etc., because various input devices eventually convert images into pixels and store them on a computer, so ppi is of course an image resolution. The unit of measurement is gone.
Scanning resolution
In the process of using the scanner, it is important to determine the scan resolution, that is, how many ppi scan images are used, so you must know the final output size of the image and the output quality of the output device before scanning. If the scan resolution is too low, there will be too few pixels in the image, making the image look rough. Conversely, if the resolution of the image is too high: the image file will be too large, affecting efficiency at work and output.
Some color publishers mistakenly believe that the more data stored in an image, the better the image quality, so all images are scanned with the highest resolution of their scanner, resulting in a large document that saves, places, and separates the color. Slow. So to understand the scanning resolution, you also need to understand the characteristics of various output devices. The following is the input resolution calculation method for the output device.
Formula: Input resolution = number of dot lines × mass coefficient × magnification
(ppi=lPi×QF×RF)
For best results, the expert proposes a quality factor of 2, which means that if the scanned image is output in the same size, its scan resolution should be twice the number of network lines it ultimately outputs. So if the number of screens used for printing is 150 lpi, then each photo copied in the same size should be scanned with a resolution of 300 ppi. In fact, the quality factor is not necessarily 2, and the coefficient is reduced to 1.8 or even 1.6, and the reduction in output quality is not easy to detect. If a large document makes your system run slowly, then a quality factor below 2 can be considered when deciding on computer scan resolution. For example, the quality factor is reduced from 2.0 to 1.6, the document size can be reduced by 36%, and the processing speed is increased accordingly. The table below shows the scan resolution for different printed network lines and different quality factor requirements.
Output resolution & graylevel
Formula for calculating the number of gray levels: gray level = (dpi / lpi) 2
On a 300dpi laser printer, if you output 60 network lines, each dot has 5 × 5 (300 / 60 = 5) print points to form, then each dot will have 25 gray scale (5 × 5=25), so if you want to get all the gray levels at the same time and keep the number of network lines high enough, you need to output on a higher resolution device.
For example, at 2400 dpi output, the printing network cable is set to 150 lines, that is, 16×16 (2400/150=16) printing dots are used to form a dot, and each dot can generate 256 gray scales (16×16=256). ), such a combination is sufficient to match the needs of the computer's 256 gray levels.
However, if 1200dpi and 150-line output are used, it means that 8×8 (1200/150=8) print dots are used to form a dot, and each dot can only produce 64 grayscale changes (8×8=64). , not enough to handle the full-tone picture. Therefore, when outputting the film, the number of dots and the number of network cables must be matched to obtain the best results. The chart below shows the combination of the best number of dots and the number of cables.
In summary, the output point directly affects the output quality and speed. Therefore, the user must pay attention to how to match the input and output points to get the best results. The gray scales that can be generated by the combination of different printed network lines and different output points in the table below. Tune.
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