Resolution and image quality
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Images are an important means of expressing and transmitting information and are the main form of mass media. Images that are published as printed desktops are mainly obtained in the following ways: digital camera shooting, optical library, online downloading, and a commonly used and important method is scanned by a scanner. No matter which way we get the image, we have to make it suitable for printing.
This paper focuses on the relationship between resolution and image quality.
1. Resolution representation
PPI (Pixei Per Inch) is the number of pixels per inch, which is the number of pixels that can be actually sampled per inch of the input device. To continuously adjust the photo to be recognized and processed by the computer, it must be digitized, and the pixel is the basis of the digital image. Each pixel is independent and has its fixed position and specific value. The word pixel can be used to describe several different phenomena: how much information can be captured by the scanning device per inch (input or scan resolution), the total number of information in a computer digital image (image resolution), and the computer monitor can The number of independent horizontal and vertical elements (screen resolution) displayed simultaneously. Therefore, the resolution of some input devices (such as scanners) is generally expressed in terms of "PPI". When you see "DPI" in the scan interface, you should think of it as "PPI." This is because when describing a digital image, the pixel is the basis rather than the point. This should be noted.
DPI (Dot Per Inch) is the number of dots per inch, which is the number of points that can be recorded per inch of the output device. Many magazines and some scanning software interfaces still use DPI to describe the scanning or input resolution, but technically, the number of dots per inch is used to describe the input device resolution, such as inkjet printers, laser imagesetters, and PostSeript lasers. The resolution of the printer.
LPI (Line Per Inch) is a unit of measurement of the number of screen lines, which refers to the number of screen lines per inch. The printing process is a binarization process, which uses a screening method to simulate various gray tones and color tones, so that the continuously adjusted images are finally represented by the size (or density) of the printed dots.
2. Scanning resolution
The resolution of the scanner is divided into optical resolution and interpolation resolution.
"Optical resolution" refers to the amount of actual information that can be sampled by the optical system of the scanner. For example, a scanner that can accept raw images up to 8.5 inches wide has a 5100-element charge-coupled device array with a maximum horizontal optical resolution of 600 PPI (5100/8.5 = 600 PPI), at which point the pixel size is 1/600 inch. The vertical resolution is primarily dependent on the mechanical design of the stepper device in this direction. Currently, many scanners have a vertical resolution that is twice the horizontal resolution. The CCD stepping mechanism of this type of scanner adopts the "half-step" method, that is, the stepping motor advances the distance of half a pixel at a time, and the scanner samples the pixels and performs mathematical calculations. In general, horizontal resolution represents the primary optical performance of the scanner.
"Interpolation resolution" refers to the resolution that the scanner obtains according to a certain algorithm. Interpolation resolution does not increase the new details of the image. When the amount of information in an image is too small, the best way is to rescan and try to avoid using interpolation resolution.
The image file size is directly related to the resolution. The file size is calculated as: file size (M) = length (inch) × width (inch) × resolution 2 (DPI) × D / 8 × 1042. The D value in the formula is related to the image storage mode. When the image is a white line, D=1; when the image is grayscale, D=8; when the image is RGB, D=24; when the image is CMYK When formatting, D=32. Some people mistakenly believe that the larger the image resolution, the better. In fact, too high scanning resolution will cause the file to be too large, which will increase the image processing time. In addition, the output capability of the output device is limited, so redundant scanning resolution is wasted. Excessive scan resolution also affects the size of the image displayed on the screen, resulting in operational inconvenience. Also, do not scan at an input fractional rate that is higher than the maximum optical resolution of the scanning device. Scanning with interpolation resolution actually degrades the sharpness and contrast of the image. So choosing a good scanner or extension is the best way to get a good image.
3. Screen (monitor) resolution
Some designers and producers always like to display images on the screen at a percentage of 100%. If you find that the images are not clear enough, you may not have enough resolution or clarity. The screen resolution refers to the total amount of information that can be displayed on the computer screen at one time (for example, 640 × 480 pixels, 800 × 600 pixels) or the number of pixels displayed per inch of the display in the horizontal direction (for example, 72 PPI or 96 PPI), the display The resolution only affects the convenience of the user when processing the image, and does not affect the output quality of the image. The size of the pixels on the display is fixed. This is different from the scanning device. The display displays all the pixels of each image in a fixed pixel size. The same size of the original is scanned at different resolutions. The resulting image is obtained. The size seen on the display is different. An image scanned at 350 DPI accuracy is displayed on the display with a size of 100% of the actual image size of 350/72, which is approximately 5 times the size. If you have chosen a good scanner or extension, you don't have to worry about it.
4. Output device resolution
The output device resolution is the resolution of a device such as a laser imagesetter or printer, and is also represented by DPI. For example, the resolution of a black and white printer can reach 600DPI, and the resolution of a hair machine can reach 3386DPI and so on. In fact, the printing copy method, the number of screen lines, and the resolution of the output device add up to determine the final resolution of the image.
5. Factors affecting scan and output resolution selection
If an original is to be reproduced by printing, it is very important to select and match the scanning resolution, the number of screen lines, and the resolution of the output device. Let's talk about how to choose the right resolution for different manuscripts, output, printing, and printing regulations.
Continuous adjustment of image resolution selection
There is no universal input resolution suitable for all images during the scan, and the ultimate goal of scanning is to obtain the right amount of information to meet the output requirements. Therefore, different scanning resolutions should be selected according to actual conditions and requirements.
1 scan resolution and number of screen lines
The number of screen lines is generally equal to the resolution of the digital image, that is, each pixel of the digital image is output to the film, but the above rules are only suitable for horizontal and vertical screening (ie, the angle of the network line is 0 or 90 degrees). When the angle of the network cable is not equal to 0 or 90 degrees, in the case where the pixel is insufficient, the least ideal is 45 degrees: assuming that the digital image is 10 pixels in the vertical and horizontal directions, the image length is L, The resolution is R=10/L. When the screen angle is 45 degrees, the diagonal length is 1.414L, and the number of pixels is 10 in this length. The resolution is 10/1.414L=0.707(10/ L) = 0.707R. Obviously, when the screen angle is 45 degrees, the image resolution is not enough in the diagonal direction, so the image resolution needs to be increased by 1.414 times (about 1.5 times) regardless of the gray image or the color image. The desktop publishing system Scanning originals usually uses 2x magnification, which is called the quality factor.
The scan resolution can be expressed by the following formula:
Scan Resolution (DPI) = Screen Lines (LPI) × 2 × Image Magnification (The precondition for this formula is that the optical resolution of the scanner is high enough).
For example, when you use 175LPI printing, if the magnification is 4 times, the scanning resolution is: 175 × 2 × 4 = 1400 PPI.
2 output device resolution and number of screen lines
The higher the number of screen lines, the finer the image and the better the quality. However, the number of screen lines should not be too high, otherwise the paste will be reduced due to the expansion of the dots during printing or proofing, but the quality will be reduced. Therefore, the number of screen lines should also be determined according to the paper used in printing, the condition of the printing machine, and the process.
The traditional photographic screening technology divides an image into a plurality of small squares of the same area through a screen, generates different luminous fluxes according to the brightness difference of the original, and finally forms dots of different sizes in the divided small squares. The digital screening technology adopts different methods. The image in the page adopts different methods by output resolution and screening technology. The image in the page is matched by the output resolution and the number of screen lines to generate a photo-like network. The grid dot grid lattice of the grid, when generating each dot, is controlled by the output of the output device (laser imagesetter) to record whether the spot is exposed on each cell in the grid dot matrix.
The so-called dot grid lattice is a dot matrix composed of a plurality of exposable dots, as shown in Fig. 1 is a 3 × 3 dot matrix, which is composed of 9 exposable dots. Whether the dots in the dot matrix are exposed depends on the gray value of the pixels in the digital image. Because the point has only two states of exposure and no exposure, that is, each exposable point can only represent two values of black and white. So a 3×3 dot matrix can only represent 9+1+10 different grayscale values (including whites where all dots are not exposed).
In order to increase the number of gray levels of the output image, it is necessary to increase the dimensionality of the lattice (n × n order lattice, n is called the dimension), then the gray level number N can be obtained by the following formula: N = n × n.
Since the output resolution is fixed or divided into a limited number of levels, the size of the grid lattice depends on the number of screen lines: n = output device resolution / number of screen lines. For example, on the output device of 3386DPI, using the number of screen lines of 175LPI, the number of gray levels generated is: gray level = (3386/175) 2, which is 361 gray scale, which can fully meet the printing level. Requirements.
It can be seen from the above formula that when the resolution of the output device is constant, the number of screen lines is inversely proportional to the number of gray levels, and the number of screen lines is inversely proportional to the number of gray levels. When the number of screen lines increases, the dots are small, and the level of detail of the printed products is much, but the dot matrix dimension of the dots is reduced, and the gray level of the printed products is also reduced; when the number of network lines is reduced, the dots become larger, and the image is larger. Relatively rough, but the dot matrix increases, and the number of grayscales in the print increases. In order to obtain high-quality prints, large gray scales and screen lines are often used, and high-resolution output devices are the best choice.
6. Printing conditions and resolution selection
In theory, the higher the number of screen lines, the finer the dots, and the more layers that reflect the image. However, in actual printing, it is difficult to print when the number of screen lines exceeds 200 LPI, and high-quality coated paper, fine particles of ink, and a PS plate with high resolution must be used. For example, when the number of screen lines is too high and printed on rough paper, the following situations occur: First, in the high-profile area of the image, the ink cannot be printed on the paper because the dots are too small, and the picture loses high-level Second, due to the increased amount of ink absorbed by the rough paper, the dark-adjusted dots are confused and the dark tone level is lost. It can be seen that the selection of the appropriate number of screen lines must also be determined according to the equipment, materials and processes used for printing.