"Resolution" (1)

Resolution is an important concept related to images. It is a technical parameter for measuring the expressiveness of image details. However, there are many types of resolutions with different meanings. Correct understanding of the specific meaning of resolution in various situations, and clarifying the relationship between different representation methods is a crucial step. The following describes several common image input/output resolutions and different image input/output device resolutions for your reference.
Image resolution
Image resolution: the amount of information stored in the image. This resolution can be measured in multiple ways, typically by the number of pixels per inch (PPI. The value of image resolution and image size determines the file size and output quality. The larger the value, the more disk space occupied by the image file. The image resolution affects the file size proportionally, that is, the file size is proportional to the square of the image resolution. If the image size remains unchanged and the image resolution is doubled, the file size is increased to four times the original size.
Scan resolution
Scan resolution: the resolution set before scanning an image. It affects the quality and performance of the generated image file. It determines the way in which the image will be displayed or printed. If the scanned image is used for Screen Display of 640x480 pixels, the scanning resolution does not need to be greater than the device resolution of the general display screen, that is, the scanning resolution generally does not exceed 120 DPI. However, in most cases, scanned images are output in High-Resolution devices. If the image scan resolution is too low, the output will be very rough. If the scanning resolution is too high, the digital image will generate information that exceeds the printing requirement, which not only slows down the printing speed, but also will cause the slight transition of the image tone to be lost when printing the output. In general, the image resolution should be twice the screen frequency, which is the standard adopted by most output centers and printing plants in China. However, in fact, the image resolution should be 1.5 times the screen frequency. I am afraid there will be controversy about this issue, and the specific situations of different images are indeed different. For more information, see screen angle and output resolution.
Screen Resolution
Screen Resolution (screen resolution): Also known as screen frequency, refers to the number of points per inch on the screen used to print grayscale or color-separation images. This resolution is represented by the number of lines per inch (LPI.
Bit resolution of the image
Bit resolution: Bit depth, also known as bit resolution, is used to measure the number of bits stored in each pixel. This resolution determines how many color levels can be marked. Generally, there are 8-bit, 16-bit, 24-bit, or 32-bit colors. Sometimes we call bit resolution color depth. The so-called "bits" actually refers to the square number of "2". 8 bits are the eight power of 2, that is, 8 bits are multiplied by 2, which is equal to 256. Therefore, the color level of an 8-bit color depth image is 256.
Device resolution
Device resolution: the number of points that can be generated on each inch of an output device, such as the resolution of a display, inkjet printer, laser printer, or plotter. This resolution is measured by DPI. Currently, the device resolution of a PC display is between 60 and DPI. The resolution of the printing device is between  and 1440dpi.

1. Resolution of scanners, printers, and displays
For hardware devices such as scanners, printers, and monitors, the resolution is measured by the number of dots per inch, namely, the DPI (dots per inch.
The resolution of a scanner must be determined in three aspects: Optical, hardware, and software. That is to say, the resolution of the scanner is equal to the resolution of its optical component plus the resolution obtained by processing and analyzing it through hardware and software. The optical resolution is the actual number of points of light that can be captured by the optical components of the scanner within each square inch. It refers to the physical resolution of the CCD and the real resolution of the scanner, the value is calculated by dividing the pixels of CCD by the maximum horizontal size of the scanner. The resolution of the optical part is only 400 ~ 600 dpi. The resolution of the extended part (generated by hardware and software) is produced by analyzing the image and scientifically filling the blank part (this process is also called interpolation ). Optical Scanning and output are one-to-one. What is scanned and what is output. After computer software and hardware processing, the output image will become more realistic and the resolution will be higher. Most of the scanners currently on the market have resolution-specific software and hardware expansion functions. Some scanner ads write 9600 × 9600 DPI, which is only the maximum resolution obtained through software interpolation, not the true optical resolution of the scanner. Therefore, for scanners, the resolution is optical resolution (or optical resolution) and maximum resolution.
The resolution of a scanner is as high as 4800 DPI (the 4800dpi is the sum of optical resolution and software difference processing, you can set up to 4800x4800 pixels (pixel) on a 1 square inch scanned image ). The image size generated after scanning with a resolution of 1 inch DPI is 4800 pixel × 4800 pixel. When you scan an image, the higher the Resolution, the finer the image to be generated, the larger the image file to be generated, but the more interpolation components.
We say that the resolution of a printer is  DPI, which means that when the printer is used to output an image,  color points can be printed on the printing paper per inch to characterize the image output effect. The larger the number of printer resolutions, the smaller the color point of the image output, and the finer the output image. The size of the printer's color points is only related to the printer's Hardware process, but not to the resolution of the image to be output.
We say that the resolution of a certain brand of display is 80 DPI, which means that within the effective display range of the display, the display equipment can produce 80 bits per inch on the screen. For example, if a 14 inch-inch display (with a diagonal line length of 14 inch) has a dot distance of 0.28mm, then the display resolution is 25.3995mm/inch limit 0. 28mm/DOT ≈ 90 dpi (1 inch = 25.3995mm ). When the monitor leaves the factory, the DPI value indicating the resolution of the monitor is generally not marked. Only the dot distance is given, and the resolution of the monitor can be calculated based on the above formula. Based on the calculated DPI value, we can then calculate the maximum Display Mode Supported by the display. Assume that the 14 inch display Fluorescent Screen
The diagonal line length of the valid display range is 11.5 inch. Because the horizontal and vertical display ratio of the display is 4: 3, you can set the valid display range to 4x inches in width and 3x in height, according to the math theorem, we can obtain x = 11.5 limit 5 = 2.3 inch. Therefore, the valid display range is 2.3x4 = 9.2 inch, and the vertical height is 2.3x3 = 6.8 inch. The maximum display mode is about 800 (9.2 × 90) × 600 (6.8 × 90). A dot is used to represent a pixel ).
The above mainly describes the device resolutions of scanners, printers and monitors. Strictly speaking, the device resolution is different from that of the image processed by the device. The device resolution is determined by the hardware production process. Although the resolution of some devices can be adjusted through software, they all have a limited maximum resolution, users cannot make any breakthroughs in it. The resolution of an image is a measure of the precision of the image. For images processed by scanners and printers, the resolution is measured by the number of pixels per inch, namely, the pixelsper inch. Images used for Computer Video Processing represent resolutions in the horizontal and vertical directions, such as 800 × 600, 640 × 480, and so on. Whether the image itself is fine depends only on the resolution of the image itself, but not on the resolution of the hardware device that processes the image, however, whether the image processing result is accurate is directly related to the resolution of the device that processes the image. For example, a 90ppi image is relatively fine. If it is printed on a printer with a resolution of 40 dpi, the print effect is also quite bad. For scanners, the resolution is proportional to the precision of the generated image, however, the resolution can only give an initial value for the image resolution (this PPI value is the same as the DPI setting value for the scanner resolution), and does not limit the resolution of the image, we can use the software to adjust the resolution of the images generated by the scan. In addition, it should be noted that we usually refer to an image of 640x480, which is about the image size, excluding the meaning of the image resolution.
2. Resolution of digital cameras
The resolution of a digital camera determines the final size of a high-quality image, or the size of a picture displayed on a computer display. The resolution of a digital camera depends on the number of pixels on the CCD (charge coupled device: Charge coupler) chip in the camera. The more pixels, the higher the resolution. It can be seen that the resolution of a digital camera is determined by its production process, which is fixed at the time of delivery. Users can only choose a digital camera with different resolutions, but cannot adjust the resolution of a digital camera. For similar digital cameras, the higher the Resolution, the higher the camera grade, but the data files generated by high-resolution cameras are large, high requirements on the speed, memory, hard disk capacity, and corresponding software of the computer to be processed and processed.
The horizontal level of the digital camera pixel and the size of the final resolution photo can be printed, the following method can be used for simple calculation: if the resolution of the color printer is n DPI, the horizontal pixel of the digital camera is m, the maximum number of printable photos is m inches n. For example, the resolution of a printer is 300 dpi, and the maximum size of the photo taken by a digital camera with a horizontal pixel of without interpolation ). Obviously, a digital camera with a higher pixel level is needed to print a larger digital photo. The method for calculating the display size is the same as that for printing the size.
3. projector resolution
Projector resolutions are commonly expressed in two ways: TV line (TV line) and pixel. When expressed as a TV line, the resolution is similar to that of a TV. This resolution is provided mainly to match the TV signal connected to the projector. In pixel mode, the resolution is usually expressed as 1024 × 768. In a sense, this resolution limit is required for the line frequency and field frequency of the VGA signal of the input projector. When the line frequency or field frequency of the VGA signal exceeds this limit, the projector cannot display normally. For more information about the relationship between line frequency, field frequency, and resolution, see.
4. Resolution in commercial printing
In commercial printing, the resolution is represented by lines per inch (LPI), which are arranged at the upper distance per inch. In the traditional commercial printing process, a screen must be added before the original image. The screen consists of a network cable with the same transparency and opacity. These network cables, that is, the grating, are used to cut the anatomical images of light. Light has the physical characteristics of diffraction, so after the light passes through the network cable, it forms a point that reflects the variation of the original image. These points are half-tone points. A half-tone point does not exceed the area of a grid at most. The more network cables there are, the more layers of the image, and the better the image quality. Therefore, the commercial printing industry uses LPI to represent the resolution.
5. TV resolution
In the TV industry, resolution refers to the number of black and white stripes that can be distinguished by human eyes within a distance from a fluorescent screen to an image. The unit is a TV line ).
Our country adopts the PAL standard, which requires 25 frames per second and 625 lines per frame. Due to the use of the line scan method, 625 lines of scan lines are divided into odd and even rows, which constitute the odd and even rows of each frame, respectively, in each frame, the electron beam starts scanning from the top, so there is a scanning inverse process in which the electron beam returns from the end point to the start point, during this period, invisible scan lines cannot break down the image. During the scanning inversion period, the scanning time is about 8% of the total scanning time. Therefore, only 625 rows of valid lines are used to scan the image. Therefore, the vertical resolution of the image is 576 points. According to the current 4: 3 Aspect Ratio TV standard, the image resolution in the horizontal direction should be 576x4/3 = 768 points, which gives a common image size of 768x576. In addition, when capturing computer videos, we also encounter a pal image size following the ccir601 standard, which is 720x576. For the NTSC standard that we can still access, it requires 30 frames per second and 525 lines per frame. It also uses the line-by-line scan mode, each frame is composed of two fields, the image size is 720x486.
6. Mouse resolution
The resolution of the mouse is the number of points that can be detected every 1 inch move. The higher the Resolution, the higher the quality. In the past, the resolution of the mouse was usually 100 dpi, and the current resolution of the mouse ranges from DPI to DPI. A high-resolution mouse is usually used for plotting and precise computer plotting.
7. Touch Screen Resolution
The screen resolution refers to dividing the screen into identifiable contact numbers. It is usually expressed by the number of contacts in the horizontal and vertical directions, such as 32 × 32. Some people think that the higher the resolution of the touch screen, the better. In fact, this is not the case. When using the touch screen, you should consider it based on the specific purpose. The Touch Screen with analog technology has a high resolution of 1024x1024. It can be used for screen painting and writing (handwriting recognition. In most cases, the application of touch technology only allows people to use their hands to touch the "button" of software design, and there is no need to use such a high resolution. For example, when a touch screen is used on a 14 inch display, the actual size of the display area is generally 25 cm × 18.5 cm, A 32x32 touch screen can split the screen into 1024 0.78x0.58 (smaller than a cigarette) contacts. Human fingers press the touch screen contacts more than the diameter of the cigarette, so such a contact is enough.
Difference between Dot and pixel
Dot in DPI and pixel in image resolution are two concepts that are easy to confuse. The dot in DPI can be called the smallest display unit of the hardware device, while the pixel is both a point, but also a set of multiple points. When a scanner scans an image, each sample point of the scanner corresponds to each pixel of the formed image. Therefore, the DPI value set during scanning is equal to the PPI value of the scanned image, in this case, the two can be equal. However, in many cases, the difference between the two is quite large. For example, an image with a resolution of 1 PPI is output on a 300 dpi printer. At this time, each pixel of the image corresponds to 300 x points during printing. There are also similar problems in the use of computer monitors, such as the effective display area of the 12 inch monitor is about 200mm × 160mm, if the bright spot diameter of the screen is 0.31mm, the maximum number of light points displayed on the fluorescent screen is 640 (200 bytes 0. 31) × 480 (160 bytes 0. 31), the resolution is 80 DPI. This 80 DPI is like this: 640dot notation (200mm usd25. 3995 mm/inch) ≈ 80dot/inch or 480dot notation (160mm usd25. 3995 mm/inch) ≈ 80dot/inch.
In this case, the display mode of the display card can be set to up to 640 × 480. In this case, 1 pixel is composed of 1 Dot. For example, if the display mode of the display card is adjusted to 320x200, when a 320x200 image is displayed, one pixel corresponds to four dots.

Functions of image resolution
There are many ways to represent image resolution, depending on different purposes. What we will discuss below is the role of resolution in various situations and their relationships.
1. Functions of resolution in graphic design
In graphic design, the resolution of an image is measured by PPI. The image size and image quality are determined together with the image width and height. For example, an image with a width of 8 inch and a height of 6 inch has a resolution of PPI. If the size of the image file remains unchanged, that is, the total number of pixels remains unchanged, the resolution is reduced to 50 PPI, when the aspect ratio remains unchanged, the image width will change to 16 inch, and the height will change to 12 inch. Print the two images before and after the output changes. We will find that the width of the latter is four times that of the former, and the image quality is greatly reduced. So what will happen if we send the two pictures before and after the changes to the computer monitor? For example, if you send them to a display in the display mode of 800x600, we will find that the two images have the same size and there is no difference in image quality. For a computer's display system, the PPI value of an image is meaningless, and what works is the total number of records contained in this image, that is, another resolution representation method described above: Number of vertices in the horizontal direction × Number of vertices in the vertical direction. This resolution representation method also represents the width and height of the image display. For the two images before and after the change of the PPI value, the total number of dimensions is 800 × 600. Therefore, the two images with the same resolution and width are displayed. You may try this example.
2. Effect of resolution on printed output
The image processed in the computer sometimes needs to be printed. In most printing modes, CMYK (red, green, yellow, black) Four Colors of ink are used to show a variety of colors, but the way the color is printed is different from that of TV and photo, it uses a half tone point processing method to express the Continuous Tone Changes of the image, unlike the latter two can directly show the changes of the continuous tone. In order to facilitate understanding of the processing method of Halftone points, we will analyze the processing of black and white photos below. When you carefully observe the photos in the newspaper with a magnifier, you can find that these photos are made up of black and white points, and because of the different sizes of the points, the pictures show a change in black and white colors. How are these points of different sizes formed? The answer to this question can be found from the principles of the traditional printing process. Based on the experience of the printing industry, all the LPI values in the printing industry have such a relationship with the PPI values of the original image, that is, the PI value = LPI Value × 2 × the maximum size of the printed image × the maximum size of the original image.
Generally, only by following this formula can the original image be well reflected in printing. The LPI value used in printing is relatively fixed. Generally, 75lpi is used in newspaper printing, while 150lpi or 175lpi is used in color printing. Therefore, in the case of printing, the resolution of the original image should be 150ppi, 300ppi, and 350ppi. In fact, most of our commonly used desktop printers use half-tone processing methods. The above formula is also applicable, but they do not use a physical screen during the printing process, instead, the processing of Halftone points is achieved through mathematical computing. A half-tone point generated in these printers is composed of many print points. Obviously, the more printing points that constitute a half-tone point, the larger the gray-scale variation range it can represent. For example, to simulate a gray-scale change of 256, we need 16x16 = 256 print points to form a half-tone point. On the other hand, for commonly used 16 = 22.5 lines, which makes the lines in the print image very obvious, it also affects image quality. To this end, most printers use 8x8 half-tone patterns and the corresponding row screen is 45lpi. According to the formula, the resolution of the printed image should be 90ppi for these printers.
3. Functions of resolution in the TV Industry
In the TV industry, resolutions are classified into horizontal and vertical resolutions. In most cases, the two are equal. Therefore, only horizontal resolutions are provided in technical indicators, its measurement unit TV line is also often referred to as line. From the previous definition, we can see that this resolution is based on the human eye perception. Therefore, it can be obtained through a large amount of lab statistics. According to the current national television standards, the aspect ratio is, and the number of scanned lines is 625. During the scanning back-end period, the number of valid rows scanned is 576, and the corresponding valid pixel is 768 × 576 (720 × 576). Therefore, the number of valid rows scanned is 768 × 576 (720 × 576) it is also the Standard for conversion between TV images and digital images. However, the resolution can also be said to be the resolution of the TV system, which is 625x0.7 = 438 lines.
It can also be seen that the number of valid stripes does not correspond to the number of black and white stripes in the resolution. There are many factors that affect the resolution. Generally, the resolution is estimated based on the frequency band width of the Brightness Signal in a TV device × 80 lines/MHz. For example, the most widely used video capture card has the best bandwidth of its analog signal, that is, 5 MHz, so its resolution is 400 lines. In general, the resolution of TV equipment is relatively low. The resolution of home VHS Video Recorder is slightly higher than that of 250 lines, and the TV and computer display cannot be compared. The distance between the TV set is equal to the diameter of the light point) generally 6mm ~ The DPI value is 0.8mm, and the resolution of a 29 inch TV is only about 410mm. It is worth mentioning that some foreign manufacturers claim that the horizontal resolution has reached 800 lines in the TV product promotion, which is purely nonsense. If a TV image needs hard copy output, almost all software sets the resolution of the corresponding digital image to 72ppi, which also shows the quality level of the TV image.
In general, the device resolution reflects the effect of hardware devices when processing images. The image resolution indicators reflect the quality of image resolution. Recognizing the relationship between device resolution and image resolution, and selecting the appropriate device resolution value and image resolution value in image processing can not only ensure image quality, but also improve work efficiency and reduce investment. In our work, we should pay attention to accumulating experience in this area.