Detailed Resolution (4)

Fourth, the role of image resolution

There are many ways to represent the resolution of an image, depending on the application. What will be discussed below is the role of resolution in various situations and their relationship to each other.

1. The role of resolution in graphic design In graphic design, the resolution of the image is measured in terms of PPI, which, together with the width and height of the image, determines the image file size and image quality. For example, an image is 8 inches wide and 6 inches high with a resolution of 100 PPI. If you keep the size of the image file constant, that is, the total number of pixels does not change, the resolution is reduced to 50 PPI, and the aspect ratio remains unchanged. In this case, the width of the image will be 16 inches and the height will be 12 inches.

The two figures before and after the printout change, we will find the latter's format is four times the former, and the image quality has dropped a lot. So, what happens when the figures before and after these two changes are sent to a computer monitor? For example, when they are sent to a display with a display mode of 800×600, we will find that the picture size of the two pictures is the same and the picture quality is not different. For a computer's display system, the PPI value of an image is meaningless. The function is the total number of pixels contained in the image, which is another resolution representation method mentioned earlier: Horizontal direction The number of pixels x the number of pixels in the vertical direction. This resolution representation method also shows the width and height of the image when it is displayed. The two figures before and after the change in the PPI value mentioned above have a total number of pixels of 800×600. Therefore, the two images with the same resolution and the same format are displayed.

2. The effect of resolution on print output

Images processed in computers sometimes print out. In most printing methods, CMYK (magenta, cyan, yellow, black) four-color inks are used to express rich and colorful colors, but the way in which colors are printed is not the same as that of televisions or photos. It uses a halftone dot. The processing method to represent the continuous tone changes of the image, unlike the latter two can directly show the continuous tone changes. In order to facilitate the understanding of the halftone dot processing method, we will analyze the black and white photos below. Looking at the photos in the newspaper with a magnifying glass, you can see that these photos are made up of black and white dots, and because the dot size is different, the photo shows a black and white tone change. So, how are these different sizes formed? The answer to this question can be found in the principles of the traditional printing process. According to the experience of the printing industry, all the LPI values ​​on the printing have the relationship with the PPI value of the original image, that is, the PPI value = the LPI value × 2 × the maximum size of the printed image and the maximum size of the original image.

In general, only by following this formula, the original image can be better reflected in the printing. The LPI value used in printing is relatively fixed, usually 75LPI for newspaper printing and 150LPI or 175LPI for color prints. Therefore, in the case of 1:1 printing, the resolution of the original image should be 150PPI, 300PPI, and 350PPI, respectively, for different purposes. In fact, most commonly used desktop printers also use halftone dot processing methods. The above formula is also applicable, but they do not use a physical screen during the printing process, but rely on mathematical calculations to achieve halftone dots. Processing. A halftone dot produced in these printers consists of a number of printed dots. The more printed dots that make up a halftone dot, the greater the range of grayscale that it can represent.

For example, to simulate 256 grayscale changes, 16×16=256 print dots are required to form a halftone dot. But on the other hand, for the commonly used 360DPI printers, the line screen at this time is just 360/16=22.5 lines, which makes the lines in the printed image very obvious, which also affects the image quality. For this reason, most printers use an 8×8 halftone pattern and the corresponding line screen is 45 LPI. According to the formula, for these printers, the resolution of the printed image should be 90 PPI.

3. The role of resolution in the television industry

In the television industry, the resolution is divided into the horizontal resolution and the vertical resolution. In most cases, the two are equal. Therefore, the horizontal resolution is generally given in technical indicators. The measurement unit TV line is also often referred to simply as line. From the previous definition, we can see that this resolution is based on the perception of the human eye, so it depends on a large amount of experimental statistics. According to the current television standard in our country, the aspect ratio is 4:3 and the number of scanning lines is 625 lines. Remove the scanning retrace period, the effective number of scanning lines is 576 lines, the corresponding effective pixel is 768 × 576 (720 × 576), so 768 × 576 (720 × 576) is also a TV image and digital image conversion standards. However, the resolution at this time can also be said to be the limit resolution of the television system, which is 625×0.7=438 lines.

It can also be seen that the number of effective pixels and the number of black and white stripes in the resolution are not a 1:1 correspondence. There are many factors that affect the resolution, and the resolution is usually estimated by the bandwidth of the luminance signal in the television device × 80 lines/MHz. For example, the video capture card that we use extensively, the bandwidth of its analog signal is the best 5MHz, so its resolution is 400 lines too. The resolution of television equipment is generally lower, and the resolution of home VHS recorders is only slightly higher than 250 lines. TVs and computer monitors cannot be compared, and the distance between TV sets (a spot diameter) is generally small. It is 0.6mm to 0.8mm, its DPI value is below 40, and the resolution of a 29-inch TV is only about 410 lines.

In general, the device resolution reflects the effect of the hardware device when processing the image, and the level of the image resolution index reflects the image clarity. Recognize the relationship between the resolution of the device and the resolution of the image. Select the appropriate device resolution value and image resolution value in the image processing to ensure the image quality, but also improve work efficiency and reduce investment.