The halftone screen and the principle of digital halftone

Digital halftones are generally similar to conventional halftones in that computer replaces film and halftone screens in digital halftones, and digital continuous imagery uses digital image processing techniques to "scrambling" its pixels to create halftones. A computer algorithm based on a series of numerical formulas is the core of the image halftoning, of course, some correspond to the FM network, and some correspond to the AM network. The printing industry as a whole is a pioneer in the transition from film technology to digital technology. The use of computer technology in the halftone field will provide us with more opportunities for control and experimentation.

Desktop Publishing and Postscript pioneered new ways of digital image reproduction. Digital images are often obtained with scanners, and sometimes from digital cameras. When we produce a halftone image, two factors play an important role: the quality of the scanner and the performance of the press for final output. They determine the size of halftone dots and the number of grayscale levels that are created.

The production of traditional halftones

The image that needs to be half-toned is placed on the copy plate of the camera, and a strong light is projected on it. The high-light area of ​​the image reflects more light, while the dark area absorbs more light. When the reflected light passes through the camera and is filtered by the halftone screen or the touch screen, the image is meshed and a halftone negative image of the image is formed on the film. The area where the original image reflects more light forms a large dot, and the area that reflects a smaller amount of light generates a small dot. The negative film is then printed to produce a positive image, which results in a halftone image.

Halftone screen

There are two types of halftone screens. The old screen consists of two glass plates with parallel lines drawn in them. They are glued together and those lines cross to form a suitable screen angle. However, many printers now prefer to use a touch screen made of film. However, the dots are different from those of glass screens. The sharpness of the dot in the middle area is high, and the opacity at the edge of the dot is increased.

When reflected light of different intensities passes through the screen, a halftone formed with a dot different from the size is created on each channel, and when combined with other dots, it represents a different grayscale range on the original image. If you look closely, it's not difficult to figure out that dots that represent a gray value less than 50% look like black dots on a white background, and dots that represent a gray value greater than 50% appear to appear as white dots on a black block.

The frequency of the screen refers to the number of halftone dots on the screen or image measured in 1pi. The image quality, dot quality, and line number are directly proportional to each other. High screen lines generate images containing many details, but due to the influence of other factors at the same time, the image quality has certain limitations. In addition to the mesh frequency to be considered, the printer must take into account the screen angle. Generally, for a printed continuous tone image, the visual effect is best when the screen angle is 45°.

All images in the figure have a resolution of 751pi, but the screen modules are very clearly visible at 0° and 20°, and 45° is the best screen angle for grayscale halftones, while also being used in four-color printing. The angle is generally 45°.

Creating a halftone dot matrix is ​​the basis for digital halftoning. Digital halftoning requires consideration of the number of screen lines, but it should also be noted that the output resolution of the printer is dpi. The halftone dot matrix is ​​superimposed on the output resolution grid. Screen grids are generated. A single dot matrix module can be any size determined by the grid screen grid. It can be divided into many grids with output resolution.

This division within the halftone dot pattern represents the smallest precision the printer can achieve, known as the device's pixels. With the increase of each halftone dot in-module printing module, the quality of the halftone dots will also increase. The halftone dot matrix mode must be uniform and can be more accurately adapted to other dot matrix modes. The result of the formula may be a non-integer value, but its value should be an integer, and the printer's precision can not be less than the device's pixels. Therefore, the dot matrix creation depends on the device. The Postscript language of interpretation.