Q

Why is screening necessary for prepress images?

A

Because printing technology dictates that printing can only use halftone dots to reproduce the continuous tone levels of the original, if the image is magnified, it will be found to be composed of countless halftone dots of different sizes. Although the sizes of the halftone dots we see are different, they all occupy the same spatial position, because once the original image is screened, the image is divided into countless regularly arranged halftone dots, that is, the continuous tone image information is converted into discrete halftone dot image information. The larger the halftone dot, the darker the color and the darker the level; the smaller the halftone dot, the lighter the color and the brighter the level. The size of the fixed spatial position occupied by each halftone dot is determined by the screen frequency. For example, if the screen frequency is 150lpi, there are 150 halftone dots in one inch of length or width. The spatial position of the halftone dot and the size of the halftone dot are two different concepts. For example, C50% means that the size of the halftone dot occupies 50% of the spatial position of the halftone dot, 100% means that the size of the halftone dot completely covers the spatial position of the halftone dot, which is the so-called "solid" in printing, and 0% means that there is no halftone dot, only the spatial position of the halftone dot, so no ink is printed on this area. Obviously, the higher the screen frequency, the smaller the spatial position occupied by the halftone dot, the more levels that can be described, and the finer the detail. In fact, the levels and colors of the original are reproduced by this screening method.

Q

What is printing color?

A

Printing colors are composed of different percentages of C, M, Y, and K, so it is more reasonable to call them mixed colors. C, M, Y, and K are the commonly used four printing primary colors. When printing primary colors, each of these four colors has its own color plate, which records the dots of that color. These dots are generated by halftone screens. Combining the four color plates together forms the defined primary color. Adjusting the size and spacing of the dots on the color plate can form other primary colors. In fact, the four printing colors on the paper are separate, but very close. Because of the limitations of our eyes' resolution, we cannot distinguish them. The visual impression we get is the mixed effect of various colors, thus producing various different primary colors. Y, M, and C can synthesize almost all colors, but black is also needed because the black produced by Y, M, and C is impure. A purer black is needed in printing, and using Y, M, and C to produce black will cause excessive ink in some areas.

Q

Why do computer design systems require large-capacity storage? What are some commonly used large-capacity storage devices?

A

The design will use many photos or computer-generated images. Due to the needs of print output, the resolution of the images is usually high, about 300Dpi. Therefore, the disk space occupied by the images is very large. For example, a 4K imposition file usually has a total storage file size exceeding 100M. The commonly used 3.5-inch floppy disk cannot meet the storage requirements of images, so large-capacity storage devices must be used. Commonly used large-capacity storage devices include: ① Optical disk (MO) drive, with disk capacities of 230M and 460M or larger. ② Removable hard drive. ③ ZIP (100M) drive. ④ Rewritable optical disc.

Q

How to choose the correct output angle?

A

To avoid screen interference, theoretically, the angle between colors should differ by 22.5 degrees. However, practice has shown that angles of 15, 45, 75, and 90 degrees work well. Yellow has a relatively weak visual stimulus and lower visual sensitivity, so it is generally set to 90 degrees. The vision is most sensitive to a 45-degree angle, so the main color tone of the original, magenta or cyan, is generally set to 45 degrees. For example, for a picture of blue sky and white clouds, cyan should be set to 45 degrees; for a picture of sunrise or sunset, magenta should be set to 45 degrees. One of the non-45-degree angles in magenta and cyan can be set to 15 degrees, and black is set to 75 degrees.