Why FM screening technology is currently difficult to spread

FM screening, also known as random screening (usually called "FM screening"), that is, all halftone micro dots have very small and the same size, each dot is as small as a single element of the recording device, and is a graphic recorder or The minimum number of dots that the printer can produce, but the average number (or frequency) of dots in the surface area of ​​the image varies with the value of the copied tone. In addition, their spatial distribution is carefully assigned by an algorithm, and the points are distributed according to the statistical estimates of hue and the details of the neighboring parts of the image, without significant accumulation or accumulation of unwanted micro-points. In fact, the dot position is based on "calculated randomness." In conventional AM screening (commonly referred to as "AM screening") method, the distances between the dots are the same and the dot sizes are different; the FM screening changes the distance between the centers of the dots and the dot size according to the brightness and blackness of the input pixels.

The concept of FM screening was proposed many years ago, but until 1993, lithographers were able to get their commercialized products. Since the first manufacturer announced its FM network products in the spring of 1993, many new manufacturers have also introduced themselves. The corresponding product. Although the merits of FM screening are commendable, the actual use of this technology has been slow. Many printing companies are very interested in FM screening, but few printers and plate makers make this process a fixed part of production. .

As a newer technology that appears later than the AM screening technology, FM screening technology has many advantages:

1 The very small dot image generated by FM screening looks smoother, enhances the boundary definition, and can show more detail;
2 The image file size is 25% for AM screening files. (Same image quality);
3 will not appear screen moiré, the color definition is more clear;
4 low scan and lower output resolution;
5 Faster printing on printers that do not need to be adjusted;
6 easier to maintain the ink balance;
7 thicker ink coverage can get a higher dynamic range;
8 Short drying time;
9 More uniform ink distribution in thinner layers;
10 registration problems do not affect the color balance;
(11) The screen color version of the screen plus the overprint of the original color version will not appear moiré, and high-fidelity color (HiFi color) and spot color printing plates can be realized to improve the color gamut of the printed matter;
(12) Elimination of midtone tone jumps.

Because of these advantages, FM screening technology is considered a better alternative to traditional AM screening. However, looking at the current situation in China, we will find that the application of frequency modulation and screening technology is still thundery and rainy. To make full use of the potential advantages of FM network technology, it still takes a long and difficult exploration, due to the frequency-modulated screening algorithm itself. The reason, as well as other factors, make it difficult to popularize FM screening technology:

1. Network expansion. Dot expansion occurs around the perimeter of the outlets, but a large outlet is divided into smaller outlets. The perimeter and area ratio increase. The larger the perimeter, the more the outlet expansion value increases. For any given outlet density, the perimeter in the FM network is It is larger than the perimeter in traditional screens. The extra dot gain is an inherent characteristic of small dots. The perimeter of dots in the middle tone is larger than the one-quarter tone, and thus the dots are also expanded. In the 200l screen, the perimeter of 40% of the outlets increased by 1μm, and the outlet size increased by 1%.

The dot enlargement range is in the range of 18%-25% in the halftone image according to the SWOP (web offset press standard). The use of FM screen images will have particularly large dot gains, ranging from 25% to 35% on coated paper and up to 50% on non-coated paper. The challenge is that FM screening is difficult to overcome, although some printing machinery manufacturers provide a software-based conversion curve so that their equipment can compensate for the expected dot gain, it is to change the rendering curve of the imagesetter itself, and This is only one of the links that leads to the expansion of outlets. The basis of the compensation curve is only "expected", and it cannot always be compensated according to the specific features of each manuscript and the specific printing press, resulting in errors at the same time.

2. Has a grainy feel. The appearance of “granules” or sandpaper is the result of random distribution of FM mesh points, especially in the quarter-tone region of the screen. The amplitude modulation network is regular, so the noise generated by it is easily filtered out, and the FM network is Random, random noise is non-filterable. The ideas of the quarter-tone region are easy to notice and give a grainy appearance, but not to the extent that they merge with each other. This is more likely to occur in image regions with a dot size greater than 20 μm and a very smooth image.

Combining AM and FM in an image may be the best way to reduce graininess in the future. The smooth tone region of the FM network image is preferably an amplitude modulation network to maintain a smooth appearance of the screen. The tone transition area preferably uses a frequency modulation network to reproduce as much detail as possible, particularly in the highlight portion and the tone of about 25%. Since there is no special standard limit for manuscripts to be adapted to these two screening technologies, this method of integration is not professional, and according to some professionals, the output errors caused by this combination are more difficult to troubleshoot.