Thick Cardboard Gift Boxes Visual noise in printed FM dot images

Thick Cardboard Gift Boxes Visual noise in printed FM dot images

Since the advent of the first FM network products, more than a dozen spring and autumn have passed, its development is not as predicted by the relevant people to replace the AM network to unify the world, practical applications, especially in the domestic traditional printing field of practical application development is slow. The reasons are many, both in terms of process and equipment conditions, but also due to insufficient research caused by people's understanding of the reasons, naturally there are problems in frequency modulation network technology itself, such as frequency modulation network does not produce the moire in the mode of amplitude modulation network, but the visual noise (granular) is more obvious. Where the visual noise comes from, how to detect and evaluate the noise, how to reduce the noise to make the FM dot image really close to the photo, this is an urgent theoretical and technical problem to be studied and solved.

The distribution characteristics of frequency-modulated network and the advantages of frequency-modulated network

The structure characteristics of the traditional amplitude modulation dot are as follows: the inking points (exposure points) in the grid are centrally distributed according to certain rules; The center distance of dot is unchanged and evenly distributed. The scale level is represented by the dot size (dot area rate). The inking points (exposure points) of frequency-modulated dot are randomly distributed in the grid. Dot spacing varies randomly; The size of the dot does not change, and the frequency (number) of the dot appears in the unit area represents the level of the tone.

The reason why the frequency modulation screening technology has been widely concerned by everyone, and has been applied in a certain range, is that it has many advantages, such as better reproduction of the level of light and dark tones of the original manuscript, uniform reproduction of the level, does not produce moire, is not limited by the Angle of the dot, and expands the scope of the step tone reproduction. However, FM screen technology also brings some problems to the printing process and printing quality, such as because the dot is small, it is easy to lose the dot when printing; The requirements for plate material, plate printing conditions and printing conditions are harsh; The dot expansion value is high and the expansion law is special. In addition, the visual noise generated by FM screening has become one of the key problems that limit the popularization and application of FM screening technology.

Visual noise of FM dot images

The term "noise", or noise, was originally used to characterize the properties of sound in the absence of a sound signal during radio broadcasting. Similarly, it can also be used in the transmission of television signals to describe the painting characteristics of a channel without a signal. In this case, the image displayed on the television screen resembles the particles of the photographic emulsion, so the word noise can be used to describe the graininess of the image, and the noise level becomes a measure of graininess. Due to the smoothness of the paper and other reasons, the micro-density (micro-reflection coefficient) of the printed image is not uniform, so that the printed image has a certain degree of "rough" feeling, that is, the existence of granularity. The inhomogeneity of granularity or density (light and dark), which is the visual noise of the printed image.

The visual noise (graininess) of printed images is not unique to FM dot images; traditional AM dot images can also be produced. For a long time, the center of attention for AM dot images is to prevent moire, and less attention is paid to reducing noise, because the visual noise of AM dot images is small. The noise of FM dot image is easy to occur, and it is obvious and cannot be ignored. The visual noise of FM dot image is easy to produce in the high tone and the middle tone. When the dot area coverage is 20%, the sensitivity of the human eye to visual noise peaks, especially in the same level of flat network area is easier to produce. Compared with two FM dot images with different noise levels, the less noisy picture is more delicate, and the noisy image is rougher. It is worth mentioning that visual noise is not moire, is not formed by optical interference between color separation images, monochrome printed images can also produce noise.

Analysis of the causes of noise

Generally speaking, the visual noise of FM dot print images is greater than that of AM dot images. And the same FM network, the image content is the same, the visual noise is different. Although the small printing process of the image may also increase the noise of the printed image, the main factor affecting the noise level is obviously the screening method.

The production and magnitude of visual noise in an FM dot image can be characterized by the point density in the image (the number of device pixels or ink points per unit area), the average value of dot diameter change and the average value of dot spacing change rate. When the dot diameter and spacing change rate are small, there will be no visual noise regardless of the density. When the point density is large to a certain extent, although the dot diameter and spacing rate of change is large, it will not produce noise, because the human eye can not distinguish particularly small point structures at this time. When the value of the point density is less than the critical state, and the mean value of the dot diameter and spacing rate of change is greater than its critical state, visual noise is generated. In this range, when the rate of change of dot diameter and spacing is small, the point density is small pink noise, and the point density is large blue noise. As the dot diameter and spacing rate of change further increase, white noise appears, this situation is also the most common. Because the visual characteristics of the human eye belong to a low-pass filter, it is not sensitive to blue noise and is not easy to detect, while white noise will produce a dazzling feeling.

Originally FM dot size is the same, how can there be dot diameter change? The main reason lies in the random distribution of FM nodes. In the process of screening, the irregular distribution of the location of the network is easy to cause the bonding of the network, resulting in the change of the size and shape of the network, the change of the perimeter of the network, and the change of the optical expansion of the network. As a result, some of the frequency-modulated network points are bonded to each other, some are separated, some are close to each other, and some are far apart, so that the diameter and spacing of the network points are changed. This causes noise in the FM dot image.

Possible ways to reduce noise

The structure of FM nodes is characterized by the irregular distribution of nodes. From the above analysis, it can be seen that the irregular distribution of nodes will bring visual noise, so efforts should be made to reduce the adhesion of nodes and the deviation of the distance of dot distribution. After many years of research, China has achieved remarkable results in reducing the visual noise of FM network images.