Visual noise in printed FM dot images
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
Since the first FM screening products have been available, more than a dozen Spring and Autumn Periods have passed, and its development has not replaced the AM and the Internet as the people predicted, and the actual application, especially in the domestic traditional printing field, has been slow to develop. . The reasons are manifold, both in terms of process and equipment conditions, as well as the reasons for people's understanding due to insufficient research. Naturally, there are problems with FM screening technology itself, such as FM screening, although it does not produce amplitude modulation. The moiré in the screening mode, but the visual noise (granularity) is more obvious. Where does the visual noise come from? How to detect the noise level and how to reduce the noise so that the FM network image can really approach the photo. This is a theoretical and technical problem that needs to be solved urgently.
FM distribution characteristics and advantages of FM screening
The structural characteristics of the traditional AM dot are: the inking points (exposure points) in the grid are concentrated according to certain rules; the center distance of the dots is constant and evenly distributed; and the dot level is represented by the dot size (the dot area ratio). The inking point (exposure point) of the FM network dot is randomly and randomly distributed in the grid; the spacing of the dots is randomly changed; the size of the dot is unchanged, and the frequency (number) of the dot appears in the unit area represents the tone level.
The reason why FM screening technology has received wide attention from everyone and is applied within a certain range is because it has many advantages, such as better reproduction of the layer of the original bright and dark tone, uniform reproduction of the layer, no moire. It is not limited by the angle of the dot, and the range of tone reproduction is expanded. However, FM screening technology also brings some problems to the printing process and the quality of the printed products. For example, because the dots are small, the dots are easily lost when printing, and the requirements for the plates, printing conditions and printing conditions are more demanding; The value is higher and the law of expansion is special. Moreover, the visual noise generated by FM screening has become one of the key issues limiting the promotion and application of the FM screening process.
Visual noise of FM dot images
The term "noise" or noise, originally used to characterize the sound characteristics of a radio broadcast without a sound signal. Similarly, it is also possible to describe the drawing characteristics when a certain channel has no signal in terms of transmission of a television signal. In this case, the image displayed on the TV screen is similar to the particles of the emulsion, so the term noise can be used to describe the graininess of the image, and the noise level becomes a measure of granularity. Due to the smoothness of the paper and the like, the unevenness of the micro density (small reflection coefficient) of the printed image is caused, so that the printed image has a certain degree of "roughness", that is, there is granularity. The non-uniformity of graininess or density (shading), which is the visual noise of the printed image.
The visual noise (graininess) of the printed image is not unique to the FM dot image, and the conventional AM dot image is also generated. For a long time, for the AM image, the center of attention is to prevent the moiré, and to reduce the noise less, because the visual noise of the AM image is small. The noise of the FM screen image is relatively easy to occur, and it is obvious and cannot be ignored. The visual noise of the FM dot image is easily generated in the high-key and mid-tone parts. When the coverage of the dot area is 20%, the sensitivity of the human eye to visual noise peaks, especially in the flat area of the same level. Compare the two FM image points with different noise levels. The less noisy picture should be more delicate, while the louder picture should be rougher. It is worth mentioning that the visual noise is not a moiré, not formed by optical interference between the color separation images, and the monochrome print image can also generate noise.
Analysis of the causes of noise
In general, the visual noise of the FM dot print image is larger than the AM dot image. And the same as FM screening, the image content is the same, the visual noise is also different. Although it is possible to increase the noise of the printed image when the image-printing process is small, the main factor affecting the noise level is obviously the screening method.
The generation and size of the visual noise of the FM dot image can be characterized by the dot density (the number of device pixels or ink dots per unit area) in the image, the mean value of the dot diameter change, and the average value of the dot pitch change rate. When the change rate of the dot diameter and the pitch is small, no visual noise is generated regardless of the dot density; when the dot density is large to a certain extent, although the dot diameter and the pitch change rate are large, no noise is generated, because this At the time, the human eye could not distinguish the particularly small dot structure. When the value of the dot density is less than the critical state, and the average value of the dot diameter and the pitch change rate is greater than its critical state, visual noise is generated. In this range, when the rate of change of the dot diameter and the pitch is small, the dot density is small to pink noise, and the dot density is large to blue noise. As the rate of change in dot diameter and spacing further increases, white noise occurs, which is also the most common. Since the human visual characteristics are low-pass filters, they are insensitive to blue noise and are not easily noticeable, while white noise produces a glare.
Originally, the size of the FM network is the same. How can there be a change in the diameter of the dot? The main reason is the random distribution of the FM dots. In the process of screening, the irregular distribution of the dot positions can easily cause the bonding of the dots, which causes the size and shape of the dots to change, the perimeter of the dots also changes, and the optical expansion of the dots also changes, resulting in FM outlets. Some are bonded to each other, some are separated, some are close to each other, and some are far apart, which causes the diameter and spacing of the dots to change, thereby causing noise of the FM dot image.
Possible ways to reduce noise
The structural characteristics of the FM network point are the irregular distribution of the network points. It can be seen from the above analysis that the irregular distribution of the network points will bring visual noise, so the noise reduction must be done in the direction of reducing the dot bonding and reducing the deviation of the distribution distance of the dots. After years of research, China has achieved remarkable results in reducing the visual noise of FM network images.