Printing and Screening Process Talk--FM Networking (3)

7. The number of equivalent network lines of the FM network

There is no obvious relationship between the number of network lines in the traditional network and the resolution of the teletext recorder that produces the FM network. If the 12μm site is equivalent to a 150-line screen, this is just a repeated subjective evaluation that is widely recognized.

8. Pit etching FM film

If the dot size is too small, it is difficult to ensure the accuracy of the dot-flip point and the volatility of the soft spot of the graphic recorder, making it difficult to control the dry or wet output.

9. Error diffusion and threshold

Error diffusion is part of the process of randomizing computing sites. When imaging any particular pixel, it can only be black or white, no other form. Error diffusion is to add the difference between the black and white value of the printing dot and the gray value of the image pixel it represents to the adjacent image pixels, so that the dot error is compensated when imaging its neighboring dots. The "error" in the error diffusion refers to the difference between the black and white values ​​of the printed dot and the gray value of the image pixels.

For example, the current image gray value of the image pixel is 113, the threshold is 127, the printing dot will be inked, its value is 0, and the error is 113 (113-0). The next pixel has a gray value of 120. This grayscale value plus the previous error value gives 233. The printing idea is none and its value is 255. The error is 22 (255-233).

The threshold is used to determine the presence or absence of printing points. Although the image is a smooth midtone, the printed dots can only be present or absent to accurately reproduce the grayscale. The gray value of the current image pixel is compared with a certain threshold, and the point is determined as yes or no according to whether the gray value is larger or smaller than the threshold. The threshold can be a single value for each pixel, or it can be a list of values.

Due to the fact that the size of the FM-screened image file is sometimes smaller than conventional screening, it is not possible to use images for FM screening in conventional separations. Images scanned for regular color separations are easily resampled for FM screening, and the highest resolution can be used for FM screening. Although the resolution improves, there are some benefits, but processing time increases.

Prior to rasterization, the image transfer effect must be considered to compensate for the excessively large dot gain inherent in the FM network. The same process as conventional screening. No special settings or adjustments for outlets are required.

Before the image is ready to be delivered to the RIP, there is no need to make a decision whether to screen it in a random way or in a traditional way. Change the amount of ghost mask to achieve the best image quality based on the selected screening type.

10. "Particles" in the "quarter tone" area

The appearance of “particles” or sandpaper is the result of the random distribution of FM mesh points, especially in the quarter-tone areas of the screen. The human eye can filter or ignore certain types of noise. The amplitude of the AM network can be easily filtered out, so it can not be noticed in normal observation environments. Because the amplitude modulation network is regular, it can be filtered, but the FM network is random and the random noise is wood filterable.

The ideas in the quartertone region are easy to notice, giving a grainy appearance, but not yet to the extent that they merge with each other. This is more likely to happen when the point size is larger than 20 μm and in very smooth image areas.

This phenomenon can be weakened by using smaller micro-points and soft soft originals. Now, some manufacturers are trying to use the amplitude modulation network in the FM network image to reduce this problem. Use amplitude modulation networks in areas where there may be granular FM networks or other undesirable artifacts.

Some of the problems seem to be related to the algorithm that determines the connection of the micro points. By stretching the contrast of the quarter to the key region of the midtone transition, a more accurate calculation of the transition curve will smooth this transition.

Combining AM and FM in an image may be the best way to reduce graininess in the future. It is preferable to use an amplitude modulation network in the smooth tone region of the FM network image to maintain a smooth appearance of the screen. The tone transition area is best to use a frequency modulation network to copy as much detail as possible.

11. Dot expansion

The dot expansion takes place around the outlets. When a large network is divided into a number of small outlets, the ratio of perimeter to area increases, and the larger the perimeter, the more the outlet expansion value increases. For any given dot density, the perimeter in the FM network is larger than the perimeter in a traditional screen. Extra dot gain is an inherent feature of small dots.

The median length of outlets in the middle tone is larger than that of the quarter tone, so the number of outlets increases. In the 200-line screen, the perimeter of 40% of the outlets increased by 1μm, and the size of outlets increased by 1%.

12. Transfer curve

The transfer curve is a mathematical conversion factor that adjusts the ratio of the input value to the output value. This transfer curve is used after performing all color corrections before rasterizing the image. When using the transfer curve to adjust the dot gain on the printing press, the percentage of each dot in the file minus the corresponding amount calibrated by the curve, for example, according to the curve, when the input value is 50%, the output value should be 32% when the file is When grid processing is performed, those dots that should be printed at 50% will be duplicated on the film as 32% dots. On the printing press, 18% of outlets will increase 32% of outlets to the desired 50% of outlets.

In the process of establishing the transfer curve, a signal strip consisting of at least 10 stages is rasterized and printed, and the density of each level of the signal strip is measured on the printed substrate and the expanded value is injected. For example, the image should be 50%. In places where the value was 73%, outlets expanded to 23%; in places where 50% of the curve was passed, there was a reduction of 23%. This procedure was done at each level of the ladder.

The correct transfer curve is very important for the successful use of the FM network. Specialized production processes use specialized transfer curves. It is very difficult to use the manufacturer's transfer curve as a "black box". (to be continued)