Hybrid Screening Algorithms
Bruno Schrappe
Keywords: Adaptation, Graininess, Halftone, Modulation, Screening
Abstract: A computer program that creates halftone screens using both amplitude modulation (AM) and frequency modulation (FM) techniques was developed. The objective was to eliminate the characteristic graininess in flat areas found in FM screens, while still maintaining the detail rendering capabilities of frequency modulation. So as to decide where to apply each screening technique, a detail detecting program was also developed, which scans the image prior to screening. The detailed areas are screened using FM, graininess not being an issue due to the amount of detail. However, the flat or low detail areas, which are subject to graininess or objectionable patterns in FM screens, are screened using conventional AM dots. A steep transition between AM and FM screened areas is not desirable, so algorithms that blend these techniques and create “Hybrid” dots were developed and are disclosed here. The addition of noise in FM screens is evaluated and optimized, allowing for minimum perturbation, thus increasing the detail rendering capabilities of FM screens. Images generated using this technique proved to have outstanding quality, with reduced graininess and excellent detail. An alternative application for the detail detecting algorithms was found to be in determining the required amount of noise to be added to pure frequency modulation algorithms.
Detail Rendering in AM and FM Screens
Although it is true that computer-generated AM dots or contact screen dots can follow the patterns of the original image, this is only effective in very high contrast areas on the image. Early halftones created using glass screens could not reproduce as much detail as contact screens, because the size of each dot represented the average gray level of a given part of the image.
To illustrate the capabilities of each method, the target shown in figure 1 was specially designed. The target has areas with high contrast, such as part of the text, as well as intermediate contrast in the text as well. An object with increasing frequency of detail was added, with varying contrast.
Figure 1. Target to assess detail rendering characteristics
The grayscale target was converted to a halftone using frequency modulation, shown in figure 2. Most details in the original can be seen in the halftone image.
Figure 2. Frequency-modulated halftone
Figure 3 shows the same grayscale target converted to a halftone using amplitude modulation. Clearly, there is less detail in areas of lower contrast. One important observation is that the conventional dots can follow the patterns of the original as long as there is high contrast, such as in the black text. In the gray text, the edges are not as well defined as they are in the frequency-modulated screen. It can also be seen that in the gray areas, the dots still can follow the pattern of the original, but with limited results. Clearly, FM screens are more efficient in determining the boundaries of lower contrast objects.
Figure 3. Amplitude-modulated halftone
Observing the rendering of the halftone steps in figure 3, it becomes evident that the detail rendering characteristics of conventional screening depend on image contrast. The high contrast areas (black or white only) were screened in the same way as in the FM screened image.
Susceptibility to Subject Moiré
Another drawback of AM screens is the appearance of interference patterns between the image and the AM screen, also called subject moiré. This problem can appear when rendering highly detailed images such as cloth or grids.
Again, to illustrate this problem, an image with a repeating pattern was created and screened using AM and FM. Figure 4 shows the grayscale image, whereas the halftones are shown in figures 5 and 6 for FM and AM screening.
Figure 4. Target to assess subject moiré susceptibility
Figure 5. Frequency-modulated halftone with no interference pattern
Figure 6. Amplitude-modulated halftone showing subject moiré
Interference Patterns Between Colors
Although FM screens have the potential to render more detail than conventional screens, one major drawback of using frequency modulation is the objectionable interference between dots when two or more colors are printed. Since FM dots are irregularly spaced, the interference pattern will also have an irregular distribution, resulting in white noise.
Even in conventional screens, interference patterns are unavoidable, unless dot on dot registration is achieved. However, the interference in AM screens is regularly distributed (rosette), which is more pleasant to the eyes compared to the irregular one found in FM screens, also referred to as graininess. In detailed areas, the graininess is less noticeable, since it is mixed with the detail. In flat areas, or areas without a significant amount of detail, it is more visible. In these areas, a fine halftone screen may perform better, rendering smooth tones without graininess.
