Of all the images, solid tones seem the most difficult to present smoothly with a halftoning pattern. Thus, simple computer graphics may be more of a challenge for a printer than cmmplex photos.
The color error diffusion algorithm can effectively limit the number of colors used for a given region. Its execution time is only margianally longer than that of regular error diffusion. The pattern produced is slightly smoother than the regular error diffusion results, though unless closely examined in these monitor examples, the differences in dot brightness and color is easy to miss. Depending in it's use with actual inks, tradeoffs might have to be made between the appearances of colors in grayscale images and this smoothing effect.
Multi-level halftoning seems to offer considerable image quality improvement without expensive algorithms. Although the expenses for realizing this fuctionality come from other areas (cost of extra inks, complexity of multi-drop or variable drop print head), the results would probably justify the extra overhead.
Model-based halftoning seems like an interesting way to make use of our understanding of the human visual system, but the complexity of these algorithms seems to limit their usefulness for the time being.
I would recommend future projects further investigate model-based color
halftoning systems and look for other simple ways to improve on these basic
algorithms -- for example, use a differnet different color system vs. the
rgb space used here to produce gradients.
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