5 Sep 2000 raph   » (Master)

mathieu wrote:

Now that I spent my day reading my computer graphics, principles and practice, I have a few questions: 1) what is the difference between dithering and halftoning (it seems the 2 terms are used interchangably. Or I missed something most likely) ? 2) for printers, you do not want to use dithering only. You should also support gammut compression. (is this right ?) 3) Does the gammut of printers is printer independant or is it a specificity of a peculiar printer model ? 4) any good biblio on gammut compression ?

1. The terms are used interchangeably. To confuse things even further, in graphic arts the word "screening" is also used.

2. Gamut compression is part of color management. It's a more or less orthogonal problem to dithering. For example, for a continuous tone printer such as an Iris, you may want to do gamut compression but not dithering. Conversely, if you're printing an image already gamut-optimized for the printer at hand, you may want to do dithering but not gamut compression.

But yes, for highest quality reproduction on typical devices, you want to do both dithering and gamut compression.

3. The gamut is highly dependent on the printer, the ink, and (perhaps most especially) the paper. Shiny papers can print much darker, richer colors than non-shiny. This is a fundamental physical property of paper and derives from the mismatch in index of refraction - any time you have a discontinuity in this, you'll have some reflection. What shiny papers do is concentrate this reflection into an area of glare, which when reading you are trained to avoid. In non-shiny papers, the reflection is diffuse, which simply limits the amount of light that can be absorbed. As a consequence, matte finish papers require more gamut compression than do shiny ones.

The other major tradeoff of color gamuts is durability vs color saturation. Extremely brightly colored dyes tend to degrade pretty quickly when exposed to light (especially the UV present in sunlight). Thus, inkjet dyes and pigments designed for "archival" tend to have a more limited color gamut.

4. The best book on color management in general is: Edward J. Giorgianni and Thomas E. Madden. Digital Color Management: Encoding Solutions. Addison-Wesley, 1998. ISBN 0201634260. You might also want to read Ján Morovic's PhD thesis on Gamut Mapping.

I talk about some of these topics in my color management page. There are a bunch more links, as well.

The diary format, while it works, is really not ideal for this kind question and answer format. It's not obvious to me what I can do on Advogato that would be better. Perhaps some kind of Wiki?

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