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Dithering, Halftone & Continuous Tone

Dithering, Halftone & Continuous Tone. The Different Ways To Print Greyscales & Colour Graduations. Dots, Lines and Greyscales. Good image quality is related to the physical printer resolution (dots per inch) the resulting optical resolution (lines per inch)

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Dithering, Halftone & Continuous Tone

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  1. Dithering, Halftone& Continuous Tone The Different Ways To Print Greyscales & Colour Graduations

  2. Dots, Lines and Greyscales • Good image quality is related to • the physical printer resolution(dots per inch) • the resulting optical resolution(lines per inch) • the number of printable greyscales • the quality of graduations

  3. The “Digital” Problem • most printer engines cannot print ‘real’ graduations • they have to print or not print a dot,like laser, ink jet, matrix, thermo • so, how do we print greyscales?

  4.      Perfect Printing consists of: Writing: Images:solid black greyscalessmooth edges graduationshigh contrast low contrast

  5. Dithering (Halftone) A physical pixel can be Black or White (on or off)

  6. Welcome: The ‘Super-Dot’ Text-mode: use physical dot Graphic mode:create a ‘super’-dot

  7. 16 “Greyscales”

  8. The Resolution in Text-Mode physical: 300 dots per inch optical: 300 lines per inch

  9. But In Halftone Printing physical: 300dpi optical: 300 / 4 = 75 lpi

  10. Simple Arithmetics • if you want to show 256 greyscales: • the ‘super-dot’ must be16 x 16 pixels • if you have a 600 dpi printer engine • your optical resolution will be600 / 16 = 37,5 lpi • conclusion: more greyscales means less optical resolution

  11. Dithering It Is Not That Easy • dithering is a complex MATHEMATICAL process done on the interface • the quality of the print-out depends on the way the interface processes • poor image reproduction very often is caused by wrong dithering(f.e. moiree-patterns)

  12. All 50% Black!!

  13. in your driver menu you may find a similar setting: this influences the way of dithering (mathematically)

  14. The Funny Moiree Effect When Your Scanner Mis-reads And Your Printer Mis-types!

  15. What Happens? An Explanation moiree is a kind of interference • where overlapping pixels produce regular patterns • a picture is printed in a different than its initial resolution • or a scanner reads a raster-printed original

  16. Continuous Tone (Con-Tone) Each pixel shows a specific number of greyscales.

  17. Continuous Tone Printing • each physical dot varies in a number of greyscales • the bit-count tells the greyscales:4-bit = 24 = 16 greyscales8-bit = 28 = 256 greyscales • there is no difference between physical and optical resolution(for con-tone printer: dpi = lpi)

  18. Advanced Arithmetics You want to print 256 greyscales with 400 lpi on a halftone printer.What physical resolution is necessary? The ‘Super-Dot’ must be 16 pixels 400 lpi * 16 dots =6,400 dpi

  19. Printing Colours

  20. Colour Printing vs. Black&White • there is no difference, except that there are three (CMY) colours • colour printers also use dithering or continuous-tone • but be aware of the data volume:expect 20 MB up to 120 MB to be normal file sizes (con-tone printing)

  21. Toner: How Many Colours Are Possible? Cyan ? Magenta Yellow Black

  22. 1 Cyan Toner: 2 Magenta 3 Yellow + = 4 Red + = 5 Green + = 6 Blue + + = 7 Black WithoutDithering:

  23. Oh No, Is It True? • Do you really spent so much money for a 7 colour printer? • but you already know the solution: colour dithering • and you will have the same problems as in black&white: dpi vs. lpi

  24. Expert Arithmetic Continuous-Tone Printing • we are using 3 colours, remember: black is our key-colour • each colour uses 8 bit colour-depth: 28 = 256 scales • 28 * 28 * 28 = 224~ 16.7 milion.

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