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Screen Ruling, Print Resolution AM, FM and Hybrid Halftoning

Screen Ruling, Print Resolution AM, FM and Hybrid Halftoning. Sasan Gooran Linköping University LiU-Norrköping. DIGITAL IMAGES (pixel based). Scanning. Photo. Digital image. ppi (pixels per inch): Number of sample s per inch. ppi (pixels per inch).

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Screen Ruling, Print Resolution AM, FM and Hybrid Halftoning

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  1. Screen Ruling, Print ResolutionAM, FM and Hybrid Halftoning Sasan Gooran Linköping University LiU-Norrköping

  2. DIGITAL IMAGES (pixel based) Scanning Photo Digital image ppi (pixels per inch): Number of samples per inch

  3. ppi (pixels per inch) • ppi (scanning resolution): Number of samples per inch • The higher ppi the better the representation of the con-tone image (Photo) • Higher ppi requires more memory • ppi should not be unncessarily high • Choice of ppi????

  4. ppi = 72

  5. ppi = 36

  6. ppi = 18

  7. DIGITAL IMAGESMemory bits/pixel • Grayscale 8 256 tones • RGB 3*8=24 256^3=16.7 million colors

  8. DIGITAL HALFTONING • Since most printers and image setters are not able to reproduce different shadows of gray the original digital image has to be transformed into an image containing white (0’s) and black (1’s)

  9. Halftoning

  10. Con-tone Halftoned Prepress Halftoning Print Image Image DIGITAL HALFTONING

  11. DIGITAL HALFTONINGExample Periodic and clustered dots

  12. DIGITAL HALFTONINGExample Non-periodic and dispersed dots

  13. HALFTONE CELL Pixel (/a number of pixels) Halftone cell The fractional areacovered by the ink corresponds to the value of the pixel (or the area)

  14. Halftone cell HALFTONE CELL Halftoned image Original image

  15. SCREEN RULING/FREQUENCY • lpi (lines per inch): Number of halftone cells per inch • The higher lpi the better the print (?!) • High lpi requires more stable print press etc. • Does a higher lpi always lead to a better print? (to be answered later)

  16. RULE OF THUMB Ex. A 10 x 15 cm2 photo that is supposed to be 20 x 30 cm2 when printed at 150 lpihas to be scanned with a ppi about 2*2*150 = 600.

  17. Micro dot HALFTONE CELL dpi: Number of micro dots per inch This halftone cell represents at most 82 + 1= 65 graytones

  18. HALFTONE CELL Screen ruling: number of halftone cells per inch (lpi) Micro dot Resolution: number of micro dots per inch (dpi) Halftone cell In this case: 17 gray tones

  19. lpi & dpi • lpi: Number of halftone cells per inch • A halftone cell consists of micro dots • dpi: Number of micro dots per inch • The ratio dpi/lpi decidesthe size of the halftone cell

  20. lpi & dpi

  21. lpi & dpi(Example) • Assume that dpi is fixed at 600 • lpi = 150 only gives 17 gray tones • lpi = 100 only gives 37 gray tones • lpi = 50 gives 145 gray tones • Does a higher lpi always lead to a better print?Not necessarily!

  22. High lpi, few gray tones

  23. Lower lpi, more gray tones

  24. Low lpi, more gray tones but large halftone dots, (not satisfying)

  25. AM & FM HALFTONING • AM (Amplitude Modulated) • The size of the dots is variable, their frequency is constant • FM (Frequency Modulated) 1st generation • The size of the dots is constant, their frequency varies • FM (Frequency Modulated) 2nd generation • The size of the dots and their frequency vary

  26. FM, 1st FM, 2nd AM AM & FM (1st & 2nd Generation) Halftone

  27. AM & FM Halftone AM FM

  28. FM Halftone, 1st and 2nd generation First Second

  29. FM_2 FM_1 Hybrid Halftoning AM

  30. THRESHOLDING g and b are the original and the halftoned image, respectively. t is the threshold matrix.

  31. 0.6 1 0.1 0.3 0.2 0 Original Image Threshold matrix Halftoned image THRESHOLDING This threshold matrix represents 10 gray tones

  32. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 THRESHOLD MATRIXExample: Line

  33. 1 2 3 4 12 13 14 5 11 16 15 6 10 9 8 7 THRESHOLD MATRIXExample: Spiral

  34. 14 12 13 16 19 21 20 17 5 4 3 10 28 29 30 23 6 1 2 11 27 32 31 22 9 7 8 15 24 26 25 1819 21 20 17 14 12 13 16 28 29 30 23 5 4 3 10 27 32 31 22 6 1 2 11 24 26 25 18 9 7 8 15 1 30 8 28 2 29 7 2717 9 24 16 18 10 23 15 5 25 3 32 6 26 4 3121 13 19 11 22 14 20 12 2 29 7 27 1 30 8 28 18 10 23 15 17 9 24 16 6 26 4 31 5 25 3 32 22 14 20 12 21 13 19 11 THRESHOLD MATRIXClustered & Dispersed, 45 degrees Clustered Dispersed

  35. 0.6 Mean 0.2 0 Original Image Halftoned image TABLE HALFTONING

  36. TABLE HALFTONING Dispersed Clustered

  37. 0.3 Threshold 0.5 0.7 error-Filter Original image Halftoned Image 7 (*1/16) 3 5 1 FM HALFTONINGError Diffusion

  38. COLOR PRINT Additive color mixing, TV, Computer screen

  39. COLOR PRINT Subtractive color mixing, Printers

  40. Three primary colors COLOR PRINT • CYAN (C) • MAGENTA (M) • YELLOW (Y)

  41. Three secondary colors And Black COLOR PRINT • RED (R,MY) • GREEN (G, CY) • BLUE (B, CM) • BLACK (K, CMY)

  42. COLOR PRINT Original

  43. COLOR PRINT AM

  44. COLOR PRINT FM

  45. AM HALFTONEsame angle for C, M, Y & K

  46. ConventionalColorHalftoningSame raster angle Error in position can cause color shift

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