1. Screen Ruling, Print Resolution AM, 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 IMAGES Memory Grayscale 8 256 tones RGB 3*8=24 256^3=16.7 bits/pixel 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. DIGITAL HALFTONING Prepress HalftoningPrint Con-tone Image Halftoned Image

11. DIGITAL HALFTONING Example Periodic and clustered dots

12. DIGITAL HALFTONING Example Non-periodic and dispersed dots

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

14. HALFTONE CELL Original image Halftoned image Halftone cell

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 cm 2 photo that is supposed to be 20 x 30 cm 2 when printed at 150 lpi has to be scanned with a ppi about 2*2*150 = 600.

17. HALFTONE CELL Micro dot dpi: Number of micro dots per inch This halftone cell represents at most 8 2 + 1= 65 gray tones

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

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 decides the 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. AM & FM (1st & 2nd Generation) Halftone AM FM, 1stFM, 2nd

27. AM & FM Halftone AMFM

28. FM Halftone, 1st and 2nd generation FirstSecond

29. Hybrid Halftoning AM FM_1 FM_2

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

35. THRESHOLDING This threshold matrix represents 10 gray tones

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

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

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

39. TABLE HALFTONING 0.6 0.2 0 Original ImageHalftoned image Mean

40. TABLE HALFTONING Clustered Dispersed

41. FM HALFTONING Error Diffusion 0.7 Threshold 0.5 Original imageHalftoned Image 0.3 error-Filter 7 351 (*1/16)

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

43. COLOR PRINT Subtractive color mixing, Printers

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

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

46. COLOR PRINT Original

47. COLOR PRINT AM

48. COLOR PRINT FM

49. AM HALFTONE same angle for C, M, Y & K

50. Conventional Color Halftoning Same raster angle Error in position can cause color shift

51. Conventional Color Halftoning Same raster angle Error in raster angle can cause Moiré

52. Conventional Color Halftoning Different raster angle, 0, 15, 75 and 45 degrees AM different angles Rosette patterns FM

53. AM HALFTONE different angles for C, M, Y and K 15, 75, 0 and 45 degrees

54. ROSETTE PATTERN