DIGITAL HALFTONING Sasan Gooran Digital Halftoning Digital Images

DIGITAL HALFTONING Sasan Gooran Digital Halftoning

Digital Images Pixel based n n n They consist of a number of square image elements, which are called pixels. Each pixel has a position in the image and a value that describes its tone value in that position. The number of pixels per inch is called resolution. (ppi = pixels / inch) Another way of mentioning resolution is to say how many pixels an image totally consists of. Ex. 1024 x 768 pixels. 2/27/2021 Digital Halftoning 2

Digital Images Pixel based n Bit depth: Number of bits that are used in order to represent the tone value in computer. Commonly we use 256 levels of tone, i. e. Bit depth = 8. 2/27/2021 Digital Halftoning 3

Digital Images How much memory? bit/pixel n n n Line 1 Grayscale 8 Indexed color 8 RGB 3*8=24 CMYK 4*8=32 2/27/2021 2 tones 256 colors 256^3=16. 7 millions colors 256^4=4. 3 billions colors Digital Halftoning 4

Halftoning n Because printing is generally not able to reproduce different tones of a color, the original image has to be transformed to a binary image. This is called halftoning. 2/27/2021 Digital Halftoning 5

Halftoning Prepress 2/27/2021 Con-tone Image Halftoned Halftoning Image Digital Halftoning Print 6

Halftone cell Halftone dot The fractional area of the cell covered with ink is decided by the pixel value. (Or by the mean of the pixel values if the small square shown in the image to the left consists of more than one pixel). 2/27/2021 Digital Halftoning 7

Line screen frequency In conventional halftoning the distance between the centers of two neighboring dots is constant. n Number of halftone dots per inch is called line screen frequency. It is denoted by lpi (lines per inch). n 2/27/2021 Digital Halftoning 8

Line screen frequency & Resolution Micro dot Resolution: number of micro dots per inch (dpi) Halftone cell In this case: 17 levels of gray 2/27/2021 Digital Halftoning 9

Levels of gray Ex, 600 dpi and 100 lpi gives 37 levels of gray Ex, 600 dpi and 75 lpi gives 65 levels of gray Ex, 600 dpi and 50 lpi gives 145 levels of gray Which one to choose? ? 2/27/2021 Digital Halftoning 10

lpi & ppi When scanning an image, for practical reasons and for better reproduction, you normally choose a ppi that is twice as high as lpi. n A sampling factor of 1. 5 is acceptable in most cases. n A sampling factor higher than 2 is unnecessary. n 2/27/2021 Digital Halftoning 11

Lpi & ppi Ex. If your original image is (24 x 36 mm) and you want the printed image to be (80 x 120) mm when lpi = 133, you should choose a ppi around 887 lines/inch. 2/27/2021 Digital Halftoning 12

AM and FM Halftoning n AM (Amplitude Modulated) Halftoning – The size of the dots varies while their frequency is kept constant. The darker the gray tone the bigger the dot. n FM (Frequency Modulated) Halftoning – The size of the dots is constant. Their frequency (number) is changed. The darker the gray tone the more the number of micro dots. 2/27/2021 Digital Halftoning 13

AM and FM Halftoning 2/27/2021 Digital Halftoning 14

AM and FM Halftoning 2/27/2021 Digital Halftoning 15

AM and FM Halftoning FM AM 2/27/2021 Digital Halftoning 16

Thresholding 0. 6 1 0. 3 0. 2 Original Image 0 Threshold matrix Halftoned Image This threshold matrix represents 10 levels of gray. 2/27/2021 Digital Halftoning 17

Threshold matrix Example: Line 1 2 5 6 9 10 13 14 2/27/2021 3 7 11 15 4 8 12 16 Digital Halftoning 18

Threshold Matrix Example: spiral 1 12 11 10 2/27/2021 2 13 16 9 3 14 15 8 4 5 6 7 Digital Halftoning 19

Threshold Matrix Example: spiral Line 2/27/2021 Spiral Digital Halftoning 20

Error Diffusion FM 0. 3 Threshold 0. 5 0. 7 Original Image Error Filter 3 2/27/2021 Halftoned Image 7 5 1 (*1/16) Digital Halftoning 21

Error Diffusion FM 2/27/2021 Digital Halftoning 22

Hybrid Halftoning Example 1 n AM+FM – Homogeneous parts by AM – Details by FM 2/27/2021 Digital Halftoning 23

Hybrid Halftoning Example 1 2/27/2021 Digital Halftoning 24

Hybrid Halftoning Example 2 Some printing methods are not able to produce the dots sufficiently small. n Ex. Flexography. n 5. 2% 2/27/2021 Digital Halftoning 25

Hybrid Halftoning Example 2 n Utilize a FM method for the highlights and shadows using the smallest possible dot. n Use an AM method for the rest of the image. 2/27/2021 Digital Halftoning 26

Hybrid halftoning Example 2 AM Hybrid 2/27/2021 Digital Halftoning 27

Hybrid Halftoning Example 2 AM 2/27/2021 Hybrid Digital Halftoning 28

Hybrid Halftoning AM 2/27/2021 Hybrid Digital Halftoning 29

Hybrid Halftoning Hybrid, The FM method is Error diffusion 2/27/2021 Hybrid, The FM method is. . . Digital Halftoning 30

Color Mixing (Additive) 2/27/2021 Digital Halftoning 31

Color Mixing (Subtractive) 2/27/2021 Digital Halftoning 32

Color Mixing n Three primary colors n n n 2/27/2021 CYAN (C) MAGENTA (M) YELLOW (Y) Digital Halftoning 33

Color Mixing n Three secondary colors n n n 2/27/2021 RED (R, MY) GREEN (G, CY) BLUE (B, CM) Digital Halftoning 34

Color Mixing n And Black 2/27/2021 n BLACK (K, CMY) Digital Halftoning 35

3 -Color Print Original 2/27/2021 Digital Halftoning 36

AM Halftone Konventionellt raster 2/27/2021 Digital Halftoning 37

FM Halftone FM- raster 2/27/2021 Digital Halftoning 38

AM Halftone, same angle for C, M, Y & K 2/27/2021 Digital Halftoning 39

AM Halftone, same angle for C & M Miss-registration Correct registration 2/27/2021 Position error Digital Halftoning 40

AM Halftone, same angle for C & M Miss-registration Correct registration Angle error Moiré pattern 2/27/2021 Digital Halftoning 41

AM Halftone, different angles for C, M, Y & K 15, 75, 0 and 45 degrees 2/27/2021 Digital Halftoning 42

Rosette Pattern 2/27/2021 Digital Halftoning 43

Rosette Pattern 2/27/2021 Digital Halftoning 44

FM (Stochastic) Halftone, 4 -color print “Moiré pattern” and “Rosette pattern” disappear 2/27/2021 Digital Halftoning 45

NUEGEBAUER’S Equations X, Y, Z are the tristimulus values for the average color of a surface ai is the fractional area covered by color Xi, Yi, Zi 2/27/2021 Digital Halftoning 46

Demichel’s Equations (3 -color print) Aw =(1 -ac)(1 -am)(1 -ay) Ac =ac(1 -am)(1 -ay) Am =am(1 -ac)(1 -ay) Ay =ay(1 -ac)(1 -am) Ar =amay(1 -ac) Ag =acay(1 -am) Ab =acam(1 -ay) Ak=acamay 2/27/2021 Digital Halftoning 47