Grafisk teknik IMCDP IMCDP IMCDP. IMCDP(filter) Sasan Gooran (HT 2006) Assumptions:

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1 IMCDP Grafisk teknik The impact of the placed dot is fed back to the original image by a filter Original Image Binary Image Sasan Gooran (HT 2006) The next dot is placed where the modified image has its largest density value Iterative Method Controlling Dot Placement (IMCDP) Assumptions: The original continuous-tone image is scaled between 0 and 1 0 and 1 represent white and black respectively The binary/halftoned image is totally white to begin with g + Filter + - IMCDP Position of Maximum Filter b(i,j)=1 The algorithm is terminated when the binary image has the same mean as the original b IMCDP Both images should have the same mean, i.e. the number of dots to be placed in the binary image is determined by the sum of the pixel values of the original image. Original Image Binary Image IMCDP(filter) A Gaussian filter is used Experiments show that an 11 x 11 Gaussian filter leads to satisfactory results in most cases The first dot is placed where the original image has its largest density value The size of the filter should be changing for the light and dark parts of the original image 1

2 IMCDP(filter) For halftoning of a constant image with a coverage of p% the size of the filter is decided by: FM Raster (Färg) a = 100 / p a is the distance between the dots if they are homogenously placed The size of the filter is (2a + 1) x (2a + 1) rounded Independent Dependent IMCDP(filter) p=1% Beroende rastrering (dot-off-dot) Den gula kanalen kan rastreras oberoende av de andra två (?!) c och m betecknar täckningen av cyan och magenta. c + m <= 1, dot-on-dot kan undvikas c + m > 1, det förekommer blåa punkter 11 x 11 filter 21 x 21 filter IMCDP Beroende rastrering (dot-off-dot) 50% cyan 50% magenta 2

3 Beroende rastrering (dot-off-dot) Oberoende 50% cyan 50% magenta 51% cyan 36% magenta Summa: 100% Summa: 87% Beroende rastrering (dot-off-dot) Beroende c + m >1, Det förekommer blåa punkter b= c+m-1 (andelen av blåa punkter) c =c-b=1-m och m =m-b=1-c => c +m <1 Rastrera c och m som förut Ersätt de vita punkterna med blå Beroende rastrering vs Oberoende Uppgift Färgkanaler till en färgbild med 20%, 30% och 0% täckning i dess cyan, magenta och gul kanaler rastreras med hjälp av en FM metod. Vi vet X, Y och Z värdena för primära och sekundära färger och det vita papperet, se tabellen nedan. Papper Cyan Magenta Blå X Y Z a) Vilket XYZ-värden har ytan om kanalerna rastreras oberoende? b) Vilket XYZ-värden har ytan om dot-on-dot undviks så mycket som möjligt? c) Vilket XYZ-värden har ytan om dot-off-dot undviks så mycket som möjligt? 3

4 Diploma work (D. Byström) IMCDP in C++ Diploma work Pärlplattor Image size 1.6 GHZ, 256 MB 3 GHZ, 1GB DBS (in best case) 512 x sec 0.9 sec 40 sec 1024 x sec 3.9 sec 180 sec 2048 x sec 17.4 sec 4096 x sec Diploma work Pärlplattor Diploma work Pärlplattor Diploma work (B. Crona) Pärlplattor Hybrid Raster FM_1 FM_2 AM 4

5 Hybrid Halftoning FM Halftoning Use a FM method in the Highlights and shadows, with the smallest possible dot. Use an AM method in the rest of the image. FM Halftoning FM AM & Hybrid Halftone AM Hybrid FM Halftoning 5

6 Reflectance Reflectance Reflectance of a halftone image: (No optical dot gain) Murray-Davis formula, R=a*R i +(1-a)*R p or X=a*X i +(1-a)*X p a- fractional area covered by ink, R i - reflectance of the full ink coverage, R p - reflectance of the white paper Dot Gain Physical dot gain: The difference between the printed dot area (a prnt ) and the commanded dot area (a comd ), a phy =a prnt -a comd ; Optical dot gain: the difference between the optically measured (a optm ) dot area and the printed dot area (a prnt ) a opt =a optm -a prnt ; 6

7 Optical Dot Gain Dot Gain Curve Yule Nielsen Yule-Nielsen modification: R 1/n =a*r i 1/n +(1-a)*Rp 1/n, or W 1/n =a*w 1/n i +(1-a)*Wp 1/n where (W=X,Y,Z) n - the Yule-Nielsen factor, for optical dot gain (1<=n<=2), n can be >2 when physical dot gain is also involved! Dot Gain Curve The dot gain curve is usually found by experiment. Print a number of tints with different commanded coverage and measure the density in print. Dot Gain Dot Gain Curve Commanded Print Measurement Dot Gain MD: Murray-Davis 0% 0% 0% exp: Experiment 2% 8% 8-2=6% pap: paper 50% 80% 80-50=30% 100% 100% =0% 7

8 Dot Gain Curve Compensation for Dot Gain By using these measured data a dot gain curve can be obtained Dot gain 30% Print coverage or the measured data We want 30% in print Follow the arrow 100% 30% 6% 2% 50% Commanded coverage Read the commanded coverage For example 12% 100% Commanded coverage Dot Gain Curve Another useful curve: Print coverage or the measured data 100% 80% Ideal (no dot gain) Measurement 8% 2% 50% 100% Commanded coverage Compensation for Dot Gain The curve shown in the previous page can be used for dot gain compensation Ex. If we want 30% coverage in print due to dot gain we cannot print 30%. What should the commanded coverage be? By using this curve you can find the commanded coverage, see the figure in next slide. 8

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