am-fm screen design using donut filters niranjan damera-venkata and qian lin hewlett-packard...
TRANSCRIPT
AM-FM Screen DesignUsing Donut Filters
Niranjan Damera-Venkata and Qian LinHewlett-Packard Laboratories1501 Page Mill Road, MS 1203Palo Alto CA 94304
page 2SPIE Electronic Imaging Conference - 01/22/04
Outline
• Halftoning methods– why use green-noise (AM-FM)
halftoning?• Statistical properties of green-noise
patterns– spatial statistics– spectral statistics
• Design of optimal green-noise screens– optimized “donut” filters
• Green-noise error diffusion– analysis of Levien error diffusion– noise shaping: role of the “donut”
filter– new green noise error diffusion
methods• Status
page 3SPIE Electronic Imaging Conference - 01/22/04
Conventional AM halftoning
• Indigo’s “Sequin” halftoning– Dot frequency is fixed– Dot size varies to
represent tone• Disadvantages
– Rosette patterns– Tone jumps– Detail rendition suffers– Scan quality suffers
page 4SPIE Electronic Imaging Conference - 01/22/04
FM halftoning (a.k.a. “blue-noise”)
• Photo printers– Dot size is fixed– Dot frequency varies
• Advantages– Does not cause Moiré – Rosette patterns
eliminated– Tone jumps not abrupt– Better detail rendition– Better quality at consumer
grade scan resolutions• Disadvantages
– Depends heavily on fidelity of isolated dot reproduction
dot dropout andbanding inhighlights
clumping in themidtones results
In grain
page 5SPIE Electronic Imaging Conference - 01/22/04
AM-FM halftoning (a.k.a. “green-noise”)
• AM-FM– Dot size and dot
frequency varies• Advantages
– Promise of the “best of both worlds”
• Disadvantages– Design depends on
particular dot formation characteristics
– Difficult design problem
highlights
midtones
page 6SPIE Electronic Imaging Conference - 01/22/04
Green-noise statistics [Lau et al.]
Spectral Spatial
G=0.10 G=0.25
page 7SPIE Electronic Imaging Conference - 01/22/04
AM-FM Screen design
• Filter based methods– Donut Filters [Lin]
• Optimal void and cluster approach
• Donut filter parameters must be empirically chosen for each graylevel
– ColorSmooth Dither [Lin and Allebach]• Computationally very expensive
• Handles joint design for a set of colorants
• Several parameters must be set empirically
• Optimal method – Green Noise Mask [Lau, Arce and Gallagher]
• Approximates green-noise Markov statistics in Maximum Likelihood sense
page 8SPIE Electronic Imaging Conference - 01/22/04
Donut filters
• Dots added one at a time• Start off with well distributed dot centers and
then grow them– Filter existing pattern with filter– Choose minimum location and add a dot there– Donut filter’s characteristic promotes dot clustering
Spatial
2
2
2 rree
Spectral
page 9SPIE Electronic Imaging Conference - 01/22/04
Donut filters
• Dots added one at a time• Start off with well distributed dot centers and
then grow them– Filter existing pattern with filter– Choose minimum location and add a dot there– Donut filter’s characteristic promotes dot clustering
page 10SPIE Electronic Imaging Conference - 01/22/04
Optimal donut filters
• Maximum Likelihood solution for dot placement problem is equivalent to finding min/max of filter output
3y
x
2y
4y
1y
page 11SPIE Electronic Imaging Conference - 01/22/04
Deriving the optimal donut filter
page 12SPIE Electronic Imaging Conference - 01/22/04
Results
G=22/255 G=42/255 G=62/255 G=82/255
page 13SPIE Electronic Imaging Conference - 01/22/04
Results
page 14SPIE Electronic Imaging Conference - 01/22/04
Results
page 15SPIE Electronic Imaging Conference - 01/22/04
Grayscale design algorithm
Existing minority dots from level G1
Generate parametric linear filter FG2 for level G2
Filter existing dot pattern for level G1 using FG2 and circular convolution
Find majority pixel where the result is minimum and convert it to a minority pixel
Add shifted version of FG2to earlier filtered result
Desired concentration of minority pixels?
No
stopYes
page 16SPIE Electronic Imaging Conference - 01/22/04
Optimal Multifilters for color screens
page 17SPIE Electronic Imaging Conference - 01/22/04
Optimal Multifilters for color screens
page 18SPIE Electronic Imaging Conference - 01/22/04
Add circularly shifted versions of impulse response vector (for impulses in C and M) of FG2 and add to earlier filtered vector result for C,M
Existing minority dots from level G1 in C and M
Generate parametric matrix-valued linear filter FG2
Filter existing dot pattern for level G1 using matrix-valued filter FG2.
Find majority pixel in C, M where the result is minimum and convert it them to a minority pixel
Desired concentration of minority pixels?
No
stopYes
Color design algorithm
page 19SPIE Electronic Imaging Conference - 01/22/04
Results
C,M=62/255
page 20SPIE Electronic Imaging Conference - 01/22/04
Results (epxand images to see dot structure)
Optimized donut filter Gaussian donut filter [Lin]
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