Title: Tone%20Dependent%20Color%20Error%20Diffusion
1Tone Dependent Color Error Diffusion
ICASSP 2004
Vishal Monga and Brian L. Evans
May 20, 2004
Embedded Signal Processing LaboratoryThe
University of Texas at AustinAustin, TX
78712-1084 USA vishal, bevans_at_ece.utexas.edu
2Outline
- Introduction
- High Quality Halftoning Methods
- Error Diffusion
- Direct Binary Search (DBS)
- Grayscale Tone Dependent Error Diffusion
- Different error filter for each input gray-level
- DBS halftone(s) used for filter design
- Color Tone Dependent Error Diffusion
- Perceptual Model
- Error Filter Design
- Conclusion Future Work
3Introduction
Digital Halftoning Examples
Direct Binary Search
4Grayscale Error Diffusion Halftoning
Background
- 2- D sigma delta modulation Anastassiou, 1989
- Shape quantization noise into high freq.
- Several Enhancements
- Variable thresholds, weights and scan paths
Error Diffusion
current pixel
weights
Spectrum
5Direct Binary SearchAnaloui, Allebach 1992
Background
- Computationally too expensive for real-time
applications e.g. printing - Used in screen
design - Practical upper bound for achievable
halftone quality
6Tone Dependent Error DiffusionLi Allebach,
2002
Grayscale TDED
- Train error diffusionweights and
thresholdmodulation
Highlights and shadows (0-20, 235-255)
FFT
Graylevel patch x
Halftone pattern for graylevel x
FFT
7Tone Dependent Color Error Diffusion
Color TDED
- Extension of TDED to color
- Goal e.g. for RGB images obtain optimal (in
visual quality) error filters with filter weights
dependent on input RGB triplet (or 3-tuple) - Applying grayscale TDED independently to the 3
(or 4) color channels ignores the correlation
amongst them - Processing channel-separable or vectorized
- Error filters for each color channel (e.g. R, G,
B) - Matrix valued error filters Damera-Venkata,
Evans 2001 - Design of error filter key to quality
- Take human visual system (HVS) response into
account
8Color HVS Model
Perceptual Model Poirson, Wandell 1997
- Separate image into channels/visual pathways
- Pixel based transformation of RGB ? Linearized
CIELab - Spatial filtering based on HVS characteristics
color space
9Linearized CIELab Color Space
Color TDED
- Linearize CIELab space about D65 white point
Flohr, Kolpatzik, R.Balasubramanian, Carrara,
Bouman, Allebach, 1993 - Yy 116 Y/Yn 116 L 116
f (Y/Yn) 116 - Cx 200X/Xn Y/Yn a 200
f(X/Xn ) f(Y/Yn ) - Cz 500 Y/Yn Z/Zn b 500
f(Y/Yn ) f(Z/Zn ) - where
- f(x) 7.787x 16/116 0 x lt
0.008856 - f(x) x1/3
0.008856 x 1 - Color Transformation
- sRGB ? CIEXYZ ? YyCx Cz
- sRGB? CIEXYZ obtained from http//white.stanford.e
du/brian/scielab/
10HVS Filtering
Color TDED
- Filter chrominance channels more aggressively
- Luminance frequency response Näsänen and
Sullivan, 1984 -
- L average luminance of display
- weighted radial spatial frequency
- Chrominance frequency response Kolpatzik and
Bouman, 1992 - Chrominance response allows more low frequency
chromatic error not to be perceived vs. luminance
response
11Tone Dependent Color Error Diffusion
Color TDED
- Design Issues
- (256)3 possible input RGB tuples
- Criterion for error filter design
- Solution
- Design error filters along the diagonal line of
the color cube i.e. (R,G,B) (0,0,0) (1,1,1)
(255,255,255) - 256 error filters for each of the 3 color planes
- Color screens are designed in this manner
- Train error filters to minimize the visually
weighted squared error between the magnitude
spectra of a constant RGB image and its
halftone pattern
12Perceptual Error Metric
Color TDED
13Perceptual Error Metric
Color TDED
- Find error filters that minimize TSE subject to
diffusion and non-negativity constraints, m
r, g, b a ? (0, 255)
(Floyd-Steinberg)
14Results
Color TDED
(a) Original Color Ramp Image
(b) Floyd-Steinberg Error Diffusion
15Color TDED
Results
(c) Separable application of grayscale TDED
(d) Color TDED
Halftone in (c) courtsey Prof. J. P. Allebach
and T. Chang at Purdue University
16Color TDED
Results
- Halftone Detail
- Blue section of the color ramp
Floyd-Steinberg
Grayscale TDED
Color TDED
17Color TDED
Conclusion Future Work
- Color TDED
- Worms and other directional artifacts removed
- False textures eliminated
- Visibility of halftone-pattern minimized (HVS
model) - More accurate color rendering (than separable
application) - Future Work
- Incorporate Color DBS in error filter design to
enhance homogenity of halftone textures - Design visually optimum matrix valued filters
18Back Up Slides
19Original House Image
20Floyd Steinberg Halftone
21Color TDED Halftone
22Floyd Steinberg Yy component
23Floyd Steinberg Cx component
24TDED Yy component
25TDED Cx component
26Color TDED
HVS Filtering contd
- Role of frequency weighting
- weighting by a function of angular spatial
- frequency Sullivan, Ray, Miller 1991
where p (u2v2)1/2 and
w symmetry parameter
reduces contrast sensitivity at odd multiples of
45 degrees
equivalent to dumping the luminance error
across the diagonals where the eye is least
sensitive.