got colors?
Color Quantization Techniques
Joo Hyun Song
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Outline
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•
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Overview
Implementation
Results and Discussion
Concluding Remarks
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Overview
• What is color quantization?
– “Color quantization is applied when the color
information of an image is to be reduced.“ 1
– “The most common case is when a 24-bit
color image is transformed into an 8-bit color
image.” 1
– Used when high-depth color is not supported
or necessary.
1
http://www.dai.ed.ac.uk/HIPR2/quantize.htm
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Overview (cont’d)
• Main issues concerning color quantization
– What are the criteria for colors that are
retained in the resulting image?
(How are the ‘important’ colors selected?)
– How ‘accurate’ is the resulting image?
(How well are the important features of the
image preserved in the resulting image?)
– How fast is the quantization process?
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Implementation
• Diversity Algorithm 2
– Color quantization algorithm devised by John Bradley,
the creator of the popular UNIX-based imaging
software xv.
– The algorithm starts by picking the most populous
color (the overall color) of the original image.
– Then the most ‘distant’ colors from the colors in the
new color table are picked until the new color table is
filled.
– Results in the most ‘diverse’ selection of colors
surrounding the overall color.
2 http://www.trilon.com/xv/manual/xv-3.10a/diversity-algorithm.html
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Implementation (cont’d)
• Modified Diversity Algorithm 2
– Improvement over the original Diversity Algorithm
suggested by Tom Lane of the Independent JPEG
Group.
– The modification aims to better balance the allocation
between diverse colors and populous colors.
– The alternation strategy is subjective – some strategy
works better in certain images while not as good in
other.
– Examples in this presentation use the 10-div-pop/div
rule. 
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Implementation (cont’d)
• ImageMagickTM 3 libraries and tools
– Provides readily available libraries and tools for
reading, manipulating and writing most of the popular
image formats.
• Programming Language of choice: C++
– C++ has std::vector and std::map datatypes that
made the histogram and colorMap implementation
simpler.
– C++ has a built-in optimized sort() function that can
be used for sorting elements in std::vector datatypes.
– Not Java. ;-)
3
http://www.imagemagick.org/
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Test Images
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Results
• Tested Algorithms
– Diversity Algorithm
– Modified Diversity Algorithm
• Tested Color Spaces
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–
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RGB
YUV
YIQ
XYZ
U*V*W*
(not really)
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Original Diversity vs. Modified Diversity
Original Diversity Algorithm (RGB)
Modified Diversity Algorithm (RGB)
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Original Diversity vs. Modified Diversity
(cont’d)
Original Diversity Algorithm (RGB)
Modified Diversity Algorithm (RGB)
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Original Diversity vs. Modified Diversity
(cont’d)
Original Diversity Algorithm (RGB)
Modified Diversity Algorithm (RGB)
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Color Space Comparisons
Original
YUV
RGB
XYZ
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Color Space Comparisons
Original
YUV
RGB
XYZ
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Color Space Comparisons
Original
YUV
RGB
XYZ
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Weird…
U*V*W* (supposedly)
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My U*V*W* Implementation
double DecodeUVW(const unsigned int color, const char opt)
{
double X = DecodeXYZ(color, 'x');
double Y = DecodeXYZ(color, 'y');
double Z = DecodeXYZ(color, 'z');
double x = X/(X+Y+Z);
double y = Y/(X+Y+Z);
double u = 4*x/(-2*x+12*y+3);
double v = 6*y/(-2*x+12*y+3);
double W = 25*pow(100*Y, 1/3)-17;
// Get reference white
X = DecodeXYZ(0xFFFFFF, 'x');
Y = DecodeXYZ(0xFFFFFF, 'y');
Z = DecodeXYZ(0xFFFFFF, 'z');
x = X/(X+Y+Z);
y = Y/(X+Y+Z);
double u0 = 4*x/(-2*x+12*y+3);
double v0 = 6*y/(-2*x+12*y+3);
if(opt == 'u') {
return 13*W*(u-u0);
}
if(opt == 'v') {
return 13*W*(v-v0);
}
if(opt == 'w') {
return W;
}
}
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Future Interest
• Investigate more advanced color spaces
(such as L*u*v* or IHS).
• Investigate other color metrics (e.g.
Riemannian color space).
• Investigate more advanced color
quantization algorithms (such as the
Neural Networks color quantization
algorithm).
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Performance of other programs
• ImageMagick’s built-in quantizeColors() algorithm
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Performance of other programs
(cont’d)
• Gimp
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What I Have Learned
1. Start EARLY 
2. Subjectivity of image quality.
3. Different Color Spaces.
4. Don’t use ImageMagick
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Any Questions?
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