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got colors?

Color Quantization Techniques

Joo Hyun Song

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Outline

• 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– 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 ComparisonsOriginal RGB

YUV XYZ

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Color Space ComparisonsOriginal RGB

YUV XYZ

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Color Space ComparisonsOriginal RGB

YUV XYZ

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Weird…U*V*W* (supposedly)

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My U*V*W* Implementationdouble 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?