Color2Gray Conversion

Chroma Contrast and Detail Preserving in Color to Grayscale Conversion

Color to Grayscale Conversion:

• In Color Space

  • Linear

  • Nonlinear

  • In Image Space

    • Pixels (RGB)

      • Using colors in the image

      • Different gray for different color

• Relative difference

  • Using colors in the image and their position in image space

  • Colors can map to same gray…..

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Fig. 1, Color to Grayscale Conversion

Some Aspects of Concern:

• Dimension reduction 3D to 1D;

  • Information loss

• Color to Gray;

  • Luminance vs. chrominance

• Display has less than [0,100] Y-range.

  • • Contrast

Fig. 2, Color-to-Grayscale – Extreme Case: Constant luminance [Neumann et al. 07]

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Fig. 3, Isoluminant Colors

Local Method:

• Local changes, contradictions, comput. costs;

• Color contrast map to enhance gray image;

• Same color may output different gray value;

• Appearance of constant color regions distort.

  • Haloing artifacts may produce.

Global Method:

• Same luminance for the same RGB triplets;

• Speed, naturalness, luminance range;

• Color feature optimized in conversion;

  • Mostly, color order is strictly satisfied, might be ambiguous (culture, person).

Bala & Eschbach [2004, Color Imaging Conf.]:

• Local enhancement via high-frequency chrominance information in the luminance;

• Spatially adaptive;

  • Image enhancement, possible artifacts.

Rasche et al. [2005, IEEE CG&A and EG]

• Global technique maintain luminance consistency;

• Constrained MDS with color quantization;

  • Enormous computational demands.

Gooch et al. [2005, ACM Siggraph]

• Color2Gray: Saliency preserving based on local contrasts;

• Express locally visible changes;

  • Use conjugate gradient iterations.

Grundland & Dogson [2005, PR]

• Globally decolorize algo for contrast enhancing;

  • Express grayscale as continuous, image dependent, piecewise linear mapping.

Neumann et al. [2007, CAGVI]

• Perceptually based color to grayscale transform;

• Gradient-inconsistency (COLOROID ) correction;

  • Simple iteration and the 2D integration.

Smith et al. [2008, Eurographics]:

• Globally assign grey values, determine color ordering;

  • Helmholtz-Kohlrausch color appearance effect;

• Locally enhance greyscale to reproduce original contrast;

  • • Introduces lost discontinuities in regions of color contrast.

Perceptual Evaluation of Color2Gray: Cadik, [2008, Pacific Graphics]

• No conversion produces universally good results;

• Decolorize: good for images with narrow gamuts;

  • Smith08: good for colorful images.

Fig. 4, Evaluation example.

Latest: Lu, Xu & Jia [2012, ICCP]

• Relax the color order constraint based on human perception;

• Automatic selection of suitable gray scale;

  • Bimodal distribution constrains spatial difference.

(a) Original color image example. (b) Luminance only (CIELab) Conversion. (c) Lu, Xu, Jia's Conversion.

(d) Color2Gray(Gooch)05's conversion. (e) Decolorize05(Grundland)'s Conversion. (f) Smith08's Conversion.

(d) Rasche(MDS)05's conversion. (e) Bala04(HF chroma)'s Conversion. (f) Neumann07(Coloroid)'s Conversion.

Fig. 5, Latest Color2Gray Method's Result.

References

1. R Bala,R Eschbach, Spatial Color-to-Grayscale Transform Preserving Chrominance Edge Information, Color Imaging Conference, 2004.

2. K. Rasche, R. Geist, J. Westall, Re-coloring images for gamuts of lower dimension, Computer Graphics Forum 24 (2005) 423–432.

3. A. Gooch et al., Color2Gray: salience-preserving color removal, ACM Trans. Graphics 24(2005) 634–639.

4. M. Grundland, N. Dodgson, The decolorize algorithm for contrast enhancing, color to grayscale conversion, Tech. Report, Computer Laboratory, Cambridge University, 2005.

5. Neumann et al., An Efficient Perception-based Adaptive Color to Gray Transform, Computational Aesthetics in Graphics, Visualization, and Imaging, 2007.

6. K. Smith, et al., Apparent greyscale: A simple and fast conversion to perceptually accurate images and video, Computer Graphics Forum 27, 3 (2008).

7. M Cadik, Perceptual Evaluation of Color-to-Grayscale Image Conversions, Pacific Graphics, 2008.

8. Lu, Xu, Jia, Contrast Preserving Decolorizatio, ICCP, 2012.