Lightness filtering in color images with respect to the gamut

1
Lightness filtering in color images with respect
to the gamut
Fourteenth Color Imaging Conference Judith Dijk
and Piet W. Verbeek
Presented by Soo-Jin Sung

• School of Electrical Engineering and Computer
  Science
• Kyungpook National Univ.

2
Abstract

• Proposed method
• A generic method that allows grey image
  processing for lightness processing on color
  images
• without exceeding the limits of the gamut of the
  technique or device
• The results of the proposed method
• Sharpening improvement
• Contrast improvement

3
Introduction

• A difference between grey images and color images
• Grey images
• Having a simple scale between black and white
• Color images
• The range of colors
• Proposed method
• A generic method
• Appling grey image processing on the luminance of
  color images
• Without exceeding the limits of the output gamut

4
The generic method

• The base
• Image processing of color images
• Hue, chroma or saturation, and lightness
• The constraint
• The colors of the input image are located within
  the gamut
• Definition for colors as points in the CIELAB
  space

5

• The desired lightness correction
• The move way of the original color point along a
  fixed path in a constant hue plane
• The path
• The color of the original point ( position in the
  gamut and the relation between the lightness and
  chroma change)
• The fixed hue of the point
• Keeping the same chroma before and after the
  image processing
• The prevention of the colorfulness of the image

Figure 1. Illustration of a path for a given
point x with chroma constant.
6
Gamma manipulation

• The effect
• The lightness values are distributed nonlinearity
  over the range
• Contrast improvement
• More contrast in the higher colors, and decreased
  mean lightness
• More contrast in the darker colors, and increased
  mean lightness

(1)
7
Figure 2. Gamma manipulation.
Figure 3. Gamma manipulation, while keeping
chroma constant. (a) the normal, not
gamut-limited, gamma manipulation (b) the gamma
manipulation as defined in equation 1
8
Recipes for gamut-limited gamma manipulation

• Possible recipes
• The manipulated lightness while keeping the
  chroma constant
• The manipulated lightness while keeping the ratio
  constant
• Mapping towards black and white
• Movement toward black for a lightness decrease
• Movement toward white for a lightness increase
• Mapping away from black and white
• Movement away from black for a lightness increase
• Movement away from white for a lightness decrease

9
(a)
(b)
(d) Mapping away from black and white
(c) Mapping towards black and white
Figure 4. Paths along which a color point may
move within the constant hue plane, when applying
different recipes for gamma manipulations.
10
Results for gamut-limited gamma manipulation

• In figure 5
• Clipped out-of-gamut pixels
• For low gamma values, too yellow in a skin color
• In figure 6
• Having the smaller effect, because the mean
  lightness change is smaller
• The more natural skin tones than the figure 5

Figure 5. The result for normal gamma manipulation
11

• In figure 7
• chroma increase,
  chroma decrease
• Having the somewhat less quality than the images
  in figure 6
• In figure 8
• mixed white,
  mixed black
• Less vivid and less than quality of other recipes
  
• In figure 9
• No difference very much from each other

Figure 8. The results for the mapping towards
black recipe.
Figure 9. The results for the away from black
recipe.
12
Gamut limited sharpening

• Process of sharpening
• Changing the lightness of a point
• Adding a high frequency filtered version of the
  image to the original image
• Used unsharp masking
• The high frequency filtered version
• The unsharp masking sharpening filter
• Inverse unsharp masking filter
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

(2)
where the fourier transform
(3)
(4)
13
Recipes for gamut-limited sharpening

• Tested recipes
• Sharpening within the gamut
• The maximum and minimum sharpening for each point
  within the gamut
• Gamut limited sharpening
• The desired lightness difference adjusted so that
  the maximum lightness difference
• Adjustment of the value of
• Mapping towards black and white
• Selection of the new point through the original
  point and white or black
• Mapping halfway towards black and white
• Mapping away from black and white

14
Figure 10. Different paths for sharpening
manipulation.
15
Results for gamut-limited sharpening
16

• The sharpest images ((b) and (d))
• The effect on the location of the eyes and the
  mouth
• The more colorful image (b) than the image (d)
• The quite visible chroma change in the low
  frequency for the (d)
• (e) recipe
• Adding chroma on places where the lightness is
  changed
• More colorful than the colors in the original
  image
• In the colors of the mouth of the woman

17

• (a) and (c) recipe and the normal sharpening
• Adjustment of the size of for these recipes
  by the user
• Having somewhat greyish results for
• Shift towards the achromatic axis in proportion
  to
• Causing a chroma reduction for all points ((c)
  recipe)

Figure 12. The result for sharpening within the
gamut (a) for different values of , with
.
18
Conclusions

• Proposed method
• A generic method that allows grey image
  processing for lightness processing on color
  images
• Without affecting color rendering
• Staying within the gamut of the apparatus in
  question
• Results
• Sharpening improvement
• The chroma change in proportion to the lightness
• Improvement for the most images
• Contrast improvement
• The chroma reduction in proportion to the
  lightness difference
• Mapping towards black and white recipe
• The chroma increase in proportion to the
  lightness difference
• Mapping away from black and white recipe