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Color Coordinate Systems for Accurate Color Image Editing Software

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Color Coordinate Systems for Accurate Color Image Editing Software

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  1. International, Inc. W K E Color Coordinate Systems for Accurate Color Image Editing Software Sergey Bezryadin, KWE International Inc, San Francisco, USA Pavel Bourov, UniqueIC’s, Saratov, Russia

  2. What if we significantly change contrast?

  3. PhotoShop gives us a MESS Over-lightening and disappearance of details on the wall Chromatic changes of colors and disappearance of details on the flowers Over-darkening and disappearance of the details in the shadow Chromatic changes of colors on the flower box

  4. Is it what YOU want to happen … .

  5. Is it what YOU want to happen with ORIGINAL?

  6. THIS is what you WANT!

  7. Is it possible? • Is this result really possible? • Yes • And there are TWO ways: • A lot of operations in PhotoShop by a highly advanced user. • Use of alternative algorithms and ONE operation. Let the computer do the routine Let the artist to work wonders

  8. Plan • In this presentation, we will… • Formulate some of the current statements about what a color is, and state which of those we use and which we reject in our project. • Give new definitions for Brightness (B), Chroma (C), and Hue (H). • Present 3 new color coordinate systems DEF2, Bef and BCH. • Define main operations and recommend systems in which they work the best. • Show how to prepare a photo with a high dynamic range for printing.

  9. Statements that we accept • Color is a sensation, which cannot be recorded nor can it be reproduced. • Color is caused by stimulus, some characteristics of which can be measured, in order to create another stimulus such that a human (not necessarily a dog or a monkey) is not able to distinguish it from the original stimulus (both cause the same sensation). • Stimulus can be described by Tristimulus Values RGB or XYZ and for each stimulus a 3-dimentional vector can be associated. All stimuli that cause the same sensation are associated with the same vector. • That is what we ACCEPT

  10. Statements that we reject • Brightness, Chroma, Hue are not measurable. • If we want to change their value, we need to know what they are and how to measure them. • Definitions of Brightness and Chroma used in Television, HSV model, and others. • Primary Stimulus RGB or XYZ are orthonormal basis. • Pythagorean Theorem can be used for calculation of a vector’s length in any widely-used color coordinate system. • That is what we REJECT

  11. Our Definitions of Brightness, Chroma, and Hue • B –Brightnessa norm of the color vector S. • C – Chromaan angle between the color vector S and an axis D. • Axis D is a color vector representing Day Light (for example D65, D55, EE etc.). • H– Huean angle betweenaxis E and the orthogonal projection of the color vector S on the plane orthogonal to the axis D. • Axis E - the orthogonal projection of a color vector, corresponding to some fixed stimulus (for example, a monochromatic light with wavelength 700 nm), on the same plane. S B=||S||

  12. Linear Color Coordinate System • Linear CCS is the color coordinate system in which each coordinate of two stimuli mix is equal to a sum of corresponding coordinates of those stimuli. • CCS is Color Coordinate System. • Only Linear CCS may be used for image resize because the use of non-linear CCS (such as sRGB IEC/4WD 61966-2-1 or CIE L*a*b*) for image resize leads to the violation of energy conservation law and results in visual image artifacts. • CIE XYZ is the primary linear CCS in Colorimetry. There are two standards: CIE XYZ 1931 and CIE XYZ 1964, which basis vectors span different subspaces.

  13. Linear CCS DEF2 • Linear CCS DEF2 is designed based on the CIE 1931 data. • Digit “2” indicates 2º Standard Colorimetric Observer. • DEF2 is orthonormal, its design is based on the following restrictions: • D > 0 and Е = F = 0 for standard Day light D65. • E > 0 and F = 0 for monochromatic stimulus with 700 nm wavelength. • F > 0 for yellow stimulus. • CCS DEF2 is an orthonormal coordinate system according to J. Cohen metrics. We have done a research for design different orthonormal coordinate system. We tell you about it in next two presentation today. • The above restrictions uniquely determine coordinate transformation between CIE XYZ 1931 and DEF2.

  14. Coordinate Transformation: CIE XYZ 1931 ↔ DEF2 • Coordinate transformation between CIE XYZ 1931 and DEF2is performed though the matrices of transformation. • XYZis essentially anon-orthonormal system according to J. Cohen metrics.

  15. Plane D = 1 • Plane, where D= 1, is convenient for depicting Gamut of various image reproduction devices, for example, for Gamut of sRGB monitor. sRGB Monitor Gamut White Light

  16. Plane Y = 1 • Plane, where Y = 1, is a plane of constant brightness according to CIE • It is much less convenient for Gamut representation. • This an additional illustration of the fact that XYZ is not orthonormal. White Light sRGB Monitor Gamut

  17. Chromatic Coordinates (x,y) • Chromatic coordinates (x,y) and coordinate system Yxy are widely used for Gamut depicting and for illustration of some color image transformations. • We believe that it is very important to preserve chromatic coordinates unchanged when manipulating Brightness and/or Contrast. • All image editing software (as far as we know) does not meet the above requirement. • Introducing similar chromatic coordinates in DEF is not appropriate. • Coordinates E and F might take negative values. • There are stimuli for which (D + E + F) = 0.

  18. CCS Bef and Chromatic Coordinates e& f • There is an alternative way of defining chromatic coordinates. • B is Brightness. • e and f are chromatic coordinates. • Defining Vector direction through its interception with a unit sphere has more geometrical sense than coordinates of its intersection with any plane.

  19. Chromatic Coordinates e&f sRGB Monitor Gamut White Light

  20. Spherical CCS BCH • Variables B, C, and H, defined earlier, are spherical coordinates related with D, E, F through the following equations: • With this definition, Brightness, Chroma and Hue have a clear physical meaning. • This helps to effectively modify image editing algorithms.

  21. Main Operations • We will cover 6 main operations: • Brightness editing • Contrast editing • Saturation editing • Hue editing • Color to monochrome transformation • Global dynamic range modification without affecting local dynamic range

  22. Brightness Editing • Brightness modification should not affect chromatic coordinates. • In CCS BCH it might be made as follows: • f(B) is a non-negative monotone increasing function. • For example • k is a positive number. • In some graphic editors this transformation is named as program exposure compensation.

  23. Contrast Editing • Contrast modification should not affect chromatic coordinates. • In CCS BCH it might be made as follows: • f(B,B0) is a non-negative monotone increasing function of B. • For example • γ is a positive number. • Bavgis an average brightness in some neighborhood of point.

  24. Saturation Editing • Saturation modification should affect neitherBrightness, nor Hue. • In CCS BCH it might be made as follows: • f(B) is a non-negative monotone increasing function. • For example • k is a positive number. • This operation decreases saturation in two times (colors become more faded). C'=k ·C

  25. Hue Editing • Hue modification should affect neitherBrightness, nor Saturation. • In CCS BCH it might be made as follows: • h(H) is some function. • Usually, in order to make this transformation, graphic editors apply a turn on a fixed angle α. • In CCS BCH it might be made as follows:

  26. Color to Monochrome Transformation • Color-to-monochrome transformation should not affect Brightness. • It can be made, for example, as follows: • C0is Chroma of the chosen color. • H0is Hue of the chosen color. • For grey image, C=H= 0

  27. Global Dynamic Range Modification • Global dynamic rangemodificationshould not affect chromatic coordinates. • Global dynamic rangemodification should not affect Local dynamic range. • For example • γ is a positive number. • Bavgis an average brightness in some neighborhood of point. • B0is a chosen fixed level of brightness. • This transformation allows for preparation of a photo with a high dynamic range for printing (paper has a dynamic range about 30). • High dynamic range my happen due to big parts of an image being lightened by sources of different brightness. • The brightness of different parts may differ in hundreds or thousands times.

  28. Original High-Dynamic Range Photo

  29. Photo Prepared for Printing

  30. Thank You!