CS6825: Color

1. CS6825: Color

2. Light and Color • Light is electromagnetic radiation • Visible light: 400-700nm. range

3. Color: • It is determined by light source, surface properties (what light is absorbed & reflected) and the sensor.

4. The Sensor - Human Vision • Human eyes have 2 types of sensors: • CONES • Sensitive to colored light, but not very sensitive to dim light, Curves like Red, Green, Blue • RODS • (very) Sensitive to achromatic light

5. Human Cones SG • We perceive color using three different types of cones: • 440 nm (BLUE) • 545 nm (GREEN) • 580 nm (RED) SR SB

6. Illumination Source - Sunlight • Sunlight under different conditions…is not pure white light

7. Color Models They provide a standard way of specifying a particular color using a 3D coordinate system. • Hardware oriented: • RGB • CMY • YIQ (luminance, inphase, quadrature) • Image processing oriented: • HIS (hue, intensity, saturation) • XYZ

8. CIE primaries: XYZ • The Commission Internationale de l’Eclairage defined 3 standard primaries: X, Y, Z that can be added to form all visible colors. (better at this than Red, Green, Blue space ) • Y was chosen so that its color matching function matches the sum of the 3 human cone responses.

9. CIE primaries: XYZ • All positive spectral matching curves • Y corresponds to brightness • Equal energy white: X=Y=Z • Relationship to RGB

10. x,y,z normalize X,Y,Z s.t. x+y+z=1 • Chromaticity is given by • HUE • SATURATION • Pure colors are at the curved boundary • White is (1/3,1/3,1/3) Image Gonzales and Woods

11. Mixing Colors on CIE diagram • Any color along a line can be obtained by mixing the colors of the endpoints. • Any point in a triangle can be obtained by mixing the colors of the vertices.

12. RGB Model • Based on human visual system cones. • Additive model. • An image consists of 3 BANDS: Red (R), Green (G), Blue (B) • Used in computer monitors, televisions, consumer digital cameras. • Hence often used in media vision systems.

13. RGB convert to XYZ

14. CMY Model • Cyan-Magenta-Yellow is a subtractive model which is good to model absorption of colors. • Appropriate for paper printing. Subtractive model

15. Typically used for projected/ emitted light like TV / computers RGB Is Additive CMY Is Subtractive Typically used for reflected light like in printers

16. YIQ Model • Used by NTSC TV standard • Separates Hue (I,Q) from Luminance (Y) (so for old black and white TVs can transmit/use just Y).

17. Luminance vs Intensity

18. HSI Model Multiple views of the HSI space

19. RGB-HSI Conversion(See Gonzalez and Woods) Note: R, G and B are in [0,1] range

20. Color constancy • Ability to perceive the colors of objects in a scene independent for the most part from the color of the ambient illumination • Research into how can from a measurement such as R,G,B extract the information from the true color of the object (surface) and REMOVE the factors of the illumination source and the sensor limitation/properties.

21. Color Balancing by von Kries method • balancing is the global adjustment of the intensities of the colors • Refer to wiki(http://en.wikipedia.org/wiki/Color_balance) for details of this simple algorithm that uses XYZ space. • Here are the results:

22. Conclusion • Color is an important visual clue – how would you know how to grab a red pen without the red cap? • Color can and is represented in many different “color spaces”…like RGB, CMY, XYZ,etc. • We typically use different color spaces for different purposes (i.e. CMY for printers, RGB for monitors and cameras). • Color correction looks at trying to reduce or if possible eliminate the effects the illumination has on the appearance of the color of an object. This is an active area of research in color. We saw a simple color balance method.