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Color models in Digitel image processing
- 2. Roadmap 2 Image Acquisition Image Enhancement Image Restoration Image Compression Image Segmentation Representation & Description Recognition & Interpretation Knowledge Base Preprocessing – low level Image Coding Morphological Image Processing Wavelet Analysis
- 3. Color spectrum 3 When passing through a prism, a beam of sunlight is decomposed into a spectrum of colors: violet, blue, green, yellow, orange, red 1666, Sir Isaac Newton
- 4. Electromagnetic energy spectrum 4 Ultraviolet visible light infrared The longer the wavelength (meter), the lower the frequency (Hz), and the lower the energy (electron volts) The discovery of infrared (1800, Sir Frederick William Herschel) What is infrared? http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/
- 5. Hyperspectral imaging 5 AVIRIS (Airborne Visible-Infrared Imaging Spectrometer) Number of bands: 224 Wavelength range (mm): 0.4-2.5 Image size: 512 x 614 Spectral range visible light (0.4 ~ 0.77mm) near infrared (0.77 ~ 1.5mm) medium infrared (1.5 ~ 6mm) far infrared (6 ~ 40mm)
- 6. Some questions 6 What does it mean when we say an object is in a certain color? Why are the primary colors of human vision red, green, and blue? Is it true that different portions of red, green, and blue can produce all the visible color? What kind of color model is the most suitable one to describe human vision?
- 7. Primary colors of human vision 7 Cones are divided into three sensible categories 65% of cones are sensitive to red light 33% are sensitive to green light 2% are sensitive to blue light For this reason, red, green, and blue are referred to as the primary colors of human vision. CIE standard designated three specific wavelength to these three colors in 1931. Red (R) = 700 nm Green (G) = 546.1 nm Blue (B) = 435.8 nm Detailed experimental Curve available in 1965 Detailed experimental curve available in 1965
- 8. Some clarifications 8 No single color may be called red, green, or blue. R, G, B are only specified by standard.
- 9. Secondary colors 9 Magenta (R + B) Cyan (G + B) Yellow (R + G)
- 10. Primary colors of pigment 10 A primary color of pigment refers to one that absorbs the primary color of the light, but reflects the other two. Primary color of pigments are magenta, cyan, and yellow Secondary color of pigments are then red, green, and blue
- 11. 11
- 12. Additive vs. Subtractive color system involves light emitted directly from a source mixes various amounts of red, green and blue light to produce other colors. Combining one of these additive primary colors with another produces the additive secondary colors cyan, magenta, yellow. Combining all three primary colors produces white. Subtractive color starts with an object that reflects light and uses colorants to subtract portions of the white light illuminating an object to produce other colors. If an object reflects all the white light back to the viewer, it appears white. If an object absorbs (subtracts) all the light illuminating it, it appears black. 12
- 13. Color characterization 13 Brightness: chromatic notion of intensity Hue: dominant color perceived by an observer Saturation: relative purity or the amount of white mixed with a hue R G B H S 0o 120o 240o
- 14. Some clarifications 14 So when we call an object red, orange, etc. we refer to its hue
- 15. Chromaticity 15 Chromaticity: hue + saturation Tristimulus: the amount of R, G, B needed to form any color (X, Y, Z) Trichromatic coefficients: x, y, z 1=++ ++ = ++ = ++ = zyx ZYX Z z ZYX Y y ZYX X x
- 16. Chromaticity diagram 16 Specifying colours systematically can be achieved using the CIE chromacity diagram On this diagram the x-axis represents the proportion of red and the y-axis represents the proportion of red used The proportion of blue used in a colour is calculated as: z = 1 – (x + y)
- 18. This means the entire colour range cannot be displayed based on any three colours The triangle shows the typical colour gamut produced by RGB monitors The strange shape is the gamut achieved by high quality colour printers 18
- 19. Color models 19 RGB model Color monitor, color video cameras CMY model Color printers HSI model Color image manipulation
- 20. RGB model 20 Color monitor, color video cameras (additive color system) Pixel depth – nr of bits used to represent each pixel Full color image (24 bits)
- 21. RGB 21
- 22. CMY model 22 Color printers and copiers (subtractive color system) CMYK color model Four color printing Deposit colored pigment on paper ú ú ú û ù ê ê ê ë é - ú ú ú û ù ê ê ê ë é = ú ú ú û ù ê ê ê ë é B G R Y M C 1 1 1
- 23. HSI model 23 The intensity component (I) is decoupled from the color components (H and S) Ideal for developing image processing algorithms H and S are closely related to the way human visual system perceives colors
- 26. RGB-to-HSI conversion 26 BG BG H BGBRGR BRGR BGR I S BGRI 2 2 1 cos ],,[min 3 1 3 1 2 1 Given a color as R, G, and B its H, S, and I values are calculated as follows
- 27. HSI-to-RGB conversion (*) 27 Given a color as H, S, and I it’s R, G, and B values are calculated as follows: -RG sector (0o <= H < 120o) -GB sector 120o <= H < 240o For 240o <= H < 360o )(3,1, 60cos cos 1 BRIGSIB H HS IR )(3,1, 180cos 120cos 1 GRIBSIR H HS IG )(3,1, 300cos 240cos 1 BGIRSIG H HS IB