1. Capturing Light… in man and machine 15-463: Computational Photography Alexei Efros, CMU, Fall 2008

2. Image Formation Digital Camera The Eye Film

3. Digital camera A digital camera replaces film with a sensor array Each cell in the array is light-sensitive diode that converts photons to electrons Two common types –Charge Coupled Device (CCD) –CMOS http://electronics.howstuffworks.com/digital-camera.htm Slide by Steve Seitz

4. Sensor Array CMOS sensor

5. Sampling and Quantization

6. Interlace vs. progressive scan http://www.axis.com/products/video/camera/progressive_scan.htmSlide by Steve Seitz

7. Progressive scan http://www.axis.com/products/video/camera/progressive_scan.htmSlide by Steve Seitz

8. Interlace http://www.axis.com/products/video/camera/progressive_scan.htmSlide by Steve Seitz

9. The Eye The human eye is a camera! Iris - colored annulus with radial muscles Pupil - the hole (aperture) whose size is controlled by the iris What’s the “film”? –photoreceptor cells (rods and cones) in the retina Slide by Steve Seitz

10. The Retina

11. Retina up-close Light

12. © Stephen E. Palmer, 2002 Cones cone-shaped less sensitive operate in high light color vision Two types of light-sensitive receptors Rods rod-shaped highly sensitive operate at night gray-scale vision

13. Rod / Cone sensitivity The famous sock-matching problem…

14. © Stephen E. Palmer, 2002 Distribution of Rods and Cones Night Sky: why are there more stars off-center?

15. Electromagnetic Spectrum http://www.yorku.ca/eye/photopik.htm Human Luminance Sensitivity Function

16. Why do we see light of these wavelengths? © Stephen E. Palmer, 2002 …because that’s where the Sun radiates EM energy Visible Light

17. The Physics of Light Any patch of light can be completely described physically by its spectrum: the number of photons (per time unit) at each wavelength 400 - 700 nm. © Stephen E. Palmer, 2002

18. The Physics of Light Some examples of the spectra of light sources © Stephen E. Palmer, 2002

19. The Physics of Light Some examples of the reflectance spectra of surfaces Wavelength (nm) % Photons Reflected Red 400 700 Yellow 400 700 Blue 400 700 Purple 400 700 © Stephen E. Palmer, 2002

20. The Psychophysical Correspondence There is no simple functional description for the perceived color of all lights under all viewing conditions, but …... A helpful constraint: Consider only physical spectra with normal distributions area mean variance © Stephen E. Palmer, 2002

21. The Psychophysical Correspondence MeanHue # Photons Wavelength © Stephen E. Palmer, 2002

22. The Psychophysical Correspondence VarianceSaturation Wavelength # Photons © Stephen E. Palmer, 2002

23. The Psychophysical Correspondence AreaBrightness # Photons Wavelength © Stephen E. Palmer, 2002

24. Three kinds of cones: Physiology of Color Vision Why are M and L cones so close? Why are there 3?

25. More Spectra metamers

26. Color Sensing in Camera (RGB) 3-chip vs. 1-chip: quality vs. cost Why more green? http://www.coolditionary.com/words/Bayer-filter.wikipedia http://www.cooldictionary.com/words/Bayer-filter.wikipedia Why 3 colors? Slide by Steve Seitz

27. Practical Color Sensing: Bayer Grid Estimate RGB at ‘G’ cels from neighboring values http://www.cooldictionary.com/ words/Bayer-filter.wikipedia Slide by Steve Seitz

28. RGB color space RGB cube Easy for devices But not perceptual Where do the grays live? Where is hue and saturation? Slide by Steve Seitz

29. HSV Hue, Saturation, Value (Intensity) RGB cube on its vertex Decouples the three components (a bit) Use rgb2hsv() and hsv2rgb() in Matlab Slide by Steve Seitz

30. Programming Project #1 How to compare R,G,B channels? No right answer Sum of Squared Differences (SSD): Normalized Correlation (NCC):