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Course 15: Computational Photography

Course 15: Computational Photography. Course 15: Computational Photography. A.3: Understanding Film-like Photography Tumblin. Computational Photography. Computational Photography. A3: Understanding Film-Like Photography or ‘ from 2D Pixels to 4D Rays ’ (10 minutes).

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Course 15: Computational Photography

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  1. Course 15: Computational Photography Course 15: Computational Photography A.3: Understanding Film-like Photography Tumblin

  2. Computational Photography Computational Photography A3:UnderstandingFilm-Like Photographyor ‘from 2D Pixels to 4D Rays’(10 minutes) Jack Tumblin Northwestern University

  3. Naïve, Ideal Film-like Photography Well-Lit 3D Scene: Sensor: a film emulsion, : or a grid of light meters (pixels) Ray ‘Center of Projection’ Position (x,y) Angle(,) 2D Sensor: Pixel Grid,Film,…

  4. Rays and the ‘Thin Lens Law’ • Focal lengthf: where parallel rays converge • Focus at infinity: Adjust for S2=f • Closer Focus ? Larger S2 f Sensor S2 Thin Lens Try it Live! Physlets… http://webphysics.davidson.edu/Applets/Optics/intro.html

  5. Rays and the ‘Thin Lens Law’ • Focal lengthf: where parallel rays converge • Focus at infinity: Adjust for S2=f • Closer Focus ? Larger S2 f f Sensor Scene S2 S2 Thin Lens Try it Live! Physlets… http://webphysics.davidson.edu/Applets/Optics/intro.html

  6. Not One Ray, but a Bundle of Rays Lens Scene Sensor Aperture • BUT Ray model isn’t perfect: ignores diffraction • Lens, aperture set the point-spread-function (PSF) (How? See: Goodman,J.W. ‘An Introduction to Fourier Optics’ 1968)

  7. Basic Ray Optics: Lens Aperture For the same focal length: • Larger lens • Gathers a wider ray bundle: • More light: brighter image • Narrower depth-of-focus • Smaller lens • dimmer image • focus becomes less critical • more depth of focus

  8. Film-like Optics: Thin Lens Flaws • Aberrations: Real lenses don’t converge rays perfectly • Spherical: edge rays  center rays • Coma: diagonal rays focus deeper at edge http://www.nationmaster.com/encyclopedia/Lens-(optics)

  9. Lens Flaws: Chromatic Aberration • Dispersion: wavelength-dependent refractive index • (enables prism to spread white light beam into rainbow) • Modifies ray-bending and lens focal length: f() • color fringes near edges of image • Corrections: add ‘doublet’ lens of flint glass, etc. http://www.swgc.mun.ca/physics/physlets/opticalbench.html

  10. Chromatic Aberration • Lens Design Fix: Multi-element lenses Complex, expensive, many tradeoffs! • Computed Fix: Geometric warp for R,G,B. Near Lens Center Near Lens Outer Edge

  11. Radial Distortion (e.g. ‘Barrel’ and ‘pin-cushion’) straight lines curve around the image center

  12. Vignette Effects Bright at center, dark at edges.Several causes compounded: • Edge pixels span smaller angle and center pixels • Ray path length is longer off-axis • Internal shadowing • Compensation: • Use anti-vignetting filters, (darkest at center) • OR Position-dependent pixel-detector sensitivity. http://homepage.ntlworld.com/j.houghton/vignette.htm

  13. Film-like Color Sensing • Visible Light: narrow band of e’mag. spectrum •   400-700 nm (nm = 10-9 meter wavelength) • (humans:<1 octave honey bees: 3-4 ‘octaves do honey bees sense harmonics, see color ‘chords’ ? • Equiluminant Curvedefines ‘luminance’ vs. wavelength http://www.yorku.ca/eye/photopik.htm

  14. Film-like Color Sensing • Visible Light: narrow band of emag spectrum •   400-700 nm (nm = 10-9 meter wavelength) • At least 3 spectral bands required (e.g. R,G,B) • RGB spectral curves Vaytek CCD camera with Bayer grid www.vaytek.com/specDVC.htm

  15. Color Sensing • 3-chip: vs. 1-chip: quality vs. cost http://www.cooldictionary.com/words/Bayer-filter.wikipedia

  16. 1-Chip Color Sensing: Bayer Grid • Estimate RGBat ‘G’ cels from neighboring values http://www.cooldictionary.com/words/Bayer-filter.wikipedia

  17. Polarization Sunlit haze is often strongly polarized. Polarization filter yieldsmuch richer sky colors

  18. RAYS and PROCESSING • ONE Ray carries doubly infinitesimal power: Ray bundles with finite, measurable power will: • Span a non-zero area • Fill a non-zero solid angle • Everything is Linear: (HUGE win!) Ray reflectance, transmission, absorption, scatter*… • Rays are REVERSIBLE. Helmholtz reciprocity Ray bundles? Not so much: falls quickly with angle,area growth…

  19. Film-like Photography:Many Limitations • Optics: Single focus distance, limited depth-of-field, limited field-of-view, internal reflections/flare/glare • Lighting: Camera has no knowledge of ray source strength, position, direction; little control (e.g. flash) • Sensor: Exposure setting, motion blur, noise, response time, • Processing: • Quantization/color depth, camera shake, scene movement…

  20. Conclusions • Film-like photography methods limit digital photography to film-like results or less. • Broaden, unlock our views of photography: • 4-D, 8-D, even 10-D Ray Space holds the photographic signal. Look for new solutions by creating, gathering, processing RAYS, not focal-plane intensities. • Choose the best, most expressive sets of rays, THEN find the best way to measure them.

  21. Useful links: Interactive Thin Lens Demo (or search ‘physlet optical bench’) www.swgc.mun.ca/physics/physlets/opticalbench.html For more about color: • Prev. SIGGRAPH courses (Stone et al.) • Good: www.cs.rit.edu/~ncs/color/a_spectr.html • Good: www.colourware.co.uk/cpfaq.htm • Good: www.yorku.ca/eye/toc.htm

  22. Course 15: Computational Photography Course 15: Computational Photography

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