摄影教学：计算摄影学Computational Photography.ppt
A.3:Understanding Film-like Photography Tumblin,Jack TumblinNorthwestern University,A3:UnderstandingFilm-Like Photographyor from 2D Pixels to 4D Rays(10 minutes),Nave,Ideal Film-like Photography,Angle(,),Position(x,y),2D Sensor:Pixel Grid,Film,Well-Lit 3D Scene:,Center of Projection,Sensor:a film emulsion,:or a grid of light meters(pixels),Ray,Rays and the Thin Lens Law,Focal length f:where parallel rays convergeFocus at infinity:Adjust for S2=fCloser Focus?Larger S2,Try it Live!Physlets,http:/webphysics.davidson.edu/Applets/Optics/intro.html,Thin Lens,f,S2,Sensor,Rays and the Thin Lens Law,Focal length f:where parallel rays convergeFocus at infinity:Adjust for S2=fCloser Focus?Larger S2,Try it Live!Physlets,http:/webphysics.davidson.edu/Applets/Optics/intro.html,Scene,Thin Lens,f,S2,f,S2,Sensor,Not One Ray,but a Bundle of Rays,BUT Ray model isnt perfect:ignores diffraction Lens,aperture set the point-spread-function(PSF)(How?See:Goodman,J.W.An Introduction to Fourier Optics 1968),Scene,Sensor,Aperture,Lens,Basic Ray Optics:Lens Aperture,For the same focal length:Larger lensGathers a wider ray bundle:More light:brighter imageNarrower depth-of-focusSmaller lensdimmer imagefocus becomes less criticalmore depth of focus,Film-like Optics:Thin Lens Flaws,Aberrations:Real lenses dont converge rays perfectlySpherical:edge rays center raysComa:diagonal rays focus deeper at edge,http:/,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 imageCorrections:add doublet lens of flint glass,etc.,http:/www.swgc.mun.ca/physics/physlets/opticalbench.html,Chromatic Aberration,Lens Design Fix:Multi-element lensesComplex,expensive,many tradeoffs!Computed Fix:Geometric warp for R,G,B.,Near Lens Center,Near Lens Outer Edge,Radial Distortion(e.g.Barrel and pin-cushion),straight lines curve around the image center,Vignette Effects,Bright at center,dark at edges.Several causes compounded:Edge pixels span smaller angle and center pixelsRay path length is longer off-axisInternal shadowingCompensation:Use anti-vignetting filters,(darkest at center)OR Position-dependent pixel-detector sensitivity.,http:/,Film-like Color Sensing,http:/www.yorku.ca/eye/photopik.htm,Equiluminant Curvedefines luminance vs.wavelength,Visible Light:narrow band of emag.spectrum 400-700 nm(nm=10-9 meter wavelength)(humans:1 octave honey bees:3-4 octavesdo honey bees sense harmonics,see color chords?,Film-like Color Sensing,RGB spectral curves Vaytek CCD camera with Bayer grid,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),Color Sensing,3-chip:vs.1-chip:quality vs.cost,http:/,1-Chip Color Sensing:Bayer Grid,Estimate RGBat G cels from neighboring values,http:/,Polarization,Sunlit haze is often strongly polarized.Polarization filter yieldsmuch richer sky colors,RAYS and PROCESSING,ONE Ray carries doubly infinitesimal power:Ray bundles with finite,measurable power will:Span a non-zero areaFill a non-zero solid angle Everything is Linear:(HUGE win!)Ray reflectance,transmission,absorption,scatter*Rays are REVERSIBLE.Helmholtz reciprocityRay bundles?Not so much:falls quickly with angle,area growth,Film-like Photography:Many Limitations,Optics:Single focus distance,limited depth-of-field,limited field-of-view,internal reflections/flare/glareLighting: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,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.,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.htmGood:www.yorku.ca/eye/toc.htm,
