《点扩散函数》PPT课件.ppt

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1、Course 15:Computational PhotographyCourse 15:Computational PhotographyA.3:Understanding Film-like Photography TumblinA.3:Understanding Film-like Photography TumblinJack TumblinJack TumblinNorthwestern UniversityNorthwestern UniversityA3:UnderstandingFilm-Like Photographyor from 2D Pixels to 4D Rays(

2、10 minutes)Computational PhotographyComputational PhotographyNave,Ideal Film-like PhotographyAngle(Angle(,)Position(x,y)Position(x,y)2D Sensor:2D Sensor:Pixel Grid,Film,Pixel Grid,Film,Well-Lit 3D Scene:Well-Lit 3D Scene:Center of Center of ProjectionProjectionSensorSensor:a film emulsion,:a film em

3、ulsion,:or a grid of light meters(pixels):or a grid of light meters(pixels)RayRayRays and the Thin Lens Law Focal lengthFocal length f:f:where parallel rays converge where parallel rays converge Focus at infinity:Adjust for Focus at infinity:Adjust for S S2 2=f=f Closer Focus?Larger Closer Focus?Lar

4、ger S S2 2Try it Live!PhysletsTry it Live!Physletshttp:/http:/Thin LensfS2SensorSensorRays and the Thin Lens Law Focal lengthFocal length f:f:where parallel rays converge where parallel rays converge Focus at infinity:Adjust for Focus at infinity:Adjust for S S2 2=f=f Closer Focus?Larger Closer Focu

5、s?Larger S S2 2Try it Live!PhysletsTry it Live!Physletshttp:/http:/SceneSceneThin LensfS2fS2SensorSensorNot One Ray,but a Bundle of Rays BUT BUT Ray model isnt perfect:ignores diffraction Ray model isnt perfect:ignores diffraction Lens,aperture set the point-spread-function(PSF)Lens,aperture set the

6、 point-spread-function(PSF)(How?See:(How?See:Goodman,J.WGoodman,J.W.An Introduction to Fourier Optics 1968).An Introduction to Fourier Optics 1968)SceneSceneSensorSensorApertureLensBasic Ray Optics:Lens ApertureFor the same focal length:Larger lensGathers a wider ray bundle:Gathers a wider ray bundl

7、e:More light:brighter imageMore light:brighter imageNarrower depth-of-focusNarrower depth-of-focusSmaller lensdimmer imagedimmer imagefocus becomes less criticalfocus becomes less criticalmore depth of focusmore depth of focusFilm-like Optics:Thin Lens FlawsAberrations:Real lenses dont converge rays

8、 perfectlySpherical:edge rays center raysComa:diagonal rays focus deeper at edgehttp:/encyclopedia/Lens-(optics)Lens Flaws:Chromatic AberrationDispersion:wavelength-dependent refractive indexDispersion:wavelength-dependent refractive index(enables prism to spread white light beam into rainbow)(enabl

9、es prism to spread white light beam into rainbow)Modifies ray-bending and lens focal length:f(Modifies ray-bending and lens focal length:f()color fringes near edges of imagecolor fringes near edges of imageCorrections:add doublet lens of flint glass,etc.Corrections:add doublet lens of flint glass,et

10、c.http:/http:/l lChromatic Aberration Lens Design Fix:Multi-element lensesComplex,expensive,many tradeoffs!Computed Fix:Geometric warp for R,G,B.Near Lens CenterNear Lens CenterNear Lens Outer EdgeNear Lens Outer EdgeRadial Distortion(e.g.Barrel and pin-cushion)straight lines curve around the image

11、center straight lines curve around the image center Vignette EffectsBright at center,dark at edges.Bright at center,dark at edges.Several causes compounded:Several causes compounded:Edge pixels span smaller angle and center pixelsEdge pixels span smaller angle and center pixelsRay path length is lon

12、ger off-axisRay path length is longer off-axisInternal shadowingInternal shadowingCompensation:Compensation:Use anti-vignetting filters,Use anti-vignetting filters,(darkest at center)(darkest at center)OR Position-dependent OR Position-dependent pixel-detector sensitivity.pixel-detector sensitivity.

13、http:/http:/Film-like Color SensingEquiluminant CurveEquiluminant Curvedefines luminance defines luminance vs.wavelengthvs.wavelength Visible Light:narrow band of Visible Light:narrow band of emagemag.spectrum.spectrum 400-700 nm400-700 nm(nm=10-9 meter wavelength)(nm=10-9 meter wavelength)(humans:1

14、 octave (humans:1 octave honey bees:3-4 octaveshoney bees:3-4 octavesdo honey bees sense harmonics,see color chords?do honey bees sense harmonics,see color chords?Film-like Color SensingRGB spectral curves RGB spectral curves Vaytek CCD camera Vaytek CCD camera with Bayer gridwith Bayer grid Visible

15、 Light:narrow band of Visible Light:narrow band of emagemag spectrum spectrum 400-700 nm400-700 nm(nm=10-9 meter wavelength)(nm=10-9 meter wavelength)At least 3 spectral bands required(e.g.R,G,B)At least 3 spectral bands required(e.g.R,G,B)Color Sensing3-chip:vs.1-chip:quality Color Sensing:Bayer Gr

16、idEstimate RGBat G cels from neighboring valueshttp:/words/Bayer-filter.wikipediaPolarizationSunlit haze is often Sunlit haze is often strongly polarized.strongly polarized.Polarization filter yieldsPolarization filter yieldsmuch richer sky colorsmuch richer sky colorsRAYS and PROCESSINGONE Ray carr

17、ies doubly infinitesimal power:Ray bundles with finite,measurable power will:Ray bundles with finite,measurable power will:Span a non-zero areaSpan a non-zero area Fill a non-zero solid angle Fill a non-zero solid angle Everything is Linear:(HUGE win!)Ray reflectance,transmission,absorption,scatter*

18、Ray reflectance,transmission,absorption,scatter*Rays are REVERSIBLE.Helmholtz reciprocityHelmholtz reciprocityRay bundles?Not so much:falls quickly with Ray bundles?Not so much:falls quickly with angle,area growthangle,area growthFilm-like Photography:Many LimitationsOptics:Optics:Single focus dista

19、nce,limited depth-of-field,limited field-of-view,Single focus distance,limited depth-of-field,limited field-of-view,internal reflections/flare/glareinternal reflections/flare/glareLighting:Lighting:Camera has no knowledge of ray source strength,position,Camera has no knowledge of ray source strength

20、,position,direction;little control(e.g.flash)direction;little control(e.g.flash)Sensor:Sensor:Exposure setting,motion blur,noise,response time,Exposure setting,motion blur,noise,response time,Processing:Processing:Quantization/color depth,camera shake,scene movementQuantization/color depth,camera sh

21、ake,scene movementConclusions Film-like photography methods limit digital photography to Film-like photography methods limit digital photography to film-like results or less.film-like results or less.Broaden,unlock our views of photography:Broaden,unlock our views of photography:4-D,8-D,even 10-D Ra

22、y Space holds the photographic 4-D,8-D,even 10-D Ray Space holds the photographic signal.Look for new solutions by creating,gathering,signal.Look for new solutions by creating,gathering,processing RAYS,not focal-plane intensities.processing RAYS,not focal-plane intensities.Choose the best,most expre

23、ssive sets of rays,Choose the best,most expressive sets of rays,THEN find the best way to measure them.THEN find the best way to measure them.Useful links:Interactive Thin Lens Demo(or search physlet optical bench)For more about color:Prev.SIGGRAPH courses(Stone et al.)Prev.SIGGRAPH courses(Stone et al.)Good:Good:www.cs.rit.edu/ncs/color/a_spectr.htmlwww.cs.rit.edu/ncs/color/a_spectr.html Good:Good:Good:Good:Course 15:Computational PhotographyCourse 15:Computational Photography