1Advanced Computer Graphics Advanced Computer Graphics (Fall 2009)(Fall 2009)CS 294, Rendering Lecture 12 Computational Imaging and PhotographyRavi Ramamoorthihttp://inst.eecs.berkeley.edu/~cs294-13/fa09Many slides courtesy Ramesh Raskar, SIGGRAPH 2008 courseComputational ImagingComputational Imaging Digital cameras now commonplace Can we use computation for better images Many novel capabilities relative to film And new ways of processing images Is this computer graphics, optics, or image proc? All of the above; many rendering ideas apply Application shift. Computer aided design to movies/games to photography (big market) Brief lecture. Some more on image processing by Prof. Agrawala in 2 weeksOutlineOutline Image formation, basic lens-based camera Light Field camera Coded aperture depth of field Flutter shutter (coded aperture shutter) Many many more old, new innovationsHow do we see the world?Let’s design a camera• Idea 1: put a piece of film in front of an object• Do we get a reasonable image?object filmSlide by Steve SeitzPinhole cameraAdd a barrier to block off most of the rays• This reduces blurring• The opening known as the aperture• How does this transform the image?object filmbarrierSlide by Steve SeitzPinhole camera modelPinhole model:• Captures pencil of rays – all rays through a single point• The point is called Center of Projection (COP)• The image is formed on the Image Plane• Effective focal length f is distance from COP to Image PlaneSlide by Steve Seitz2Point of observationFigures©StephenE.Palmer,2002Dimensionality Reduction Machine (3D to 2D)3D world 2D imageWhat have we lost?• Angles• Distances (lengths)Funny things happen…Parallel lines aren’t…Figure by David ForsythLengths can’t be trusted...Figure by David ForsythB’C’A’…but humans adopt!http://www.michaelbach.de/ot/sze_muelue/index.htmlMüller-Lyer IllusionWe don’t make measurements in the image planeCamera ObscuraThe first camera• Known to Aristotle• Depth of the room is the effective focal lengthCamera Obscura, Gemma Frisius, 15583From Pinhole to LensesComputer graphics assumes pinhole modelBut making aperture narrow limits lightMaking aperture large causes blurrinessReal cameras have lenses to collect more light, and focus it on the image plane(Kolb et al. 95 simulates lens effects rendering)Home-made pinhole camera http://www.debevec.org/Pinhole/Why soblurry?Shrinking the apertureWhy not make the aperture as small as possible?• Less light gets through• Diffraction effects…Less light gets throughSlide by Steve SeitzThe reason for lensesSlide by Steve SeitzFocus and DefocusA lens focuses light onto the film• There is a specific distance at which objects are “in focus”– other points project to a “circle of confusion” in the image• Changing the shape/separation of lens changes this distanceobject filmlens“circle of confusion”Slide by Steve SeitzThin lensesThin lens equation:• Any object point satisfying this equation is in focus• What is the shape of the focus region?• How can we change the focus region?• Thin lens applet: http://www.phy.ntnu.edu.tw/java/Lens/lens_e.html (by Fu-Kwun Hwang )Slide by Steve Seitz4Depth of Fieldhttp://www.cambridgeincolour.com/tutorials/depth-of-field.htmOutlineOutline Image formation, basic lens-based camera Light Field camera Coded aperture depth of field Flutter shutter (coded aperture shutter) Many many more old, new innovationsLight Field Inside a CameraLight Field Inside a CameraLensletLenslet--based Light Field camerabased Light Field camera[Adelson and Wang, 1992, Ng et al. 2005 ]Light Field Inside a CameraLight Field Inside a CameraStanford Stanford PlenopticPlenopticCamera Camera [Ng et al 2005][Ng et al 2005]4000 × 4000 pixels ÷ 292 × 292 lenses = 14 × 14 pixels per lensContax medium format camera Kodak 16-megapixel sensorAdaptive Optics microlens array 125μ square-sided microlensesDigital RefocusingDigital Refocusing[Ng et al 2005][Ng et al 2005]5Mask based Light Field CameraMaskSensor[Veeraraghavan, Raskar, Agrawal, Tumblin, Mohan, Siggraph 2007 ]1/f0Mask TileCosine Mask UsedCaptured 2D PhotoEncoding due to Mask[Veeraraghavan, Raskar, Agrawal, Tumblin, Mohan, Siggraph 2007 ]2D FFTTraditional Camera PhotoHeterodyne Camera PhotoMagnitude of 2D FFT2D FFTMagnitude of 2D FFTOutlineOutline Image formation, basic lens-based camera Light Field camera Coded aperture depth of field Flutter shutter (coded aperture shutter) Many many more old, new innovationsOut of Focus Photo: Coded ApertureEngineering the PSF when you cannot capture Lightfield6In Focus PhotoLED2D PhotoOut of Focus Photo: Open ApertureOut of Focus Photo: Coded ApertureCaptured Blurred Photo[Veeraraghavan, Raskar, Agrawal, Tumblin, Mohan, Siggraph 2007 ]Refocused on PersonIncrease DoF +large apertureOutlineOutline Image formation, basic lens-based camera Light Field camera Coded aperture depth of field Flutter shutter (coded aperture shutter) Many many more old, new innovations7Traditional CameraShutter is OPENOur CameraFlutter ShutterShutter is OPEN and CLOSEDLab SetupBlurring ==ConvolutionTraditional Camera: Box FilterSync FunctionFlutter Shutter: Coded FilterPreserves High Frequencies!!!8ComparisonInverse Filter UnstableInverse Filter stableInput ImageRectified CropDeblurred
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