Background SubtractionImage StackExampleGetting the right pixelsInput VideoAverage ImageSlide 7Slide 8Crowd Synthesis (with Pooja Nath)Background Subtraction for mattingRemoving Shadows (Weiss, 2001)Averaging DerivativesRecovering ShadowsCompositing with ShadowsFigure-centric RepresentationContext-based Image CorrectionMidterm StatsFun with Focal Length (Jim Sherwood)Background Subtraction15-463: Rendering and Image ProcessingAlexei EfrosImage StackAs can look at video data as a spatio-temporal volume•If camera is stationary, each line through time corresponds to a single ray in space•We can look at how each ray behaves •What are interesting things to ask?t0255timeExampleGetting the right pixelsAverage imageMedian ImageInput VideoAverage ImageWhat is happening?Average ImageWhat can we do with this?Background Subtraction--==Crowd Synthesis (with Pooja Nath)1. Do background subtraction in each frame2. Find and record “blobs”3. For synthesis, randomly sample the blobs, taking care not to overlap themBackground Subtraction for mattingA very hard problem.But sometimes it works:Removing Shadows (Weiss, 2001)How does one detect (subtract away) shadows?Averaging DerivativesRecovering ShadowsCompositing with ShadowsFigure-centric RepresentationContext-based Image CorrectionInput sequence3 closestframesmedian imagesMidterm Stats# of people: 11Total points possible: 120Mean: 77.1Median: 76Max: 98Min: 56Std. Div: 13.2Skewness: -0.04Kurtosis: 1.9The scores: [56 61 68 70 71 76 82 88 89 89 98]Fun with Focal Length (Jim
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