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Introduction to Image-Based Rendering Jian Huang, CS 594, Spring 2002 A part of this set of slides reference slides used at Standford by Prof. Pat Hanrahan and Philipp Slusallek.

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What is Image- Based Rendering? Not just using images on geometry (akin to texture mapping) Built on desire to bypass the manual modeling phase Use images (of some kind) for modeling and rendering

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Types of IBR Panoramas/Image Mosaics/Light Fields, Lumigraph –QuicktimeVR –Concentric Mosaics, light fields/lumigraph View Interpolation Model based methods –Depth Images –Geometry from Images

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Plenoptic Function Plenoptic function (7D) depicts light rays passing through: –center of camera at any location (x,y,z) –at any viewing angle ( , ) –for every wavelength ( ) –for any time ( t )

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Limiting Dimensions of Plenoptic Functions Plenoptic modeling (5D) : ignore time & wavelength Lumigraph/Lightfield (4D) : constrain the scene (or the camera view) to a bounding box 2D Panorama : fix viewpoint, allow only the viewing direction and camera zoom can be changed

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Limiting Dimensions of Plenoptic Functions Concentric mosaics (3D) : index all input image rays in 3 parameters: radius, rotation angle and vertical elevation

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Apple’s QuickTime VR OutwardInward

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Mars Pathfinder Panorama

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Creating a Cylindrical Panorama From www.quicktimevr.apple.com

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Commercial Products –QuickTime VR, LivePicture, IBM (Panoramix) –VideoBrush –IPIX (PhotoBubbles), Be Here, etc.

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Light Field and Lumigraph Take advantage of empty space to –reduce Plenoptic Function to 4D

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Capturing Lightfields Need a 2D set of (2D) images Choices: –Camera motion: human vs. computer –Constraints on camera motion –Coverage and sampling uniformity –Aliasing

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Point / angle Two points on a sphere Points on two planes Original images and camera positions Lightfield Parameterization

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Two Plane Parametrization Object Focal plane (st) Camera plane (uv)

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Reconstruction

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Light Field Key Ideas: n4D function - Valid outside convex hull n2D slice = image - Insert to create - Extract to display nInward or outward

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Lightfields Advantages: –Simpler computation vs. traditional CG –Cost independent of scene complexity –Cost independent of material properties and other optical effects Disadvantages: –Static geometry –Fixed lighting –High storage cost

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Concentric Mosaics Concentric mosaics : easy to capture, small in storage size

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Concentric Mosaics A set of manifold mosaics constructed from slit images taken by cameras rotating on concentric circles

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Sample Images

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Rendering a Novel View

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Construction of Concentric Mosaics Synthetic scenes –uniform angular direction sampling –square root sampling in radial direction

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Construction of Concentric Mosaics (2) Real scenes Bulky, costly Cheaper, easier

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Construction of Concentric Mosaics (3) Problems with single camera: –Limited horizontal fov –Non-uniform spatial horizontal resolution Resampling necessary –bilinear is better than point sampling Video sequence can be compressed with VQ and entropy encoding (25X) Compressed stream gives 20fpx on PII300

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Results

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Results (2)

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View Interpolation Sprites/Imposters with Depth –Better image warping: Wider range of reuse Backward mapping only with homograph –New mapping: Stored depth map Forward map depth map (approximate geometry) Backward mapping of color using depth information d d’

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Mapping with Depth Forward Mapping: –Holes and aliasing I 1 d 1 (I 2 ) I 2

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Mapping with Depth Backward Mapping: –What is d? I 1 (I 2 ) d 2 I 2

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Mapping with Depth Solution: –Forward map depth –Reconstruct approximate geometry –Backward map color I 1 (I 2 ) d 2 I 2

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Layered Depth Images Idea: –Handle disocclusion –Store invisible geometry in depth images Data structure: –Per pixel list of depth samples –Per depth sample: RGBA Z Encoded: Normal direction, distance –Pack into cache lines

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Layered Depth Images Computation: –Incremental warping computation –Implicit ordering information Process in up to four quadrant –Splat size computation Table lookup Fixed splat templates –Clipping of LDIs

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Layered Depth Images

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Model-based IBR Basic Idea: Sparse set of images [Debevec’97, Pulli’96] Overview: –Approximate Modeling Photogrammetric modeling Triangulated depth maps –View-dependent Texture Mapping Weighting Hardware accelerated rendering –Model-based Stereo Details from stereo algorithms

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Hybrid Approach Courtesy: P. Debevec

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Approximate Modeling User-assisted photogrammetry [Debevec ‘97]: –Based on “Structure from Motion” –Use constraints in architectural models Approach: –Simple block model –Constraints reduce DOF –Matching based on lines –Non-linear optimization –Initial Camera Positions

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Approximate Modeling: Block Model Courtesy: P. Debevec

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Approximate Modeling Active Light: –Calibrated camera and projector –Plane of light and triangulation –Registration of multiple views –Triangulation of point cloud Projector Camera

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Approximate Modeling

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Projecting Images Technique: –Known camera positions –Projective texture mapping –Shadow buffer for occlusions –Blending between textures –Filling in

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Visibility

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Projecting Images

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Simple compositing vs. blending Blending: –Select “best” image closeness to viewing direction distance to border sampling density [Pulli] deletion of features Some computation in HW –Smooth transition between pixels and frames Alpha blending, soft Z-buffer, confidence

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Projecting Images Closeness to viewing direction: –Triangulate the Hemisphere Delaunay triangulation of viewing directions Regular triangulation: label each vertex with best view –Interpolate based on barycentric coordinates

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Blending of Textures

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Model-Based Stereo Problems with conventional stereo algorithms: –Correspondences are difficult to find –Large disparities –Foreshortening, projective distortions Approach: –Use approximate geometry to reproject one image –Compute disparity of warped image Significant smaller disparity and foreshortening

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Model-Based Stereo

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Demos

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