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CS 551/651 Advanced Computer Graphics Warping and Morphing Spring 2002 Deforming Objects • Changing an object’s shape – Usually refers to non-simulated algorithms – Usually relies on user guidance • Easiest when shape number of faces and vertices is preserved – Define the movements of vertices Moving Vertices • Time consuming to define the trajectory through space of all vertices • Instead, control a few seed vertices which in turn affect nearby vertices Defining Vertex Functions • If vertex i is displaced by (x, y, z) units – Displace each neighbor, j, of i by • (x, y, z) * f (i, j) • f (i,j) is typically a function of distance – Euclidean distance – Number of edges from i to j – Distance along surface Vertex Displacement Function k 1 i f (i ) 1.0 ;k 0 n 1 k 1 i f (i ) 1.0 ;k 0 n 1 • i is the (shortest) number of edges between i and j • n is the max number of edges affected • (k=0) = linear; (k<0) = rigid; (k>0) = elastic • Figure 3.55 2-D Grid Deformation • 1974 film “Hunger” • Draw object on grid • Deform grid points • Use bilinear interpolation to recompute vertex positions on deformed grid • Draw example on board • Figure 3.57 Polyline Deformation • Draw a piecewise linear line (polyline) through the geometry • For each vertex compute – Closest polyline segment – Distance to segment – Relative distance along this segment • Deform polyline and recompute vertex positions Global Deformations • Alan Barr, SIGGRAPH ’84 • A 3x3 transformation matrix affects all vertices – P’=M(P) .dot. P • M(P) can taper, twist, bend… • Figure 3.65 3.66 Free-Form Deformation (FFD) • Sederberg, SIGGRAPH ’86 • Position geometric object in local coordinate space • Build local coordinate representation • Deform local coordinate space and thus deform geometry FFD • Similar to 2-D grid deformation • Define 3-D lattice surrounding geometry • Move grid points of lattice and deform geometry accordingly • Local coordinate system is initially defined by three (perhaps non orthogonal) vectors Trilinear Interpolation • Let S, T, and U (with origin P0) define local coord axes of bounding box that encloses geometry • A vertex, P’s, coordinates are: P P0 s (T U ) (T U ) S P P0 t (U S ) (U S ) T P P0 u (S T ) ( S T ) U Volumetric Control Points • Each of S, T, and U axes are subdivided by control points • A lattice of control points is constructed • Bezier interpolation of move control points define new vertex positions P P0 s S t T u U i j k Pijk P0 S T U l m n l l m m n n P( s, t , u ) (1 s) s (1 t ) t (1 u ) u Pijk l i i m j j nk k i 0 i j 0 j k k 0 Examples Using FFDs to Animate • Build control point lattice that is smaller than geometry • Move lattice through geometry so it affects different regions in sequence • Animate mouse under the rug, or subdermals (alien under your skin), etc. • Figure 3.74 Using FFDs to Animate • Build FFD lattice that is larger than geometry • Translate geometry within lattice so new deformations affect it with each move • Change shape of object to move along a path • Figure 3.75 Animating the FFD • Create interface for efficient manipulation of lattice control points over time – Connect lattices to rigid limbs of human skeleton – Figure 3.77 – Physically simulate control points Morphing • 2D image metamorphosis – Coordinate Grid Approach – Feature-Based Approach Coordinate Grid • Source and destination images – Overlay upon both a 2-D lattice of points • Points along edges must remain on edges • Internal points can be in different positions • Same number of points in both • Points define movement of pixels Two-pass Rendering Overview • Figure 3.79 Morphing Images to Intermediate • First stretch in x direction and then in y direction – Auxiliary lattice has x coordinates from source and y coordinates from intermediate lattice Morphing Images to Intermediate – Use scanline method to compute what pixels from source image map to a particular pixel of intermediate image Feature-Based Morphing • Simplest case: user draws one ray on source and destination images to define morph Local Coordinate Systems • Let the root of the ray in source image be the coordinate system origin • Let the ray in source image correspond to the v axis (unit length) • Construct perpendicular to this ray for u axis • Every pixel (x, y) of image now mapped to (u, v) using projection to u/v axes Local Coordinate Systems Local Coordinate Systems • Perform similar local coordinate system computation for destination image – Build s/t axes Mapping Destination to Source • (x, y)dest --- (s, t) • (s, t) --- (u, v) • (u, v) --- (x, y)source • Color (x, y)dest with (x, y)source Necessary Details • More than one line – Perform mapping for all line segments – Weight each line segment’s contribution to averaged color value b • Q2 – Q1 = distance of line Q2 Q1 p segment w a dist • dist is distance from pixel to line • a and b are user specified Necessary Details • Mapping from destination to pixel to source pixel will not land on pixel centers – Aliasing results… – Use quadrilateral centered at source (u, v) location to sample multiple pixels and average Assignment 2 • Due 2 weeks from today • Implement Beier-Neely morphing using fltk and OpenGL – http://www.hammerhead.com/thad/morph.html • Details online tonight

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posted: | 3/26/2011 |

language: | English |

pages: | 36 |

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