Animosaics

Animosaics Kaleigh Smith, Yunjun Liu, and Allison Klein McGill University Eurographics Symposium on Computer Animation 2005

Abstract • Animated mosaics are a traditional form of stop-motion animation. • It is time-consuming and labourious.

Outline • Introduction • Related Work • Temporal Coherence and Group Motion • Construction of a Mosaic Animation • System Details • Results • Conclusion and Future Work

Introduction • Arrange and rearrange small objects or tiles from frame to frame. • Manually placing and moving pieces is time-consuming. • Making small changes means exactly reconstructing. • Packing objects to fill a desired shape is a problem in computer science. • Create static mosaics comprised of different tile shapes using area-based centrodial Voronoi diagram.

Related Work • Simulating decorative mosaics.[Hausner A. ‘01] • Take as input a rectangular image. • Generate a mosaic composed of rectangular or oval tiles • Discuss methods for tile orientation • Jigsaw Image Mosaics. [Kim & Pellacini ‘02] • Define a metric that measures the quality of a tile packing. • Both the tiles and containers can be arbitrary shaped.

Related Work • (a) packs a single tile shape well. • (b) achieves tight packings with irregular shapes. • The CAVD approach (c) handles any variety of arbitrary shapes.

Temporal Coherence andGroup Motion • Minimize temporal discontinuities while having primitives appear attached to underlying scene object. • Even if individual primitives have temporal smoothness, uncoordinated changes among groups of NPR primitives will still yield distracting.

Construction of a Mosaic Animation • A temporally coherent sequence of mosaic images over time. • Given a container C (a closed polygon over time) and a collection of tile shapes T (called a packing of C). • Three challenges: • 1) Temporal coherence • 2) Stylistic coherence • 3) Performance

Construction of a Mosaic Animation • 1) Temporal coherence: • Tiles should move smoothly, appear attached to their underlying object • Tile appearances or disappearances should be minimized • 2) Stylistic coherence: • Tiles are evenly distributed, tightly packed with minimal overlaps, • Tiles’ orientations reflect the edge of the container shape

Construction of a Mosaic Animation • 3) Performance: • In order to support input from the animator, the first two properties should be achieved as interactively as possible • Conflict between these three goals: • Independently packing each frame will lead to high per-frame mosaic quality, but at the cost of distracting temporal artifacts. • Very smooth, coherent tile movements may not yield pleasing individual mosaic. • Performance requirements limit the amount of time that can be spent optimizing either for packing quality or temporal coherence.

Construction of a Mosaic Animation • Take as input an animated scene represented as a collection of 2D containers (polygons). • Pick the desired tile shapes and sizes, then pack the container’s first frame. • Generate the remaining frames in two step: • 1) The system automatically advects the container’s tile from the current frame to the next • 2) The animator optionally inserts new tiles and refines the current packing to reflect container changes.

Construction of a Mosaic Animation • Mosaic Packing • Tile Orientation • Tile Repositioning • Temporally Coherent Tile Movement

Construction of a Mosaic Animation • Mosaic Packing • Tile Orientation • Similar container shapes should lead to similar orientation fields. • [KP02] preserves container edges by fitting tiles against boundaries and previously placed tiles. • Reinforce container edges but do not preserve an internal orientation field. • [Hau01] aligns tiles to a continuous orientation field based on feature lines. • [EW03] align tiles along concentric contour lines to emphasize container shape.

Construction of a Mosaic Animation • Mosaic Packing • Tile Orientation • Preserve the container shape boundary by aligning each tile with its closest container edge. • To enable tighter packings and reduce image regularity, each tile shape may have a set of equivalent orientations.

Construction of a Mosaic Animation • Mosaic Packing • Tile Repositioning • Use an artist-specified collection of arbitrary tile shapes to pack a container. • centroidal Voronoi diagram vs. centroidal area Voronoi diagram

Construction of a Mosaic Animation • Temporally Coherent Tile Movement

Construction of a Mosaic Animation • Anchor-point mapping • Cause insertion or deletions tohappen only at the boundary. • Preserve a packing’s interiororganization. • The system automatically choosesthe anchor point with thesmallest displacement betweentwo frames. • The animator can choose ifdesired.

Construction of a Mosaic Animation • Nearest-edge mapping • Preserve the packing boundaryinstead of the interior. • The interior tiles move away formthe center as a container increase.

System Details • Container Specification • Containers are specified as scalable vector graphics (SVG) animation. • SVG is a human-readable format for describing 2D graphics shapes in XML and enables easy specification of vector graphic shapes and animations. • Use a graphic SVG editor (Corel WebDraw). • Translate scene elements into colored container shapes.

System Details • Packing • The artist choose the set of shapes and the number of tiles. • The system calculate the tiles size for each shape to randomly seed in the container. • Each tile is assigned its container’s color. • Tiles can be add manually or automatically. • The artist can also add noise to tile positions and orientation for a more hand-crafted look.

Result

Conclusion and Future Work • Meets the three challenges of temporal coherence, per-frame mosaic quality, and performance that supports interactive input from the artist. • Use area-based centroidal Voronoi diagrams to create mosaics comprised of different tile shapes. • Blend in other NPR animation styles. • Pack 3D volumes with 3D objects.

Animosaics

Animosaics

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