The Design and Applications of Shape Similarity in Computer Graphics
The Design and Applications of Shape Similarity in Computer Graphics
Xuemiao Xu

Shape analysis is a fundamental problem in computer vision and has been widely adopted in many applications of computer graphics. In particular, shape similarity, as one of the core techniques of shape analysis, plays an important role in shape retrieval, recognition and classification, alignment and registration, and approximation and simplification. Its applications in computer graphics include shape-based image morphing, image mosaic, 3D reconstruction, image editing, motion inference, and motion tracking, etc.

Shape similarity has been deeply studied by many researchers. However, challenges still remain. The first challenge is the invariance. Since shape in many applications is often discussed based on the property of invariance, shape similarity is expected to be invariant to the transformations (e.g. translation, scale, orientation). The second one is tolerance. As noise, blur, crack and deformation are usually introduced when extracting the shape of an object from an image, a shape similarity metric is required to be robust to these imperfections. During the design of shape similarity metrics, it is usually difficult to achieve these objectives.

Shape similarity is application dependent. Different applications may have different requirements on invariance and tolerance. Thus, it is almost impossible to design an universal metric which is suitable for all applications. Nowadays, many techniques have attempted to solve these problems to some extent. But, most of them only focus on the applications where shape is invariant to transformation. And the cases where shape transformations may be taken into account when measuring the shape similarity are seldom discussed.

We further explore the applications of shape similarity in computer graphics. And we especially concentrate on some challenging problems. Since shape similarity is application dependent, we need to tailor-make the design of shape similarity metric in order to satisfy the specific application. In this thesis, we mainly discuss two applications, animating animal motion from a still picture, and outline-based ASCII art generation. Both of them are regarded as challenging problems and hard to be solved with existing techniques. It is our great contribution that we observe that the complexity of these applications can be largely reduced by applying the concept of shape similarity, so that the corresponding problems become solvable. In addition, for the first application some existing shape similarity metrics can be directly applied, while in the second application the design of a new shape similarity metric is required.

Animating animal motion from a still picture, has been regarded as an impossible task due to the lack of temporal information in a single picture. However, considering a picture of a moving animal group, different individuals in the picture usually suggest how this species moves. This is because different individuals have similar look and behave asynchronously, so that they can be regarded as the snapshots of one individual during the motion. And these snapshots form the key frames of the motion cycle. Hence, our key contribution is to determine an optimal ordering of snapshots in order to reconstruct the motion cycle. To achieve this, we construct a snapshot graph using shape context, an invariant shape similarity metric. Then the ordered snapshots (which correspond to the motion cycle) can be deduced from this graph by minimizing an objective function. To alleviate the pose, morphology, and appearance variation of the snapshots along the motion path, we further propose to perform consistency refinement. Finally, a smooth motion sequence can be synthesized by morphing among the ordered snapshots.

Outline-based ASCII art generation is another typical application of shape similarity. The wide availability and popularity of text-based communication channels encourage the usage of ASCII art in representing images. Outline-based ASCII art approximates the major line structure of the underlying image content. We mimic how ASCII artists deform the underlying image in order to approximate the image content with the limited variety of character font structure. The generation is formulated as an optimization problem that minimizes the shape dissimilarity, the text resolution, and the text grid deformation. Shape similarity for character matching is the basic technique in this application. Here shapes are not invariant to transformations when we try to measure the similarity between the shape of characters and the underlying structure. Moreover, the compared shapes can be substantially different. Therefore, an alignment-insensitive shape similarity metric which is variant to transformations as well as robust to misalignment is also proposed.

Related publication:
  • " Animating Animal Motion from Still",
    X. Xu, L. Wan, X. Liu, T. T. Wong, L. Wang and C. S. Leung,
    ACM Transactions on Graphics (SIGGRAPH Asia 2008 issue), Vol. 27, December 2008, pp. 117:1-117:8.

  • "Structure-based ASCII Art",
    X. Xu, L. Zhang and T. T. Wong,
    ACM Transactions on Graphics (SIGGRAPH 2010 issue), Vol. 29, No. 4, July 2010, pp. 52-1-52:9.

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