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Abstract :
We present a 2D feature-based technique formorphing 3D objects represented by light fields. Existing light field morphing methods require the user to specify corresponding 3D feature elements to guide morph computation. Since slight errors in 3D specification can lead to significant morphing artifacts, we propose a scheme based on 2D feature elements that is less sensitive to imprecise marking of features. First, 2D features are specified by the user in a number of key views in the source and target light fields. Then the two light fields are warped view by view as guided by the corresponding 2D features. Finally, the two warped light fields are blended together to yield the desired light field morph. Two key issues in light field morphing are feature specification and warping of light field rays. For feature specification, we introduce a user interface for delineating 2D features in key views of a light field, which are automatically interpolated to other views. For ray warping, we describe a 2D technique that accounts for visibility changes, and present a comparison to the ideal morphing of light fields. Light field morphing based on 2D features makes it simple to incorporate previous image morphing techniques such as non-uniform blending, as well as to morph between an image and a light field.
Index Terms
3D morphing, Light field, Ray correspondence, Ray space warping, Ray resampling.
I. INTRODUCTION
METAMORPHOSIS, or morphing, is a technique for generating a visually smooth transformation from one object to another. Often morphing is performed between objects each represented as an image. This approach of image morphing which has been popular in the entertainment industry, computes a transformation based on corresponding 2D features between the images. This 2D feature-based interpolation of images can be extended to 3D geometry morphing that deals with objects whose geometry and surface properties are known. 3D geometry morphing techniques have been developed for polygonal meshes and volumetric presentations [9], [10], [11], and they allow for additional effects such as changes in viewpoint any real objects, however, have geometry and surface properties that are too difficult to model or recover by traditional graphics and computer vision techniques. Light fields are often used to model the appearance of such objects from multiple viewpoints , and a method for morphing based on the light field epresentation has recently been proposed [12]. With user specification of 3D polygonal features in the light fields, this approach effectively combines the advanced morphing effects of 3D geometry morphing with the fine appearance details of images.
The principal drawback in the light field morphing technique of [12] is its reliance on 3D feature polygons. Computing an accurate 3D position of a feature polygon requires very precise user specification of the polygon in two views, presenting a major challenge to the user. Slight misalignment of the feature polygons even by only a couple pixels in these two views can be amplified into increasingly large errors in the polygon’s image position at more divergent viewing angles, leading to prominent morphing inaccuracies. Careful avoidance of this misalignment is a cumbersome task, which involves much zooming in and out and substantial trial-and-error. In cases such as untextured regions, the exact pixel positions of corresponding polygon vertices can be far from obvious to the user, even with the computer vision aids that are incorporated into the user interface.
In this paper, we present a new approach to light field morphing based entirely on 2D features. Unlike the 3D method in [12], our 2D technique does not compute or rely on the 3D positions of feature points. Rather than warping geometrically in 3D, our technique records correspondences among multiple views and warps between these views by feature point interpolation. In this way, specification errors have only a local effect, and are not propagated and magnified globally as with 3D features. This approach significantly improves the usability of light field morphing by reducing the sensitivity of morphing quality to user specification error. Additional benefits of 2D features in light fields are that they allow for easy incorporation of prior image morphing techniques such as non-uniform blending, and for morphing between an image and a light field.
The remainder of the paper is organized as follows. Section II reviews previous morphing methods related to our technique. In Section III, we present an overview of light field morphing and outline the previous technique
based on 3D features. Section IV describes our method based on 2D features, including the user interface and the concept of ray space warping. In Section V, we
analyze our 2D technique with respect to ideal light field morphing when geometry is known, and then provide a comparison with other object morphing approaches.
Experimental results are exhibited in Section VI, and conclusions are presented in Section VII.