Baseline Extrusion (Image-Based Modeling method)

Excerpt from Chapter 4 (full text)

In our proposed baseline extrusion method, we assume that a flat ground is captured in the input image. We define the baseline of an object to be its contact line with the ground plane of the scene. An example of the baseline for the wall structure of a scene is shown in Figure 4-1. In the modeling process, the user is presented with perspective views of a panorama, just like those produced with a typical panorama viewer, with controls of camera panning and zooming. The user then identifies the objects in the scene to be modeled by vertical rectangles. These include man-made structures like walls and foreground objects that can be modeled as simple vertical billboards. When drawing the baseline, the user can pan the camera for baselines that span more then one screenful of view. As the user pans the camera to inspect the scene, baselines drawn will follow the panning. This is easily achieved when the vertices of the baselines are internally stored using a polar coordinate system with the camera position as the origin.

Figure 4-1. Baseline drawn by user on a perspective view of a panorama.

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When a ceiling is chosen to model the upper part of the background, vertical lines extrusion from the vertices of the baseline defined in the 3D scene are presented to the user on top of the perspective view of the input panorama. The user can then interactively sets the height of the ceiling by dragging any vertex of the baseline along its vertical line (see Figure 4-7). This is possible since we can map a point in the screen coordinates of a point to its 3D coordinates on a plane by ray casting. The ceiling will intersect with any vertical billboard whose height is chosen not to be determined by the mask image. The whole process of specifying an indoor background model with a ceiling is thus just as easy as in TIP, but without the restriction of a limiting box shape.

Figure 4-7. Interactively specifying the height of a ceiling. (a) Baseline already drawn by the user. (b) Vertical lines extruded from the baseline and user interactively sets a height for the ceiling. (c) Finishing the height adjustment. (d) The 3D structure inferred.

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