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PTM's (Polynomial Texture Maps) Overview | Light Space | Image Enhancement | Viewing a PTM | Building a PTM Light Space Changing the illumination direction within the PTM is accomplished by clicking and dragging the mouse in the PTM viewer's Image Display Window. Because PTMs are 2D representations the mouse can only be dragged in 2D across the image plane. Because PTMs use 3D lighting models, image content can be interactively illuminated by the user as if it occupied 3D space. This 3D space is called 'Light Space'. When examining a PTM, Light Space can be thought of as a hemispheric dome enclosing the PTM image. Visualizing the Image Display Window as a dome rather than as a plane approximates the mathematical nature of Light Space. When the mouse cursor is in the center of the Image Display Window the illumination is analogous to the sun at 'high noon'. As the mouse cursor is dragged toward the Image Display Window's edge, the sun approaches the 'horizon'. The capture of 3D normal information and its display within 3D Light Space is responsible for the three dimensionality of the user's PTM experience. Illumination in Light Space has similarities with illumination in the 'real world'. The PTM Viewer's viewing location is fixed perpendicular to the image plane, at the Light Space dome's 'high noon'. Illumination originating from the same 'high noon' location, equivalent to the center of the PTM, will be mainly reflected back to the viewing location and will be at its brightest. As the light direction approaches the horizon, equivalent to the edge of the PTM, fewer light rays will find their way to the viewing location and the PTM will be less brightly illuminated. Here are three images from the same PTM with the light source coming from three different directions. In the left image, the light is positioned at 'high noon'. In middle image , the light is positioned near the 'horizon' from the direction corresponding to the top of the image. In right image, the light is positioned near the 'horizon' from the direction corresponding from to the bottom of the image.
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