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Go back to the Ground Reconstruction example. With the cylinder casting an interesting shadow on an irregular surface, right-click Texturing/Rolling Front Projection. The mesh apparently disappears, but the irregular shadow remains. This continues even if you scrub through the shot.
In short, the image has been “front projected” onto the mesh, so that it appears invisible. But, it continues to serve as a shadow catcher.
In this “Rolling Front Projection” mode, new U,V coordinates are being calculated on each frame to match the camera angle, and the current image is being projected, ensuring invisibility.
Alternatively, the “Frozen Front Projection” mode calculates U,V coordinates only once, when the mode is applied. Furthermore, the image from that frame continues to be applied for the rest of the frames. This kind of configuration is often used for 3-D Fix- It applications where a good frame is used to patch up some other ones, where a truck drives by, for example.
Because the image is projected onto a 3-D surface, some parallax can safely be developed as the shot evolves, often hiding the essentially 2-D nature of the fix. If the mesh geometry is accurate enough, this amounts to texture-mapping it with a live frame.
The U,V coordinates of the mesh can be exported and used in other animation software, along with the source-image frame as a texture, in the rare event it does not support camera mapping. Frozen Front Projection is the prelude to the texture extraction capabilities described next.
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