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When a mesh covers only the interior of an object, the pixels underneath it repeat reliably in every frame. For example, a side of a building, a poster on a wall, etc.
However, the edge of a castle or a natural scene with rocks or trees will have an irregular border that is tough to model with mesh geometry.
If the geometry extends past the edge of the object, those pixels will vary over time, depending on what is behind the object. As the camera moves, those pixels that are not over a part of the object will sweep across different parts of the background, potentially producing a broad spread of pixel values.
We can exploit that to produce an alpha channel for the mesh that is opaque where it covers the object of interest, and transparent for the background. Such meshes and textures can then readily be used in traditional compositing, especially if the mesh is always a flat plane.
SynthEyes measures the RMS error (repeatability) of pixels, and offers the Alpha Control section of the texture control panel. To generate an alpha channel, turn on the Create Alpha checkbox.
The Low error spinner sets the level at which the alpha channel will be fully opaque (below this lower limit). The High error spinner sets the level at which the alpha channel will be fully transparent (above this upper limit). The Sharpness spinner controls what happens in between those limits, much like a gamma control.
You can increase the Low limit until portions of the alpha channel start to drop out that should not, and decrease the High limit to the point that the background is fully clear.
The alpha channel will update immediately as you do this, without having to recalculate the texture.
Important : though the texture will be updated in the viewports immediately as you adjust the alpha spinner controls, you should click the Save button when you are done, to re-write the modified textures to disk.
You should not expect this process to be perfect; it depends strongly on what the background is behind the object and how much variability there is in the background itself. For example, a green-screen background will always stick with the foreground, because it never varies!
To make it easier to see what you have in the alpha channel, you can use the Show only texture alpha checkbox and of course the Hide mesh selection checkbox.
You can also use your operating system's image-preview tools to look at the texture images that have been stored to disk.
To clean up the alpha channel, or create one from scratch, you can paint in it directly, as described in the next section.
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