Often, an outer shell of geometry that hides internal meshes from view is not completely closed. The outer geometry may contain small gaps or discontinuities that nevertheless effectively block a viewer from making out internal details. For example, in this motor, the chain passes through the exterior shell through small holes:
In cases like these, you still want to hide the internal meshes. Therefore, when determining which triangles are occluded, the jacketing algorithm can bridge small gaps, acting as if those gaps were covered by a mesh. This allows you to get the benefit of hiding the internal occluded parts, even if the occluding meshes are not completely sealed.
You can configure the maximum size of the gaps that you want to be ignored during the occlusion testing.
You may be tempted to set the gap size threshold to a very large value, just to be safe. However, this threshold is also used in the Mesh target mode (see below) to evaluate which triangles are safe to remove from the Static Meshes. If you set the gap threshold too high, the internal triangles of the geometry may not be simplified as much as possible. Set the gap threshold as close as you can to the actual size of the gaps in your occluding meshes.