jrgauthier
Can someone tell me what it means and what should I do about it please?
Increasing the value for the “Expand Bounds” parameter on the Fur Object's “Render” tab should solve the problem.
Procedurals need to inform Mantra of a bounds on the geometry they will create for efficiency reasons (to avoid rendering stuff that is not visible). If a procdural happens to create geometry outside of the bound it told Mantra – this is what the warning message is indicating, then some parts of the render image may have missing geometry.
The bounds for a patch of fur created by the Mantra Fur Procedural is not simple to compute in advance of generating the hairs because users can apply their own custom CVEX shaders to move about the vertices of each hair. The Fur Object has an “Expand Bounds” parameter to indicate how much the bounds of each patch needs to be enlarged to account for things like applied CVEX shaders. This parameter needs refining to avoid the issue for your scene.