Volume Mask Modifier
Volume Mask modifier hide a part of volume box. For now, only box-shaped region is implemented.
For now, only a singe mask modifer works simultaneously.
This modifier is applied before volume coordinate modifier regardless of its position in the stack.
In spherical coordinate (r, theta, phi), the volume is masked like this.
Mask Type
Intersect mask hides outside of the region, while difference hides withins the region.
Add mask shows the region even if it is hided by other mask modifier in the stack (but stacked modifier is not implemented yet).
[X,Y,Z] min/max
These 6 parameters determine a box shaped region for mask.
return
