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Element vs. node

In the group command, style_cg can be either element, node, or null.

For element, if the centroid of an element is inside or outside (depending on boolean_in) group_shape, this element and all its nodes belong to the group.

For node, if some nodes of an element is inside or outside (depending on boolean_in) group_shape, this element and these nodes belong to the group.

The difference between element and node is explained in the figure below, where red elements (solid lines) and nodes (small spheres) belong to the group (large sphere) with group_shape = sphere.