What is all this G1, G2, G3 stuff?

G1, G2, G3, ..., Gn are commonly used in surface modeling to describe the continuity of two surfaces that meet at an edge.

G1 - positional continuity - means two surfaces that meet along a common edge will have any point along that edge co-resident (they meet at the edge) but they do not have to be tangent/ typically you will have crease where these surfaces meet.

G2 - tangential continuity - means two surfaces will meet at a common edge and the slope of any point along that edge on either surface with be tangent. In other words their first derivatives defining the edge where the surfaces meet will be equal. No crease where the surfaces meet.

G3 - curvature continuity - means two surfaces will meet at a common edge and the rate of change of slope of any point along that edge on either surface will be equal. Or in other words the second derivative of a the function defining a point along the edge where the surfaces meet will be equal. No crease where the surfaces meet and the rate of change in the slope is uniform (think of a spiral vs a circle).

and to G4 would mean the 3rd derivative of a function defining a point along a common edge of two surfaces would be equal and so on. However, although I know packages like alias (formerly studiotools) can handle continuity beyond G3 I have never seen a case where it was used.

Originally Posted by ctbram

There is no way to check for for continuity in Maya. You can estimate G3 by eye but judging rate of change of curvature without comb plots and other continuity tools is impossible.

G2 curvature is easier to achieve by simply building the poly cage on top of a nurbs surface. But then when you cut the surface to create a seam you have to once again just play it by feel when placing the supporting edges on either side of the seam as the overall surface curvature will change when you cut it. I always have to re-snap the verts to the nurbs surface and then manually make small adjustments to maintain Tangency (and once again this is all very much by eye).

G1 is trivial and is the only form of continuity that does not require any additional tools to verify and so all you can really say with certainty of any two surfaces that meet at an edge in maya is they are always at least G1.

You can get a very good approximation of G2 and using a chrome environmental shader you can get a similar visual check as with a chrome shader in an actual surfacing application. But once again there is no zebra shader or comb plot with Maya so it's all touchy feely.

NOTE: The above applies to trying to capture Nurbs surface continuity when converting to polygons. Maya does have crude and I mean very crude Nurbs tools to create fillets and rounds and sew surfaces with G2 continuity.

Originally Posted by ctbram