Understanding A:M CP bias Gamma property.
Or why is there creases or ripples in my surface?
Note that this applies to Animation:Master previous to version 10.0 Starting with version 10.0, three important changes have made this tutorial much less relevant and usefull. The first change is that the default gamma is no longer parallel to the reference line. It is now tweaked to produce smoother curves. The second change is that when a tweaked gamma CP is moved under from under to above and vice-versa, during animation, A:M now tries to maintain the original curvature. The third change is that it is now possible to modify and animate the gamma and alpha values in actions, poses and smartskins.
Understanding what are the gammas in Animation
Master v8.5 splines is essential knowledge to anyone who want to produce
nearly perfectly smooth surfaces. They are often the cause of some
ripples that appears on surfaces even though all the CPs seems to
be optimally laid-out and a lot of time have been invested in finding
that optimal layout.
This happens because of the way that the CP bias
angles are automatically determined by the AM modeler. To get rid
of those ripples, one can modify the gamma angle of the CP bias.
However there are some danger to modifying the gamma if the resultant
model is to be animated. Here again, understanding how gamma affect
the appearance of a surface and how the gamma is affected by the
spline layout can help decide where to modify the gamma value of
any CP.
A demonstration
First let's draw a typical problematic spline layout. Such a spline
have an 'S' shape.
To most modeler, it is obvious that the spline segment that passes
through CP #3 does not have the proper curvature. When such a spline
is extruded and shaded, it will indeed present a ripple along this
position.
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This is because, the angle of the bias handle that passes through
any CP is, by default, parallel to an imaginary line that passes
through the 2 immediate neighbor CPs. This is when the Gamma is
0°. Let's call this imaginary line the "reference line".
For the vast majority of the CPs in a model, this way of determining
the angle of the spline gives the expected results.
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In cases where the CPs layout produces S shaped splines, this scheme
does not produce the expected results.
It is then necessary, in order to obtain a smooth surface, to modify
the Gamma of the offending CP.
The Gamma value represents the angle in degree between the CP bias
and the reference line.
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This will give the expected smooth splines and when extruded and
shaded, will not exhibit that ripple effect.
This little demonstration have been done with all the CPs on the
same plane. When modeling complex objects, the CPs of a spline rarely
lies on a simple plane. However, the exact same principle of Gamma
determination applies to arbitrary splines.
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The danger of Gamma tweeking
There is, however a danger to this Gamma modification
when the surface is to be animated.
It is important to understand that the Gamma angle is computed
according to which side of the reference line the CP is located.
No matter which side the CP is located, the angle will always stay
at the tweeked value.
For instance, if the CP #3 is moved one grid unit down, the CP
still lies on the same side of the reference line and the resultant
spline is still smooth.
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However, if the CP is moved one grid unit up, such that it now
lies on the other side of the reference line, AM will change the
orientation of the bias handle in order to keep the angle between
the bias and the reference line to its tweeked value which produces
a ripple worse than the one that was corrected in the first place.
The bias of the CP will snap to its new orientation whenever it
passes over the reference line.
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Animating with tweeked Gammas
It is possible to tweek Gammas and still animate the model if some
simple restrictions are observed and applied.
- Gamma are not modifiable inside smartskins. So if a tweeked gamma
gives you trouble while smartskining it, you won't be able to
correct it while smartskining. Gamma is modifiable inside poses
and actions however.
- If care is taken to always keep gamma-tweeked CPs on the same side
of the reference line during animation, posing or even smartskining,
they will not produce any noticeable problems.
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