Some things about Blender were easy to understand. Other times I have a “senior moment.” %| And anything having to do with vertex-keys seemed to demand renewed contact with Geritol. I read and re-read the tutorials at http://www.blender.org/modules/documentation/htmlI/c6321.html and my ol’ cranium just kept saying :Z . Then the little dim-bulb finally came on. This is what lit up. See if I’m on-track, and “hope this helps somebody.”
“Big Picture” Time… What vertex-keys are: Whether “relative” or “absolute,” a vertex-key is a capture of the position of the vertices (“points”) in a mesh, lattice, curve, or surface. The first vertex-key you capture is the “reference key,” and it represents the starting-position. Subsequent keys, “normal keys,” are based on that reference. To put it another way, each normal-key captures, for each selected point, “how far away” the point is from where it used to be. The information is captured when you leave Edit mode: all changes you make during Edit-mode are associated with the then-active key. The information consists of a set of deltas, representing a potentially large number of points.
Absolute vs. Relative: Use an absolute key when you want to show a progression of deformations over time, like a candle melting in half. Each new key represents a new arrangement of the melting stuff. The entire object is morphing from one arrangement to the next over time. The morph is described by the Speed-IPO as it crosses over and between vertex-keys denoted by horizontal lines. (See below.) <=#=#=#=> Use relative keys when you want to do things like create a smile… and a wink… and a nose-wrinkle… all at the same time. The deformation is described as the sum-total of several different, independent vertex-keys all being applied at the same time in an amount-of-influence (0.0 to 1.0, say…) that is controlled by an IPO; one per key.
What vertex-keys look like: This threw me for a long time. Vertex-keys show up as horizontal lines in an IPO window! If you don’t change the frame-number, several vertex-key lines might be smack on top of one another. Why is this so? And why is a “Speed” IPO superimposed on top of them? It threw me. But see the next point.
(Absolute vertex-keys) What’s that line for, and why is there a Speed IPO also? The goal of absolute-keys is to describe the progressive change of vertex-positions over time, as in a melting candle. You select a frame, define a key, then switch to Edit mode and arrange the vertices, then leave Edit to lock them in. As you place the various keys, Blender creates a horizontal blue line placed where the current frame-number intersects the Speed IPO in the window. You can, of course, move the line. You can see that as the IPO curve moves forward in time, it moves away from one vertex-key line and closer to the next. This represents how the shape of the figure should “morph” from one position to the next: if the curve is 50% of the way between two keys, the position of the vertices likewise should be halfway between one key position and the next. As you change the shape of the IPO-curve, and/or reposition the vertical lines, you change the morphing that takes place over time.
(Relative keys) Okay, how about these? The whole idea behind relative-keys is to allow you to mix several deforms at the very same time. Each vertex-key represents just one, independent, deformation, such as “left eye closed” or “smile.” Each key has an influence value, described by an IPO of its own. The IPO’s value describes how strongly that particular key influences the final position of the vertices. Each vertex-key can affect some of the same points, or entirely different points. The deformation for each point is calculated by starting with the reference-key, then adding the influences described by each relative-key that affects that point, according to the present value of that key’s respective influence-IPO. So if you want the character to wink while slowly breaking into a smile, the influence of the left-eye-closed key’s IPO will quickly rise from 0.0 to 1.0 and back again, while the influence of the smile key’s IPO will simultaneously slowly rise. Note that in this case, while the keys are still represented by horizontal lines, their vertical position no longer matters. (That threw me a lot!) What matters is their respective influence-IPOs.
Like I said… senior moments (suck…), and I hope this helps someone.
