A Basis slider??? Noooooo!!!
Hmmm, there shouldn’t be a basis slider. There could be a basis IPO, but no slider. So let’s try this:
Start with a new scene with just a cube in top view and the cursor at the centre. Add a Lattice. Resize the Lattice up so it is about 10-20% bigger than the cube (just enough to work comfortably with it). You can set the cube to deform if you wish but it doesn’t matter for the exercise.
Arrange your windows so you can see a Buttons window, an Action window and an IPO window with “SHAPE” selected in the drop-down menu - or you can switch between windows as we go if you need space.
Now, on Frame 1, with the Lattice still selected in Object mode, Press IKEY, choose Lattice. A Basis Key should appear in your IPO window with one flat orange line and a pink spline curve that does, errr, something ;).
In your Edit Panel (F9) press the “Relative” button and the word “Sliders” should appear in the Action window (The word "Basis seems to disappear from the IPO window - don’t ask! ). You’ve just set your basis key.
Now, advance 10 frames (not too important but bear with me) and press IKEY again - choose Lattice. A blue line appears in the IPO window with a Key 1 label and a Key 1 slider appears in the Action window.
Tab into Edit mode. Grab one vertex on your lattice and drag it out to a point. Tab back into Object mode. You’ve set your first real key but nothing happens yet.
Advance 10 frames again and press IKEY > Lattice. Tab into Edit mode, grab a different vertex and drag it in or out somewhere. Tab back to Object Mode. That’s 2 keys. You should now have 2 sliders in the Action window and in the IPO window there should be one orange line and two blue lines (These are spaced vertically because we changed frames before keying. If we hadn’t, they’d be on top of each other which is a bugger to deal with).
Now we animate. Go to frame 1 (Shift-Left Arrow) and to the Action Window. We want the Lattice to start in the basis position. I like to do this by clicking on the slider for each key and make sure it’s on zero. Each click should set a key at frame 1. Now advance 10 frames and slide Key 1 to 1.00 and Click Key 2 to set 0.00 again. This forces Key 2 to remain unmoved while Key 1 deforms fully at this frame. Then advance 10 frames again, Slide Key 1 back to zero and Key 2 to 1.00. Then 10 more frames and slide Key 2 to zero.
Some of the above is redundant but you can delete redundant keys later. The important thing is that you have to set keys to zero to unset them and any key will immediately begin transforming from one key to the next, even if the next key is 1000 frames away and that’s why it’s often necessary to set a zero key (or duplicate a previous key) at certain points to prevent changes starting too early.
Keys can be dragged or duplicated (Shift-D) and dragged into other positions.
Your IPO window should now have curves showing full deforms at the appropriate key positions. If you edit one it will change the slider values.
Your animation should now play with the lattice starting undeformed then one point will extend then, as this point returns to basis, the other point will extend then return to normal.
If you set your cube to deform with your lattice, then it will animate too.
For a “boingy” sort of expansion, you could probably combine this sort of shape-key deformation with a simple Size Key expansion.
As for multiple keys, Blender does an averaging of keys on common vertices. This is great when you want someone to smile and talk at the same time.
See how you go with all that.
EDIT: More info about the lines in the IPO window… If you click on the top key in the list (mines now labelled “----”) you will see your blue and orange lines. If you select one of the blue lines, you’ll see your lattice deform to that key shape. If you now enter Edit mode you can edit this shape and when you exit Edit mode, it will store the new deformation in that shape key.
If you delete (XKEY) that blue line, you will delete the key. If your blue lines are all on top of each other, this is difficult. NOTE: If you rearrange the blue lines it will look like you’ve rearranged the keys but you haven’t - you’ve only rearranged the labels and I don’t advise it. I don’t know if you can rearrange the keys in the action window. /EDIT