[Bf-animsys] f-curve modifier thoughts

[Aurel W.]

Hi,

I spend the evening reading most of the f-curve modifier and
evaluation code so far in 2.5 and I though I should share some
thoughts about it. What's currently there is pretty straight forward
and works great for the modifiers so far implemented. I want to
address some ideas for features and some issues with current design
and evaluation, which I cam across too.

- Stack
At first I was a little bit puzzled about how the evaluation of
modifiers works, since I was more expecting a real modifier stack for
f-curves, where each modifier takes a curve, modifies it and passes it
on to the next modifier in the stack. Currently this is done a bit
differently, which makes a slight difference with the current
modifiers, but a huge one for other modifiers and even renders some
things impossible to implement.

More precisely, at the moment the original curve and it's modifiers
are evaluated frame by frame. That means for the first modifier, the
entire original curve (mostly a bezier curve) is passed to
fcm_modname_evaluate(), which then calculates and returns a value for
the frame in question. This value and the original curve is then
passed to the next modifier, which performs another operation. The
value, which is evaluated by the last modifier becomes the overall
value of the curve on the current frame. I hope it is clear, that this
is more a modifier stack for single float values rather than one for
curves. Current modifiers (Generate, Noise, Envelope,...) operate on
single samples and therefore it is not a problem to stack those. But
other modifiers as the not yet ready 'Filter' will have to use more
samples then just the current one (for e.g. at frame 20, samples from
frame 15-25), to determine the value at the current frame. It gets
apparent, that such modifiers will only be able to operate on the
original curve and not on already modified ones.

So with current design, each modifier will have to be evaluated
independently from other ones and the results will have to be merged
in some way to determine the over all value. My question is, wouldn't
be a real modifier stack much better than the current solution?
Personally I can think of many situations, where modifiers would need
to operate on top of each other, rather than just using the original
curve. I am also aware, that the current implementation has advanced
quite a lot, but maybe this should be still discussed and maybe
changed.


.. I guess that's enough for now,... to be continued,...

Aurel