IPO Transparency

Hi,

Working with 2.41, getting the hang of using the interface and features
and such.  One thing I'm playing with is an animation that has objects
that fade in and out - I can get the IP curves to adjust for colours, but
it seems the filter variable (under the 'mirror transp' tab in the
materials editor) cannot have a curve assigned to it.

I tried using alpha on the colour channel, but that doesn't seem to work
correctly, either - I slide it from a value of 1 to a value of 0, and for
a 250 frame animation it moves the slider smoothly, but only between frame
249 and 250 does the object actually disappear.

Am I missing something obvious (probably)?

Thanks,

Jim