Output param and call of standard effects from API

Question

I have two questions.
1) how can i output parameters? The supervise option is usefull to make a change in parameters each time a parameter is changed... but if i want to change a value of parameter according to my image, how can i do?
I want to compute some point or color, and make them available in the parameter windows so that it can be connected afterwards with expression to other effects parameters...
Using arbitrary type + event callback would solve the problem?
2)My second question is: can I apply from the API an effect other than the one I am working on?
for example, I would like to apply a color key effect on the input layer before I apply my effect. Is it possible? or am i oblige to either program again colorkey, or apply it before outside the API?

Answer 1

about outputting parameters:
you can apply a value to any standard parameter in any effect using StreamSuite2()->AEGP_SetStreamValue() for constant values, or KeyframeSuite3()->AEGP_SetKeyframeValue() for time varying streams.
(no problem with point, and color. but I'm not sure about arbitrary)
and also set PF_OutFlag_REFRESH_UI, so the user can see the change.
as for applying a different effect:
that a tricky one.
EffectSuite3()->AEGP_ApplyEffect(), will apply any effect you want as long as you have it's AEGP_InstalledEffectKey.
the illusive install key may change between computers, installations and even re launches of AE.
so each time your plug-in launches you must ensure you have the correct install key.
to do that you need to go through every effect installed and check it's MatchName using EffectSuite3()->AEGP_GetEffectMatchName(), AEGP_GetNumInstalledEffects() and AEGP_GetNextInstalledEffect().
the match name for the color key is something like "ADBE ColorKey".
this may also change, if adobe would decide to rename their plug-in internally. (unlikely, as it would invalidate this plug-in for old projects, but don't bet your life on it)
you're not out of the woods yet.
the worst is still to come.
you can only apply effects at a higher position in the effects stack.
i.e. if your effect is 3rd in the stack, you can only apply and effect at position 4 and up.
it gets worse now.
CS3 will crash if your effect isn't the last in the stack while applying a new effect.
and that's not the worst news.
any attempt to reorder the newly applied effect using EffectSuite3()->AEGP_ReorderEffect(), to a position prior to your effect, will launch an error message.
it woun't make AE crash, but it will send a nasty message + error sound each and every time.
and even that wasn't the worst news.
the reason for all these problems is that these functions were intended to be used by AEGPs and not effects.
changing an effect stack from an effect within that stack, wrecks havoc on AE's internal checks.
that was the worst bit.
the only solution i found was using a separate AEGP, that lurks in the shadows and identifies (either by itself or by sending it a message from your effect) an event in which effects need be applied and re ordered, and does that ONLY during AEGP_IdleHook.
that means these changes don't happen while this effect stack is in use.
it's the long way around, but it's the only way that worked for me.