I was wondering if it was possible to process stencil shadows only every second frame, as this would make the program more efficient, without much noticeable decrease in image quality.

If it is possible, could you give an example action for an entity that does this?

so the shadow will follow 1 second behind?
use baked shadows if you are asking this for non-moving entites.

Not exactly 1 second behind, It would at most be 1/30th of a second behind (if the games is at 60fps). A delay of 1/30th of a second would be hardly noticeable, in comparison to the halving of the time needed to process shadows.

I am trying to figure out an efficient way of using shadows for waving trees (of which there are many)..

Maybe I could render the animated shadows from a top view in blender, then save them as an animated sprite. I could maybe then use the transparent sprite as a shadow..

I've seen games with many thousands of trees, all waving, with animated shadows (e.g AOE III). I wonder how they would do it..

Don't use complex models with stencil shadows. The fps drop makes them almost unusable in Gamestudio. If your models are high poly, use very low poly LODs for rendering the shadows.

@the_mehmaster:in such case shadow mapping shaders may perform better

That's probably correct. I'll try both methods, and see which has the best performance and quality, thanks .

But I guess that shadow mapper needs render2texture right? I dont think it will work in A7 Extra, so does custom decal shadow... You can use XDiv10's FFP projected texture, but it wont work on animated model...
Maybe you should use the ol' reliable z-buffer shadow...