Hello once again!
You can skip to the next green line if you are a tl;dr kinda person.
Time for more Ideas for more things about which people can say "But you can do it with events!" and label me a lazy bum in their heads (or maybe not)
Anyhow - right now a layer (and everything I guess) gets redrawn every frame, replaced with the new stuff. Not like the old days of, say, a ZX Spectrum where you had to erase the screen by hand to create the illusion of motion. But if you didn't there was the nice side effect of seeing all the trajectories basically burned into the screen.
I tried to achieve this effect using canvas (check it out here http://dl.dropbox.com/u/1328856/Motion%20Trails.cap), but there's a certain limitation that you need to paste sprites into it and the result leaves a bit to be desired, since there seems to be no way to affect only the tint and opacity of what's inside canvas (actually a simple opacity option for Clear canvas would be real handy).
So anyhow, right now when something moves in a layer each frame it does it like this:
What would be great would be a layer effect (so that there's no object pasting, no canvas, no events) which basically takes the previous frame, but instead of erasing it completely just lowers it's opacity, thus giving us this effect (here we could assume it lowers the opacity by 20% each frame):
Even better if it can be given a tint, like this:
I suspect it's not quite that simple - since the system probably handles the frame erasing process and you'd have to pretty much copy/paste the layer contents and adjust what's pasted.
Still, it'd be a great effect - seen a lot in those Geometry Wars style shmups so popular a couple of years ago.
As for parameters I think the best would be Time to a full fade out in ms - So if you enter 1000 then the effect will take one second to fully erase the trail. In an example of 60 fps that would be lowering each previous frame's opacity by 1.66% or so.
So either this or additions to the Canvas object such as "Paste Layer to Canvas" and "Erase Canvas with opacity" would be very handy.
Edit: Sort-of on topic - in the example wanted to give the ball a nice retro rainbow effect, but it seems there is no RGB(R,G,B) like command line command for HLS (or at least I couldn't find it). That would be a nice addition as well.