Renderable motion trails

[dferner]

Hi. I've been working on a mel script that creates lots of objects, and then makes them chase each other around, creating loads of translate and rotation keyframes for each one. Now I want to give them trails though, and I can't seem to get it to work. I mean trails like those blue glowing ones behind those spaceships in WipeOut (old video game, looks v.nice). I figured i could just use "create motion trails", then make an expression that makes the trail maybe 10frames before the current time, but I can't find a way to make the trails renderable. I can't even convert them to nurbs curves and do an extrude along them. And i don't want to emit particles from the objects cos I've got maybe 300 objects in a 500 frame sequence, all with trails, and my computer can't handle that many particles. Any ideas how to do it?
thanks!

[drblacksnake]

Can You post example image?
hard to understand from such a poor description complexity of an effect You want to acheive.
First thing that crossed my mind was doing this in post.
Second - PFX. Third - particles (300 trails is ok anyway)
Hard to advice an exact thing if task is unclear, since we don't see neither Your trail-emiting-objects nor their movement accordingly POV.

[joojaa]

Theres a trick for this render sequence without the trails with motion blur... THen over expose the frame -10 with frame -9 with frame -8 ... to frame -0 in your comp app!

[drblacksnake]

yes. that was first that came to my mind, but we can only guess what kind of trails he need.. what if partially obscured by same less mBlurred geometry? then he will have to do also some tricky masks for each object. better to wait for some image.

[joojaa]

ona can allways render 2-3 passes! one for trail!

[dferner]

Well, Joojaa's idea is a good one, but i'm not sure it would work.
Here's an image of the new wipeout game (the old one looked better but u get the idea) -

http://gamesdomain.yahoo.com/psp/wipeout\_pure/screenshots/13

Those blue trails are the kind of look i want to acheive. Trouble is, I've used loads of transparency mapping for my objects, they're all low polygon sprites with images mapped onto them, so I can't composite stuff afterwards because the Z buffer ignores the transparent areas and really messes stuff up. So if i used that motionblur trick, it would still get messed up when i want to composite it. The trails weave in and out of stationary solid objects, so you'd need a working depthmap/zbuffer. I thought maybe i could render a pass with a the transparency on but the colour of the objects set to their distance from the camera, to get a kind of home-made depth map. Not the easiest thing in the world though!

Not sure if particles would be a good idea - i'd have to parent 300 different emitters to the 300 different objects, and there'd be so many particles, don't think my puny 2.4gHz 512mb laptop could cope.

Think i've managed to get the trails using the "create animated sweep" though. But i'd still like to be able to mess with them separately in post (using AfterEffects). Is there any way i can get a depthmap that isn't messed up by the transparent areas? or maybe attach a material ID to the surfaces like you can in 3D studio max? Or even just render separate passes with proper alpha maps?

Thanks guys!

[joojaa]

actualy that particular effect is doen wth a plane that has lagged points wich most closely would be a particle thing altough not a emitter one!

Alternatively one could do this with path anim quite easily!

But the solution dpends hwavaily on wat kind of geometry you se.

but in 3d id use jsutthe comp thing sis so much easier.

[drblacksnake]

generate motionpath from Your animated objects and make trail object pathFlow. have a fun!
Ah, yeah! Also think about doing this automatically. Perhaps Joojaa cauld help with exact MEL cmd sequence. Otherwise no fun at all.