Motion blur, alpha chanels, and compositing

Just spent 2 hours digging around for solutions-- and no luck. Here’s my problem:

  • I need a method to combine animated 3D objects with video footage.

I’m trying to do this using Blender 2.25, Photoshop 6, and Premier 6.0. Maybe that’s my problem, but I’m thinking it can be done. I’m sure I can do it without motion blur by outputting from blender with a solid colored background and using Premier’s chroma key or other key functions to pull the objects from the background, but animated 3D without motion blur just doesn’t cut it.

The problem with the motion blur is that since the blurs are slightly transparent, when it’s rendered from blender some of the background color is combined with the object color. That unintended color eventually combines with the colors in the video footage and looks hokey.

So here’s question 1: Is there a way to output from blender so that there is no background color, only transperancy and the foreground object(s)?

Question 2: Has anyone else had this problem with compositing 3D animation’s motion blur?

Question 3: Am I missing something? Because I feel like I may be missing a key point in this whole thing.

Premier comes with a matte tracking function that combines with another video clip to act as a moving alpha chanel so that one clip can be over-laid and combined with another. So I was thinking that I could render an animation from Blender as a sequence of numbered TARGAs, do what I have to do in Photshop to create a animated matte for Premier, then do the compositing in Premier, but that still wouldn’t solve the problem of the hokey motion bulr around the edges of the 3D object(s).

You all are awesome and I appreciate your help!

Output to numbered frames in TGA or PNG format, switch the color mode to RGBA and the background mode to Premul or Key instead of Sky (use Key for best results). Be sure to turn on the Extension option too.

All of these settings are in the Display buttons window (F10).

Martin

Yeah, I knew that, but the problem was not that I didn’t set the alpha chanel option to on or off. The problem was in finding where that alpha chanel was stored. I ended up having to isolate the alpha chanel with Photoshop (the “Chanel” tab in the “layers” pallete) and then use the isolated alpha chanel frames as a track matte (moving mask) to set the transperancy values for the clip I wanted to combine on top of another clip in Premiere. So there ended up being 3 clips: (1) The background clip, (2) the clip I wanted to combine with the first clip, and (3) the tracking matte. STILL, in order for the composition to look right, the lights had to be set up in the Blender scene very particularly along with an oppropriate motion blur buffer setting (via the render screen). Oh yeah, maybe it’s the version I’m using, but Blender doesn’t show an output to PNG format. I know what it is and have used it before with Flash, but don’t see the option in my version of Blender.

what version are you using? It’s in there since 2.25 IIRC

Martin