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ddfletch
08-20-2007, 05:25 PM
I'm completely new to all this so forgive me if I seem a bit parochial.

I've been trying to add some airplanes flying overhead of a shot but the motion blur in shake is a bit... well, lame.

The manual spoke briefly about adding the motion blur in my 3D render prior to making the composite.

When I do this, it looks fantastic but for some reason there is a black ring around the aircraft that I can't seem to get rid of. I can see in the alpha map of the plane animation that the motion blur is more of a grey color than black. Great, this portion of the alpha channel creates a grey blur on my composite. I'm certain that there is a way to combine these two shots effectively but the manual doesn't ever get into it. Can you help me?

Seems like this would be a fairly basic thing that the manuals should have covered. Even noobs like me would be using this feature a great deal to add 3D renders to their live footage.

thehive
08-21-2007, 02:27 PM
you may want to look in to rendering out motion vectors, the ring has to do with the bluring of the image + alpha , also what program are u rendering in?

ddfletch
08-21-2007, 09:03 PM
I'm using Carrara to model, animate and render with. My goal was to show students what is possible with even a cheap piece of software.

WmH
08-21-2007, 10:49 PM
I'm completely new to all this so forgive me if I seem a bit parochial.

I've been trying to add some airplanes flying overhead of a shot but the motion blur in shake is a bit... well, lame.

The manual spoke briefly about adding the motion blur in my 3D render prior to making the composite.

When I do this, it looks fantastic but for some reason there is a black ring around the aircraft that I can't seem to get rid of. I can see in the alpha map of the plane animation that the motion blur is more of a grey color than black. Great, this portion of the alpha channel creates a grey blur on my composite. I'm certain that there is a way to combine these two shots effectively but the manual doesn't ever get into it. Can you help me?

Seems like this would be a fairly basic thing that the manuals should have covered. Even noobs like me would be using this feature a great deal to add 3D renders to their live footage.

How are you layering (what mode/node)- and do you know if the 3D generated image/alpha was premultiplied? Some 3D apps output straight alpha by default while others premultiply.
Here is an example of fringing caused by not correctly applying an alpha mask in a straight (over) comp (in this case the CG image is a straight alpha). There is no "more correct" way to output as both straight alpha and premultiplied images have specific do's and don't that can result in artifacts along the edge of the matte (where it fades) your mo-blurred plate simply made it very obvious.

A CG plate (with straight alpha) "over" a background plate (the camera is tracking a moving object)
http://idisk.mac.com/holderness/Public/Concept/nopremult.png

Same plates with premultiply active (in the over node) you can see how naturally the background pate now blurs into the cg image.
http://idisk.mac.com/holderness/Public/Concept/wpremult.png

Just my 2, this advice may turn ou to be worth what you paid for it ;-)

P.S. thought I should mention, so there is no confusion, that the BG plate is the sky and the FG plate (from EI) contains both the building (with a pan blur) and the tracked object (the oblate sphere is a place holder)

WmH
08-22-2007, 04:24 AM
I'm using Carrara to model, animate and render with. My goal was to show students what is possible with even a cheap piece of software.

I just noticed you second post. I think that Carrara does pre-multiply the alpha, so -in theory- you should be good to go with Carrara images (with imbedded alpha) straight from renderer and an over/under node. There are methods to correct (tint/choke) fringe but you shouldn't have to do it.
Very curious...

WmH
08-22-2007, 04:34 PM
It occurred to me that the most likely (perhaps only) explanation is that Carrara somehow included some of the background (black) within the extents of the alpha matte

To check the image to see if it ok, open the file (filein) in shake and then attach a mdiv (matte divide) node, this will remove the pre-multiplication of the image with the alpha channel. (you need straight alpha to check the matte, the pre-multiplication of the image will hide the error, at least till you try to comp it...)
At any rate, next load the image from the mdiv node into the viewer and flip back and forth between the alpha and the RGB (combined), the (blurred) object image should extend all the way to the (black)edge of the mate (alpha) no black background (RGB) should extend into the grey edge of the matte (alpha) if it does, you have your problem.

I can post a couple image's if you are having trouble with checking the matte

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