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Fog 3D destroys Motion Blur data in Alpha Channel
I’ve rendered out a sequence of RLA frames from Maya with Maya’s motion blur turned on and I’m using Fog 3D on them. I’ve noticed that while Maya renders the motion blur into the alpha and color channels, it renders the ZDepth channel without motion blur. I can’t find any way to persuade Maya to render the motion blur data into the ZDepth channel.
When I use Fog 3D the motion blur effectively gets destroyed since the area that is motion blurred is black in the ZDepth channel. Consequently Fog3D treats the motion blurred area as though it’s at the “Fog End Depth” and so assigns the full Fog Color to that part of the image. Since my Fog Color is blue it turns the motion blurred sections of the image fully blue no matter what the image’s Zdepth.
It seems to me the way to fix this is to somehow expand the image in the ZDepth channel so it encompasses the motion blur area. Somehow I need to extract that channel, perform a negative Simple Choker on it and reinsert it as the ZDepth channel. I can use 3D Channel Extract to get it out but what do I use to reinsert it back in?
Alternately, how can you perform an operation equivalent to Fog 3D but using another layer in place of the ZDepth channel?
Any ideas?
Thanks all,
John -
3 Replies
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since the 3D channels do not alias and (i’m guesszing here) take moption blur into account, you are better off creating a material (or downloading one) that simulates the white to black 3D depth.
If you don;t know how to do that, I’d suggest visiting the Cow forum for your software (maya, Max…etc.,) and asking other compositors how they do that.
Once you create an exact duplicate of your RGB render with the Fake Z render (This will be a seperate render pass), you can bring that white to black Z footage into AE, and use it as a Lumma Matte for the fog element/footage – that fog layer goes between the RGB layer and the matte layer (Z depth “fake” render you made). You can use the level effect on the matte layer to control how the fog appears, and even fills up the scene.
Chris meyer has a great section on this in volume 2 of her books.
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Aharon Rabinowitz
aharon(AT)yahoo(DOT)com
http://www.allbetsareoff.com
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Creative Cow Master Series DVD
particleIllusion Fusion Volume 1
available @ http://www.pIllusionFusion.com -
Thanks Aharon, but will that really look the same? I was thinking 3D Fog does something more along the lines of a Blend crossfade. If I use a Luma Matte won’t I get the same sort of additive transparency issues you get if you try to do a crossfade with transparency instead of the Blend effect.
Perhaps I could use Blend and sample the value of the greyscale ZDepth layer with an expression to control the degree of the Blend.
John
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I am not sure what you mean. Fog 3D uses the Z-Depth channel to decide where fog is more concentrated over the image – while it uses a numbers internally, that image is translated out to a white to black image for your viewing pleasure (and comprehension). Since what I suggested is using levels of lightness as an indicator of distance, it should look pretty good. In fact, I know it does, as I’ve done it myself before. It may not give you the control you want over how the fog rolls in (although with the levels effect, it can work), for example, but it does a pretty good job of simulating the foggy depths as AE does it with the fog 3D effect – not a great effect, since it is a 2D fake, but not bad.
Why don’t you render 1 frame the way I mentioned above, and see how it looks. It may be good enough for your needs.
Again, you may want to post this in the Maya or Max forums here (or whatever 3D app your using) – people have been doing this stuff for a while, and the issue probably has a solution that needs to be handled in the 3D app as opposed to AE.
Good luck.
—————————————-
Aharon Rabinowitz
aharon(AT)yahoo(DOT)com
http://www.allbetsareoff.com
—————————————-
Creative Cow Master Series DVD
particleIllusion Fusion Volume 1
available @ http://www.pIllusionFusion.com
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