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Question about Smoke/Flame compositing concepts--

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Dustin Bowser
Question about Smoke/Flame compositing concepts--
on Jan 30, 2013 at 9:57:48 pm

I come from compositing in After Effects, and just came upon a technique that I often use in AE but am not sure how to do in Smoke/Flame, due to the way that alphas and mattes are handled as elements and are brought together.

Often in a composite, as an edge treatment for keyed footage, I'll do an 'edge motion blur' so that I can get a smoother motion blur back into the edge after a key. In After Effects, the way that I can do this is by keying the footage, and then applying Reel Smart Motion Blur as an adjustment layer, and then matte that layer with an edge matte that I derive from my key. Because After Effects maintains all of the transparent pixels in the keyed part, when I precomp all of these elements together, and then place it over my background for additional compositing, I'm getting my comp, with the edge motion blurred on my FG and nothing affecting the BG.

I tried to build up a similar tree in Smoke, where I'm performing a vector based motion blur using the Motion Analysis and the Motion Blur nodes, and then compositing the motion blurred edge treatment on top of the comp, but I can't seem to get a stack that accomplishes what I want.

One option was -- Feeding the actual Comp, FG+BG as the FRONT into the Motion blur node, and then compositing that back onto itself with a LogicOP using my edge matte to Matte where the motion blur is seen. The problem with that is I'm getting motion blurring of my BG image as well as the FG image, which isn't what I want.

Another thing I tried was feeding my Comp into an ACTION node, and setting multiple outputs, one for my COMP, one for my FG keyed, and one for the BG. I tried feeding the keyed FG as the FRONT into the Motion Blur node and doing the same process, but because Smoke treats the 'black' of the keyed area as true black and not as transparent, I'm getting a black halo around my keyed FG.

The nice thing about the way that it can be built in After Effects, is that I can key my FG and have the Matte as I like, and then when I do my motion blur edge treatment, the Fill AND the Matte is receiving the motion blur, so I'm getting a much nicer and more natural edge to my composite. I can't seem for the life of me to recreate the same kind of process within Smoke, but perhaps it is just coming from not having an expert understanding of the Compositing and Matte concepts that Smoke operates by.

Sorry for the wordy explanation, but just wanted to be thorough and clear. Any help would be greatly appreciated!

Let me know if anything is unclear and needs clarification.


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Grant Kay
Re: Question about Smoke/Flame compositing concepts--
on Jan 30, 2013 at 10:47:49 pm

Hi Dustin,

I have been trying to visualise what you do in AfterFX and apply that in Smoke terms.

So far the closest you've come to it in Smoke is that you are able to key your subject and produce a keyed image over black and a matte. Both of these are fed through the motion blur node and finally you put it through a logics ops node. You say that you get a black halo on the edges this way.

The reason for the edges is that action has produced a pre-multipled image. In order to recomp it correctly, I suggest using a BLEND & COMP node instead to composite the foreground and the background. Make sure to set the blending from AS IS to DIVIDE. This will fix the issue wit the the black edges.

I am assuming that this is your issue. I did do a whole series on dealing with Alpha on the Smoke Learning Channel which addresses these issues.

If you wish, you can send me a private message through YouTube, twitter or AREA and we can discuss what you're trying to do and maybe I can produce a video of the Smoke equivalent. As a suggestion to you of course :)

Hope this helps!


Check out the Smoke Learning Channel

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or search for the Smoke Learning Channel on iTunes
Follow me on twitter @discreetuk

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