Received CG elements from client rendered over white with embedded alpha. New to smoke, normally I would bring this into After Effects and set to premultiply and choose white as the background color. The result is a shadow with no halo when comped over black (or any other color). Trying to get the same result in Smoke. Using a blend and comp node in the modular keyer gets me closer, but I still have a gray halo around the shadow. Obviously I could render it out of After Effects and bring it into Smoke, but it seems like I shouldn’t have to do that.

Any suggestions?

	Attachment

You can add a Logic Op set to Divide with you Fill and matte as inputs.  The output would then be your new fill.

IN Action there is a DIVIDE check box in the media list.

In Axis you can adjust the Blend Curve by moving the left side of the curve to the top of the graph

Brian Mulligan
[The Area Smoke Forum Assistant]
WTHR-TV
Indianapolis, IN, USA
SMOKE 2013.2.2 HP9400/FX5800 | HP z800/FX5800 | & now Smoke 2013 SP2 on MBP
[Twitter @bkmeditor] | Check out the Smoke Blog on PremiumBeat.com |

Thanks for the quick response Brian. First I brought in the fill/matte in action and checked divide and it looks worse (see attached). Then I tried the Logic Op in Modular Keyer, and when I set the blending mode to divide, it disappeared (see attached). Lastly, I tried it in axis, but couldn’t find the blend curve you referenced. No doubt I’m probably doing something wrong. Any ideas?

	Attachment

	Attachment

Yeah, see that’s the problem. In After Effects it won’t pull any white from the background, which gives me a clean black shadow with no halo.
Is there a way to get that same result in Smoke?

I think I have it… try XNOR in the logic op.

Brian Mulligan
[The Area Smoke Forum Assistant]
WTHR-TV
Indianapolis, IN, USA
SMOKE 2013.2.2 HP9400/FX5800 | HP z800/FX5800 | & now Smoke 2013 SP2 on MBP
[Twitter @bkmeditor] | Check out the Smoke Blog on PremiumBeat.com |

	Attachment

I have the answer for you guys.  The problem with doing a straight divide is it will only work with images premultiplied over black, i.e. zero.  Your image is premultiplied over white, i.e. value of 1.

John Montgomery of fxguide gave a master class at NAB several years ago on custom nodes.  He created a node to solve for this exact issue.  In fact, his formula works for images premultiplied over any color, not just white.

The formula is RGB-((1-alpha)*bg).  RGB is the fill premultiplied over color background.  bg is the background color. 

Take a frame of white(that’s the 1) and subtract the alpha using a logic op.  Then multiply by the background color using another logic op.  Subtract that from the original premultiplied fill using a third logic op.  Voila, you now have a new fill premultiplied over black which you can divide by the alpha using a fourth logic op.  Now you have your “straight” un-premultiplied fill.

This four logic-op batch can be saved and recalled as needed, or saved as a custom node.

Here is a link to John’s class:  http://usa.autodesk.com/adsk/servlet/item?siteID=123112&id=10120873 Then click on view now next to his picture, then open the folder Flame 2007, John Montgomery, Using quick nodes.

I hope this helps.

Steve Beecher
Flame Artist
Disney ABC Television

Two words; “feature request”.  I remember the days when you didn’t even have the divide option in either action or in the logic op.  You had to use a Sapphire math op to divide.

Steve Beecher
Flame Artist
Disney ABC Television

Thank you for this very informative thread.  I’m coming from After Effects and Fusion and things like premultiplication in Smoke was a head scratcher.

Ok, I watched the video and set it all up. I’m using smoke on mac, so instead of true “batch” I’m using the modular keyer to do all logic ops (math). My problem is I can’t simply add an output node to get the result out. I have to use this “result” node.

When I try to connect it to the result node I get nothing, so I’m stuck. I’m sure I am just doing something wrong, but I can’t figure out what. See attachment for details.

Thanks!

	Attachment

After watching the John Montgomery video and working with our Flame artist, we’ve finally cracked it. Screen shot attached. Glad I can save the setup so it can be easily applied to all the other shots that were rendered the same way.

Thanks for the help guys!

	Attachment

Steve B 21 May 2012 02:14 PM

Two words; “feature request”.

Maybe in the not too distant future it could be this easy? (see attached).

	Attachment