Defocusing a chroma-keyed layer

Hi!

I'm stucked again with a starting level problem, this time with alpha management in keyed footage. Hope someone has some advice for me.

EDITED: I have a simple 3d composition, foreground layer is an ImagePlane3D with chroma keyed footage assigned to it. At some point I want to defocus that fg layer. Defocus affects all RGB and alpha channels and, as the input for the defocus node has a black background, after defocusing I get darkened edges, wider as the defocus grows in size. Old classic issue, isn't it?


I've tried to divide (un-multiply) the alpha channel before the defocus, but i'ts not working, as the divide node only affects a few pixels on the borders of the matte, and later I want to make a strong defocus effect.

This only happens with 3D composition. I've just tested it with a 2D composition (simple merge node), and you can defocus a keyed layer as much as you want witout border issues going to black. Fusion manages the alpha perfectly fine within a merge node.

I have a work around this problem, by switching to real 3D multipass depth of field effect, instead of 2D defocusing the keyed footage. That way I avoid messing with any alpha channel. But render times will explode, as strong defocus via depth of field will need a LOT of passes to render clean.

Before taking that painful work-around... Anyone to give me cue on how to defocus a keyed footage in Fusion 8 without messing the matte borders to black?

Thanks in advance!

edited the description of my comp.

Can you please elaborate on how you're setting up the nodes?
Are you doing

Ld-->Chromakeyer-->defocus-->imageplane3D?
Because that shouldn't be a problem. It should work the way you want it to.
Can you perhaps share a single frame of the footage that needs to be keyed?
Usually what you describe happens if the original footage has a small black border around the edge of the frame to begin with. There shouldn't be dark edges in your key, since the key is already premultiplied. The black of the background shouldn't influence the defocus tool.

Hi, Sander, and thanks for your interest

Yes, that's the flow I have
Load -> Key -> Defocus -> Image Plane.

I attach a frame (croped and jpg to make it fit) of both the original footage and the final comp with that weird effect.

and this is the result after defocusing

bad matte after defocus.jpg (265.33 KiB) Viewed 2250 times

original bluescreen footage.jpg (417.03 KiB) Viewed 2250 times

Of course, key is fine before defocusing.

Thanks!

What's the 3Im's blend mode set to?

Thanks, Chad.

Blend mode is Additive.

I had to deliver, so finally I just 2D merged the two layers and sent the comp to a single 3Dimageplane, simulating the paralax by animating position and scale of the foreground layer.

MUCH faster than trying a real depth of field defocusing with lot of passes.

Thanks for the help anyway!

Just wondering: if you merged the two together and did the fake parallax there, then why did you need to put it on a Imageplane 3D after that?

Yeah, I could leave only the letters on 3D and then merge the 3Drender output with this background composition. Sure. This other approach was the shortest way from the composition I already had.

No big difference, I think.

Just curious....when you have the element(s) on 3d image planes,
why not let the 3D Renderer handle DOF by activating accumulation effects?