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Change fill to stroke - alpha matte?

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Mike Douglas
Change fill to stroke - alpha matte?
on Jul 21, 2017 at 8:25:43 pm

Hey there, is there any quick/easy way to change an alpha matte / luma matte from being a fill to being stroked?

For example, I have a luma matte render of a CG character from C4D - so it's white on black. I want to quickly make that into an outline instead of a fill. And yes, I realize I could render this out using the Cel Renderer in C4D. But I don't always want to do that, and that may not work in some cases.

Here's what I currently do:
- Use Set Matte (using Luminance) to change it from a luma matte to an alpha matte
- duplicate my layer, use Simple Choker with a positive value to cut into the matte
- set the Track Matte for my first layer (without the choker) to Alpha Inverted (using my 2nd layer as the matte layer)
- then, if need be, pre-comp both layers and add a Fill to change it to whatever colour I need

This seems like a lot of steps - is there an easier way? Also, it doesn't work great if your element touches the edge of the frame, as it will then add an outline around there as well. Thanks!

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Mohsen Saeedi
Re: Change fill to stroke - alpha matte?
on Jul 21, 2017 at 10:55:36 pm

Layer > Layer style > Stroke
Creates outline around the alpha, just like photoshop.
for keying out black matte, I Always use free plugin "UnMult" from Red Giant.

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Mike Douglas
Re: Change fill to stroke - alpha matte?
on Jul 24, 2017 at 5:13:18 pm

Thanks Mohsen. Good ideas there. I had forgotten about UnMult - I'll start using that again.

Re: using Stroke, that's a good technique, but I find it has some flaws. I compared the result of that vs my method, and while they're similar, using the Stroke layer style gives me a very aliased outer edge for some reason (I tried to attach a screengrab but it wasn't working).

Also, I then don't get JUST the stroke - it's still filled as well. If I want to get it to work on top of other footage, I need to fill the layer with black, then put it on Lighten/Screen/Add, which is likely fine in most cases but not all.

Thanks for the tips!

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