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keying out soft subjects

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Josh Brine
keying out soft subjects
on Jul 28, 2012 at 9:37:20 pm

Whats the best approach to keying out a very feathered soft subject?

im using primatte keyer and/or keylight.

some of the issues im having are

heavy increase in noise from the key,
denoising removes too much detail, leaving me with pretty much blocky colours

maaaasive jittering on the soft edges of the subject, i keep trying to choke the matte then feather the edges, but im always left with really jittery edges.

any help would be really appreciated.

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Barend Onneweer
Re: keying out soft subjects
on Jul 29, 2012 at 7:10:58 am

First: denoise the footage. Even if the noise isn't very apparent in the source material, keying will indeed bring out the noise - especially in semi-transparent areas.

If you use Keylight: set the output to 'intermediate' and not final (in the dropdown menu in the top). Keylight's built-in spill suppressor has a tendency to amplify noise a lot.

You may have less noise now - but more spill. So you use other ways of spill suppression. Playing with the Hue/Saturation filter is a good start, swinging the greens to another hue a little, or desaturating.

If you have Key Correct Pro, there's a useful spill killer in there.

And finally if that doesn't fix it I've got a bit of a convoluted trick:

Add a solid behind your greenscreen layer, in the complementary colour to the green. That means an orange-ish tint.

You should see that the green spill in semi-transparencies is now neutralized to a certain extent, but of course you don't have a transparent background.

So: you duplicate the greenscreen layer including all the keying plugins and you set the top layer blend mode to 'stencil alpha', which will force it's alpha to all layers below.

The blending of the orange solid around the edges should do a useful job reducing spill, but you can also reduce the layer transparency to reduce the influence.

To put the result on your background, you'll need to drag this comp into a new one and do the final composite in a separate comp. You may want to tweak the coloured solid a little depending on the colour of the background.

And finally, adding Light Wrap will top things off. If you don't have Key Correct Pro there are some tutorials around that will show you how to build your own.

Hope that helps.


Raamw3rk - independent colourist and visual effects artist

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Darby Edelen
Re: keying out soft subjects
on Jul 29, 2012 at 7:29:15 pm

Here's another way to try and suppress the spill if you're using the Intermediate result.

Add another instance of the unkeyed footage above your key. Enable this layer's "Preserve Transparency" switch and add the Channel Mixer effect to that footage. Change the Channel Mixer "Green" settings to:

Green-Red: 50
Green-Green: 0
Green-Blue: 50

This will replace Green values in your footage with an average of the Red and Blue values.

Finally set the upper layer to the Darken blend mode, this will only replace the Green values where the Green value is greater than the average of Red + Blue values, effectively you'll clamp the Green values in the footage to the Red + Blue values.

Nothing could possibly have a green tint at this point, but you may not be entirely happy with the result (colors may shift). In that case, change the Green-Red, Green-Blue or even Green-Green values to change the look.

If the Green-Red, Green-Green and Green-Blue values ever combine to more than 100 then you will be adding additional Green into the footage. That may be desirable though.

I'd recommend just playing with the values to see what you like.

Darby Edelen

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Josh Brine
Re: keying out soft subjects
on Jul 30, 2012 at 1:15:41 am

Thanks guys, it was really successful, the intermediate stage solved my noise problem, and i have key correct, so used the spill killer for a nice finish.

thanks again

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