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Quick chroma key question

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Rick Neely
Quick chroma key question
on Mar 8, 2008 at 9:33:41 pm

Hi,

I prefer to use After effects but am doing some chroma keying in motion this weekend. I'm using the rt matte effect and the result looks pretty good, but I'm still getting some slight green edging. is there a quick trick/effect to soften (slightly) the edging of the keyable source, or equivalent to the 'enhance parameter' of FCP chroma key.

thanks in advance.

Rick


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Jim Johnson
Re: Quick chroma key question
on Mar 9, 2008 at 8:38:11 pm

Howdy,

You can use the Matte Magic filter to choke and feather the edges of the key. You can also apply Spill Suppressor to help suppress green rim-lighting on the subject.



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David Hartnett
Re: Quick chroma key question
on Mar 10, 2008 at 12:36:28 pm

Hi there, I'm new to posting on this forum, but have read posts here quite a bit.

Can anyone explain why the Primatte RT produces a halo around the edge of its matte? It seems like the footage has a nice clean separation between the green screen and the subject, but as you can see from the alpha matte, a new outline is created when the key is applied.



Additionally, the keyer seems to produce extra pixelation in the subject. The border between the subject's chin and jacket clearly illustrates this.



This is 10-bit uncompressed source footage, by the way. Any help dealing with these issues would be greatly appreciated.



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Jim Johnson
Re: Quick chroma key question
on Mar 10, 2008 at 5:20:13 pm

Howdy,

What format/medium was used to acquire the footage? If it was a lossy one (DV, HDV, DVCPRO, etc.), then the compression causes artifacting the blue and green records, resulting in chunky edges that are revealed by the keyer.

Some keyers will blur the green or blue chroma (or both) to help camouflage the damage caused by the format. Primatte RT doesn't have this capability, unfortunately. If you are keying lossy footage and need a better result, I highly recommend dvMatte Blast or dvMatte pro, both of which are cheap, run extremely fast and are designed to compensate for the artifacting found in lossy formats.

If you didn't shoot on a lossy format, it's possible that there's a hard green rim on the edge of the subject, which could spoof the key. Can you post a screenshot of the footage unkeyed?



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David Hartnett
Re: Quick chroma key question
on Mar 11, 2008 at 9:49:45 am

Thanks for your input Jim. The footage was actually shot on Digibeta, so I don't think it is a DV-related compression problem. Here is an enlarged screenshot of the original footage.



I would think that a nice clean line like that should key well, but I'm having to either turn noise reduction up very high (thus losing detail in the hair) or shrink and feather the matte (same problem with detail elsewhere) in order to get rid of that halo edge.

Your thoughts are appreciated.

-david



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Jim Johnson
Re: Quick chroma key question
on Mar 12, 2008 at 4:22:47 am

Howdy,

Digibeta is 4:2:2 so it can have some impact on the image quality, but I've pulled a number of keys in Digibeta footage without any issues, so that's not likely to be the issue.

In that blow-up, I can see a little bit of a halo running along the subject's shoulder. Any chance you can post a 100%-scale PNG (no compression) of the image? Then I can take a stab at pulling a key and see if I encounter the same problem.



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David Hartnett
Re: Quick chroma key question
on Mar 13, 2008 at 12:01:38 pm

Here is a 100% scale PNG (taken as a screen grab) of my original footage. I've managed to make the key work now, but only by duplicating the layer so that I have one with blurred edges and another with a shrink matte effect so that the interior is crisp.








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Jim Johnson
Re: Quick chroma key question
on Mar 13, 2008 at 6:53:52 pm

Howdy,

That halo is definitely in the image as a hot green line that is brighter than the actual background. I'm assuming it's some sort of blooming artifact. Anyway, adjusting the Noise Removal value in Primatte cleaned most of it up. I added Matte Magic and did a tiny bit of choke and feather to remove the rest of the halo and try to soften some edges. The fine hairs on his ears picked up a lot of light and created a hard edge, as well.

Finally, I added a little bit of light wrap to make the blending a little better, including the hardness of his right cheek. Here's a screenshot:


The project, with the filter settings I used, can be found here.






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Kevin Thomas
Re: Quick chroma key question
on Mar 15, 2008 at 10:16:23 pm

This looks like camera sharpening. For greenscreen work it's recommended you
turn off any sharpening and apply when doing the composite. It's pretty easy to see
in the red channel.






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