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Green tint when using an alpha matte tracking to a keyed out layer

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Mark GinGreen tint when using an alpha matte tracking to a keyed out layer
by on Apr 6, 2012 at 7:45:02 pm

Hi,

I have, for example, 3 layers in the following order:
1.Person on a green screen. Effects: NR, Lut buddy, KeyLight. Invisible.
2.Same as [1]. Alpha matte from [1].
3.A landscape.

Since I want to work in Log Gamma I have to create layer [1] where I convert the clip to 2.2 Gamma (using Lut Buddy) and then I properly key out the green screen. Doing only this gives an expected result.
When I track alpha matte in the native Log layer [2] to layer [1] and switch off the visibility of layer [1] all the semi transparencies (like shadows) become green.
I assume it happens because it takes the green screen as a background instead of layer [3].

How can it be solved?

Thank you in advance,
Mark.


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Dave LaRondeRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 6, 2012 at 10:32:27 pm

If you're using Layer #1 as a track matte, I wouldn't think you'd need anything but keylight. You should be able to pull the key, and apply the other color correction effects to the layer below it.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Mark GinRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 7, 2012 at 8:21:34 am

Thank you for your respond

The NR (noise reduction) is needed to eliminate noise from the green screen.
Lut buddy is needed to restore proper Gamma – otherwise it is just impossible to key.
The layer below won't pass color correction – I should leave it in Log space for now. This is actually the reason why I track matte.

The key is pulled perfectly but instead to be semi transparent over the background layer; it is semi transparent over the green layer. It leads to a green tint on the background and a green spill on edges.

If I turn layer [1] visible and layer [2] invisible, the result is perfect – but it is no longer a log space and I can't go back to log in non-destructive way.

How can I force the layer or KeyLight to peak layer [3] as background for transparency transitions?


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Chris ButtacoliRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 7, 2012 at 4:48:15 pm

I think because you are using the track matte, you are circumventing the spill suppression built into Keylight. You'll need to pre-compose those layers 1 & 2 and do a spill suppression separately.


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Mark GinRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 9, 2012 at 8:29:59 pm

Hi Chris,
Tried your suggestion with no luck.
Pre-composed layers 1&2. Added Red Giant’s Spill Killer on the comp – doesn’t work at all.
I’ve tried to add Keylight on the comp, but it re-keys the footage without the ability to separate only spill suppression.

Please take a look on the original issue-
Here are only layers [1] and [3] (both visible):
http://img576.imageshack.us/img576/68/keyissue.jpg

Here what happens when I add layer [2], alpha track it to [1] and turn off visibility in [1]:
http://img4.imageshack.us/img4/8982/keyissuespill.jpg

It just looks like it blends it with the green screen instead of layer [3].


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Dave LaRondeRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 9, 2012 at 4:59:56 pm

Let's back up a bit for diagnostic purposes.
Here are a series of questions intended to help solve your problem:

What kind of camera shot the footage?
Why is the shot noisy?
Did you do something during shooting to make the shot noisy?
Are you well-versed in shooting green screen?
In what codec was the footage initially recorded?
If you transcoded, what codec are you using in AE for this footage?
If the codec of the footage doesn't demand that you do so -- it might, but we don't know that -- why do you feel the need to work in Log Gamma?

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Mark GinRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 9, 2012 at 8:52:34 pm

Hi Dave,
1. Canon 550D
2+3. Even in ISO 160 it is noisy because of native pixel noise that doesn’t get mediated (a problem caused by line skipping). In addition the ability to gather more dynamic range using the temporal space (with temporal noise filter algorithm).
4. I believe so
5. Baseline IP-only H.264 (Canon’s)
6. No transcoding
7. I usually work in Log gamma (CineStyle). At least 2-stops more DR, greater ability for color correction and grading.

Could you please take a look on the original issue-
Here are only layers [1] and [3] (both visible):
http://img576.imageshack.us/img576/68/keyissue.jpg

Here what happens when I add layer [2], alpha track it to [1] and turn off visibility in [1]:
http://img4.imageshack.us/img4/8982/keyissuespill.jpg

It just looks like it blends it with the green screen instead of layer [3].

Thanks


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Dave LaRondeRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 9, 2012 at 9:24:05 pm

In light of the H.264 footage, should I have added an additional question about the version of AE you're using? That's the only thing that really stands out to me at the moment, but I'd assume you're already aware of the problems it creates for non-64-bit versions.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Mark GinRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 9, 2012 at 9:41:21 pm

Actually it is CS5


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Dave LaRondeRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 9, 2012 at 10:34:37 pm

...a fully-updated version, I presume. If you got the chroma key shot by shooting the subject walking across a green background, there will be a TON of green spill; it can't be avoided.


Well, H.264 just does not contain a lot of color information to pull a really good-looking key. It just doesn't: that 4-2-0 color resolution puts you behind the 8-ball right away. The only thing worse is HDV. Instead of following your current AE plan, try this:

Put the chroma key clip in its own comp, add noise reducer if you like, but please forget about any other effects: they will be useless.

Nest this comp in the main comp. Apply Keylight, and pull a good-looking key according to the Status view. This is Layer 1.
Immediately below this nested comp add the original chroma key clip (layer 2), and the background at the very bottom (layer 3).

Use layer 1 as an alpha matte for layer 2. You will see green spill. To remove it, use multiple instances of AE's own spill supressor effect. Alternately, you can apply Keylight, turn off the alpha matte, and tweak keylight settings until the green background goes gray, but doesn't go away.

Now load up all the effects you want to layers 2&3.

Now -- and ONLY now -- use your LUT Buddy plugin on your main comp after you've nested it in a brand-new comp... from which you'll render the completed key shot.

Trying to create a convincing shot under the conditions you shot is as much fun as root canal, I fear.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Mark GinRe: Green tint when using an alpha matte tracking to a keyed out layer
by on Apr 15, 2012 at 2:38:40 pm

Thank you Dave.

I wasn’t happy with the result after multiple spill suppressor instances.

But I made it in other way:
Since I work in 16 bit environment and the footage is 8 bit – I built a non-destructive S-curve LUT.
8->16 bit conversion gives me additional 256 values between each step, so I just need to verify that every X value has different Y value.
So I apply this LUT, then KeyLight and then the inverted LUT. I got a perfect result without any data loss.

Another way that I haven’t tried (but I think would work) is to use the “CC Composite” effect.

Thank you for your time and help!

P.S. I was wrong; I actually can pull a pretty good key in Log Gamma.


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