Green screen & alpha gradient

Hello

I've used filters "Handling alpha channel" of KDEnLive and used a green background to cropping people (presenters of JT).
However, I would like now to embed videos like this:

http://www.youtube.com/watch?v=jpw84tj0CbA

But It's not working well... There are plenty of areas of semi-transparency in the flames which are not interpreted ...
Basically, it's as if a pixel could only be transparent or opaque (as GIF...) without alpha gradient that correspond to green gradient.
I can well add blur to slap, but it's still a flame surrounded by a large green edging ... It's the same problem with everything more or less transparent or translucent : smoke, clouds, glass, flare...
Do you know a way to convert gradients of green in degraded alpha more faithful?

Thank you

Yonnel

(Precise Pangolin + KDEnLive 0.9.4)

You can get gradual alpha using "color selection" and any edge mode except "hard". With the "Slope" edge mode, you can control the softness of the edge with the "Slope" parameter.
You can try to correct the green tint of the semi-transparent areas with "key spill mopup".

I tried all the options and I used "key spill mopup" option, but it's just correting the limits of an alpha zone.
That damages images and it don't works with a flare effect or a (breaking) glass effect...

I'm looking for a way to consider all the pixels of the picture (like "replace a color by alpha" in gimp...)
I saw it in iMovie : smokes and flares are looking well.

Thank you

http://userbase.kde.org/Kdenlive/Manual/Effects/Alpha_manipulation/Color... and http://userbase.kde.org/Kdenlive/Manual/Effects/Alpha_manipulation/Blue_... both allow you to replace a color by alpha.

In gimp you have the advantage that you are only dealing with a single frame and so it is easier to get the settings just right.

In video you have to pay a lot of attention during the shooting stage to ensure that the background you are shooting against is very uniform in colour. Shadows are a bitch. So it might be that your footage is just not going to be easily green screened.

Are you saying that you are trying to green screen flames ? That might be near impossible because the colour of the background behind semi-transparent flames is not going to the same as the colour of the bacground outside of the flames. So how is the effect supposed to deal with that. My observation is that bluescreen works well with solid objects but not well with semi-transparent ones - eg with people with a frizzy hair-do.

Another thing you could use to help somewhat is to combine bluescreen and rotoscoping http://userbase.kde.org/Kdenlive/Manual/Effects/Alpha_manipulation/Rotos...

If you are playing with fire have a look at http://userbase.kde.org/Kdenlive/Manual/Transitions/screen which is another method of compositing fire footage into a shot. You can use fire footage shot against black background and add that to your shot using the screen transition.

HM, looked in my Gimp installation (old, admittedly), but can't find "replace color with alpha". Also can't imagine what that would do, what would the use of that be.

You must more precisely describe your problem. In the first post I think it was not being able to get gradual alpha. I guess you solved that already?

Anyway, if the problem is now key color shining through the semitransparent areas, keyspill mopup should be able to help, use similarity to key color for selection, and then move away from key, or towards target as the action. In the second case, the target color should resemble the color of the background onto which you are compositing.

The youtube videos you linked in the first post, were already keyed once, and then placed on an artificial green. Not sure what the original key color was, could be that they used blue, or even a luma key (fire against the black night sky). In this case, the semitransparent areas are probably polluted with blue/black, making a second keying operation more difficult.

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