Is the blue screen functional in svn 4679? I can't get it to work.

I have one clip on top of another, with a composite transition between them.
I apply the blue screen effect to the top clip, select a color key in the top clip using the dropper tool.

What I see happening is that none of the top clip appears over the bottom clip when the blue screen effect is enabled. Playing with the variance in the blue screen effect makes no difference. If I disable the blue screen effect in the top clip then all of the top clip appears over the bottom clip as expected.

What am I doing wrong?

I have an color selection Frei0r plugin in the works, should be ready in a week or so.
Maybe this one will do the job for you....

For me Blue screen still works. Maybe this tutorial helps:
http://www.youtube.com/watch?v=c4vXxRgMiLA
If not please report back.

@Marko: looking forward to your new plugin.

I played with it some more and figured out the problem. When the blue screen effect is added to the top clip the variance is set to 15 by default. That is way too high for my top clip and makes all the pixels in my top clip become transparent. I am now getting good results with the variance at 3. Apparently I didn't try values much lower than 15 when I played with the variance before. As Homer Simpson would say, Doh!!!

Marko, can't you just improve upon the one we already have?

That would be an option, but even if I started with bluescrren0r, >90% of the code would be new...
I've implemented three color spaces, three selection subspace shapes, four thresholding options, etc.
And, huh, I must also confess that C++ is more or less Chinese for me!

So I was thinking along this way:

there probably is a need for two versions, one super simple and fast, and one more feature laden and probably slower? I guess there are at least two different types of user here (I mean Frei0r in general), the performing video DJ that can only work with real time capable stuff, and the video editor, that can afford some time to tweak the parameters, and wait for the render...
Like, a shovel and a bulldozer both serve for earth digging, but owning a bulldozer does not mean you'll never need a shovel :-)

Anyway, if you think there should really be only a single chroma key gadget, I could probably do it that way too.

Kdenlive's "Blue Screen" does not use bluescreen0r. It uses a MLT-native filter: src/modules/vmfx/filter_chroma.c.

I agree with your logic. If you already made a lot of progress with your frei0r plugin, then keep it going. I was suggesting this in case you were just beginning.

I am having a new problem with blue screen effect.

I have a title clip with various shades of gray. When I set the color key to any shade of gray, all shades of gray in the title (from completely white to completely black) become transparent whenever the variance is 1 or higher. Is that a bug or am I doing something wrong again?

Also with variance at zero none of the pixels are transparent, even the ones that exactly match the color key. Is that normal? For some reason I thought variance 0 meant exact match, while variance > 0 meant pixels whose colors are close to the color key would also match.

I am using the latest svn kdenlive, git mlt as of today.

im having the same issue with my video watermark. it has multiple colors and a black background. i can't really figure out this bluescreen thingy. i've seen tut's for using sony vegas 9.0 pro and it's as simple as blend mode and chose "screen" option. done. i understand that this isn't sony vegas, it's FOSS and the devs do very hard work. i'm just trying to understand how to make kdenlive do what i want it to do. my thread is here:
http://www.kdenlive.org/forum/video-watermark-help-please-sony-vegas-blend-mode-opacity-setting-screen-equiv

tidris769, the Blue Screen effect works using chroma information only. With grayscale, there is no distinct chroma, only luma. It only works well with highly saturated colors that are not in the foreground image you want to keep.