Composite behaviour for 3 tracks


The behaviour of the Composite transistion seems a little bit strange to me: I'm not sure if it's a bug, but I fail to grasp the logic.

Let's say you have two clips : one green on track 3, one blue on track 2, and Composite transition between the two, with 75% opacity.
The result is as expected: you can see both, the blue one being more visible.
But if you add third clip, red one, on track 1 with a Composite transition at 100% opacity, you will see the red clip at 75%, the green one (bottom) at 25% but blue one will disappear.
I expected to see only the red (top) one.

So, is this a bug or did I miss something?

You probably added a composition transition to the first track ... with which track are you composing the first track?

I have to admit that even after several months with kdenlive, its mlt-based compositing model is still unnatural to me and more than often I sit there scratching my head. Maybe part of my difficulties is that I'm accustomed to thinking in a compositing tree while the track metaphor rather appears like a layering mechanism -- which it isn't. This is probably the UI metaphor problem here...

My guess is that this will have something to do with the "Alpha Channel operation" selected. When I was playing with the alpha shapes effect and writing the manual for this effect

I found that it has a parameter called "operation" which as many (all?) the same options as the "Alpha Channel operation" on the composite transition. According to notes I found on alpha shapes the "operation" parameter "define what is to happen when you have more than one Alpha effect on the clip" And the Write on clear option is "existing alpha mask is overwritten". So I suspect you have the "Write on Clear" "Alpha Channel operation" selected on one of your clips transitions and this is causing the other transition to have its alpha channel cleared.

(alpha channel = data on transparency/opacity)

Check out if this is really what is going on by experimentation and consider writting up your findings on the manual at

@DiveO,@ttguy : thanks for your answers
@DiveO : The 3rd track is compositing with the 2nd and the 2nd with the first (the result is the same with "auto" setting for both transition)
@ttguy : "Alpha Channel operation" is set to over and from what I understand, it should be the correct one (tried them all anywway, and could get the result i wanted).
Like, @DiveO, my problem is probably that I expect a behaviour similar to other applications which have layer compositng, like Gimp.

Well, I would expect the same behavior as you did; I'm dazzled as to why I doesn't happen...

>(the result is the same with "auto" setting for both transition)

Automatic Transitions means that if you adjust the overlap between the two clips involved in the transition then the length of the transition will automatically adjust to cover the region where the clips overlap.

My bad on the operations idea then. I now note that the composite transition does not have a "write on clear" operation. So I am off base there.

I don't know how to describe how this works and what these operations do. I did figure out how the alpha operations works on alpha shapes.

If someone does know - post here or write it up in the manual.