Compositing functions counter-intuitive - please enlighten me


#1

So I have a moderately complex project of a short clip, which is assembled of a number of streams from a few cameras. As you can see, most of the tracks on a timeline are used only for editing purposes and are normally disabled; the topmost track is what represents the release video which is to be rendered. While editing, I split helper tracks and copy the needed parts into “main” topmost track.

/ Unable to put more than one image /

What I want to do is to add cool title. :slight_smile: Well, I have designed a title image using Paint.NET, exported it as PNG file and added to shotcut playlist. Then I have added a new video track above the topmost track and put the title image onto it (ignore the text on a clip itself which can be seen on a picture above, it was a desperate attempt to have at least some captions):

/ Unable to put more than one image /

What I see is:

that is, no underlying video is visible under the title image. Of course, the title image does not extend to all the frame area. Its resolution is 1920x196, so it cannot obstruct the underlying picture. Blend mode is set to “Over” on all tracks.

Changing blend mode of any track does nothing in this case.

Now the unexpected part.

When I enable the last track at bottom (labelled as “Aux cam right”), the miracle happens!

With this enabled
/ Unable to put more than one image /
All starts to work as expected:
/ Unable to put more than one image /

For some reason, the compositing function refuses to work with the bottom track video disabled, although enabling it does not change the final image (because blend mode is set to “Over”). Is it expected behavior? If so, I would be glad if someone explained me the logic of how the blend function works…

I’ve noticed that the track at bottom has no blend properties. Is it special in some way I don’t understand?


#2

Every other video track composites/blends with the bottom video track.

This is a workflow problem: the main track should be bottom-most.

I’ve noticed that the track at bottom has no blend properties. Is it special in some way I don’t understand?

It is special because it is the bottom and has nothing with which to blend.


#3

Thank you for the quick answer!

What I cannot understand is why blending does not work at all when bottom-most track is disabled?

Example 1:

[V5 - enabled, “Over”]
[V4 - enabled, “Over”]
[V3 - disabled, “Over”]
[V2 - disabled, “Over”]
[V1 - disabled, “Over”]

Here I can see V5 only. Why Shotcut does not blend [V5] with [V4]?

Example 2:

[V5 - enabled, “Over”]
[V4 - enabled, “Over”]
[V3 - disabled, “Over”]
[V2 - disabled, “Over”]
[V1 - enabled, “Over”]

And now it works as expected. Why? In both examples V1 is not visible anyway, because V4 is enabled, has the same size and blending option “Over”. Why disabling V1 results in situation where only one topmost track is visible?

By the way, apart from this blending issue, I find Shotcut a great software product, which perfectly fits my needs and is overall very easy and intuitive to use.