Understanding transitions at end of tractor

1

Hi all,
I’m studying building MLT files from code and learning how shotcut works. It’s very cool.
In general the structure is clear and sensible.
One area I don’t understand is this: if I add a video to a timeline and have a fade in/out, within the <tractor> node will be <transition> nodes:

    <transition id="transition0">
      <property name="a_track">0</property>
      <property name="b_track">1</property>
      <property name="mlt_service">mix</property>
      <property name="always_active">1</property>
      <property name="sum">1</property>
    </transition>
    <transition id="transition1">
      <property name="a_track">0</property>
      <property name="b_track">1</property>
      <property name="version">0.1</property>
      <property name="mlt_service">frei0r.cairoblend</property>
      <property name="disable">1</property>
    </transition>

This confuses me because the <chain> node has the filters clearly on it and they make sense.

Removing the <transition> nodes stops the visual fade from occurring, but doesn’t stop the audio fade!

I’m pretty sure the a_track and b_track refer to a zero-based array index into the tracks, e.g. a_track=0 means “background” and _track=1 means playlist0 in this example:

  <tractor id="tractor0" title="Shotcut version 24.11.17" in="00:00:00.000" out="01:04:55.560">
    <property name="shotcut">1</property>
    <property name="shotcut:projectAudioChannels">2</property>
    <property name="shotcut:projectFolder">1</property>
    <track producer="background"/>
    <track producer="playlist0"/>
    <transition id="transition0">
      <property name="a_track">0</property>
      <property name="b_track">1</property>
...

but can anyone help my understanding of how the transition nodes fit in?

Thanks!
Ellers

2

Study MLT docs. A MLT transition is not exactly the same thing as a video editor transition.

3

Thanks for the reply.

I’d been reading the docs, specifically MLT - Documentation

I’m struggling to grasp the concept, hence this post.

Perhaps I’ll rephrase my question. Is a transition in the context of a track about how the compositor should blend producers within the track? If so, are the track numbers (track_a=0, for example) referencing the list of producers in that track?

If this is a “read the code” question then ok, but if anyone has insights it’d be appreciated.

4

A MLT transition is an effect with 2 inputs and 1 output. The transitions on this track are doing the video track blending and audio mixing. Yes, the track indexes are to the track elements.

5

Perfect - thank you!