Adjustment Clip (FX Cut) Requires a Clip Above it to function

[doruphin]

Describe the bug
When using an FX Cut, it only applies its attached effects to the lower clips if there is a clip above it

To Reproduce
Steps to reproduce the behavior:

1. In Shotcut:
   a. Place a base clip
   b. Place an adjustment clip over it with an effect, observe that the output is unchanged
   c. Place a clip over the adjustment clip, observe that the effects are applied in the period when the adjustment layer has a clip both beneath it and above it
2. In Melt:
   a. Try the provided command in the docs: melt video.mp4 -track -blank 50 -producer color:0 meta.fx_cut=1 out=99 -attach sepia
   b. Try the workaround: melt video.mp4 -track -blank 50 -producer color:0 meta.fx_cut=1 out=99 -attach sepia -track color:transparent -transition composite a_track=0 b_track=2

Expected behavior
The adjustment clip should apply its effects to everything below without anything above it (the melt command was working on 8166aab, so most likely a regression)

Desktop (please complete the following information):

• OS: Ubuntu
• OS Version: 26.04
• MLT/melt version: 7.41.0

Additional context
I am using WSL

[doruphin]

I was working on this as part of a ticket in Kdenlive, but I figured I would post it as an issue in case somebody else was working on it as well / it was an error on my part / it's intentional.

[ddennedy]

This was caused by my commit 5864a78, and you can read about what that was trying to fix. But more testing shows that it caused more harm than good. So, I just reverted it: a720333
Not sure I will be able to fix the bug where it affects tracks above, but I will try again, and feel free to as well.

[ddennedy]

FYI, from the docs:

Its attached filters are applied to the composited video from the tracks below
Tracks above see the filtered result as their background for compositing

fx_cut does not support that when you are using transitions to composite all video tracks due to how mlt_field works. You either need to use a nested sequence containing the fx_cut on top, with tracks not to be filtered above it in the parent sequence. Or the app needs to manage this with tractors ala:

<tractor id="tractor_lower" in="00:00:00.000" out="00:00:06.375">
  <track producer="background"/>
  <track producer="playlist0"/> <!-- V1 -->
  <track producer="playlist1"/> <!-- V2 -->
  <track producer="playlist2"/> <!-- V3 adjustment -->

  <transition id="lower_mix_1">
    <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="lower_blend_1">
    <property name="a_track">1</property>
    <property name="b_track">2</property>
    <property name="mlt_service">qtblend</property>
    <property name="disable">0</property>
    <property name="1">normal</property>
    <property name="compositing">0</property>
    <property name="distort">0</property>
    <property name="rotate_center">0</property>
  </transition>

  <transition id="lower_mix_2">
    <property name="a_track">0</property>
    <property name="b_track">2</property>
    <property name="mlt_service">mix</property>
    <property name="always_active">1</property>
    <property name="sum">1</property>
  </transition>

  <transition id="lower_mix_3">
    <property name="a_track">0</property>
    <property name="b_track">3</property>
    <property name="mlt_service">mix</property>
    <property name="always_active">1</property>
    <property name="sum">1</property>
  </transition>

  <transition id="lower_blend_3">
    <property name="a_track">1</property>
    <property name="b_track">3</property>
    <property name="mlt_service">qtblend</property>
    <property name="disable">0</property>
    <property name="1">normal</property>
    <property name="compositing">0</property>
    <property name="distort">0</property>
    <property name="rotate_center">0</property>
  </transition>
</tractor>

<tractor id="tractor_final" in="00:00:00.000" out="00:00:06.375">
  <track producer="tractor_lower"/>
  <track producer="playlist3"/> <!-- V4 title -->

  <transition id="final_mix">
    <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="final_blend">
    <property name="a_track">0</property>
    <property name="b_track">1</property>
    <property name="mlt_service">qtblend</property>
    <property name="disable">0</property>
    <property name="1">normal</property>
    <property name="compositing">0</property>
    <property name="distort">0</property>
    <property name="rotate_center">0</property>
  </transition>
</tractor>

A PR that attempts to fundamentally rewire the framework internals in this area will be very difficult to accept.

[ddennedy]

FYI got "only tracks below" working in #1290