To use a Track Matte clip stack ...
- Alt-drag the clip on V1 to V2.
- Alt-drag the clip on V2 to V3.
- On V3 clip, go to Lumetri's HSL tab, and set the key. Set the HSL mask to black/color and LEAVE ACTIVE.
- Drag/drop Track Matte effect onto clip on V2.
- Set Track Matte matte to V3, Composite using Matte Luma. Now any effects applied below the Track Matte Key effect on V2 only apply to the areas passed to V2 by the HSL key in Lumetri on V3.
- Now go to Lumetri on the V2 track clip, and adjust as needed ... using any control in the Lumetri panel.
Working in this fashion ... the data for the HSL key of V3 will always be taken from the original pixel values of the clip used. The color work done below the TM Key effect on V2 will only affect the pixels selected in the HSL key on V3. And the result will be applied to the clip on V1.
You can do your normal Lumetri work on the clip on V1, and use the stacked TM Key setup to pull original pixels for the HSL key so nothing done in V1 changes the key set by the V3 Lumetri HSL tab.
I use this quite a bit, as it gives a stable key for HSL Secondary work.
There are further implications ... if you use a mask on the Lumetri on V2, you can select a key in V3, apply a Lumetri with a mask on V2, work only part of the image allowed by that mask. Create another Lumetri instance with that also having a mask set to work another section of the same keyed area.
Also ... you can create multiple stacked pairs. Alt-drag the clip on V1 to V4 and V5 ... set a Lumetri HSL Secondary effect as listed above on the clip on V5, and a TM Key effect to the clip on V4. Make sure the TM Key effect is 'linked' to the proper track, the one right above it. Then do a different section of the clip via HSL key selection from V5.
I've tested this with 5 stacked HSL/TM Key layer pairs, and it still worked as expected. Total of 11 tracks when done, all of one clip Alt/dragged up and dropped ... up and dropped ...
But I've never needed to use something that stacked.
Neil