Subtractive mask issue (newbie)

[rorycl]

Hi. I’m using Shotcut 20.07.11 on Linux, which is generally working well.

Part of the edit I’m working on is a screencapture, with some details I wish to cover, to only show the action in letterbox format.

Consequently I’m attempting to make a simple shape mask on a colour, with the operation set to “subtract”. The colour is the top track on the timeline, but I can find no way of using the black colour I have selected to mask (in subtractive or overwrite mode) the track below. The selected clip in the screenshot below is the colour element I am attempting to use as a mask.

I’ve watched the following tutorial " Mask Simple Shape with Opacity" at https://www.youtube.com/watch?v=RYSICEKsLUU
several times but the masking simply does not work for me.

Assistance gratefully received
Rory

[bentacular]

Try using the Chroma Key: Simple filter instead

[sauron]

You have a filter applied to track V4.

That might be causing the problem.

This is using mask with subtract with opacity on the black color set to 65%.

[rorycl]

Thank you for the wonderfully clear instructions, @sauron.

However, after removing the track 4 filters and adding a new black colour with opacity and masking, I still see solid black.

I made a transparent colour to see how filters might work with that, but it is still showing up as solid black, so I seem to have some sort of issue with transparency in general.

I am seeing an error log which may be related:

[Warning] <> file:///...Shotcut.app/share/shotcut/qml/filters/mask_alphaspot/ui.qml:223: TypeError: Cannot read property 'animateIn' of null

This composite image shows (top) the video below showing fine and (bottom) once the timeline hits the simple colour + mask and opacity that it only shows black. No amount of fiddling on the filter settings help.

[Hudson555x]

Unhide V1

V1 not hidden

V1 hidden

[rorycl]

Hi @Hudson55

That worked! I’m obviously missing a key principle here as I had several low-numbered video tracks hidden. Could you point me to any items in the docs where this is explained?

Thanks a ton.

[Hudson555x]

I’m not sure it’s documented anywhere. V1 has to be on for compositing.
It was a change a few years ago to Shotcut.

It’s been mentioned a few times in the forum on the use of V1.

Fade In and Fade Out filters. Adjust opacity instead of fade with black

This will not work with the primary use case of fading on V1 because V1 is not blended with the hidden black background as a performance optimization. If it did do blending, then it completely makes sense to remove the option and simply fade the opacity. However, the video processing performance is already an issue for most people in general such that I have not changed this model. If the performance improves significantly in the future then I will change this.

Compositing functions counter-intuitive - please enlighten me

In addition to what I already wrote, every track blends/composites with V1 - not V5 to V4, then V4 to V3, then V3 to V2, etc. If you want to know more you need to read the source code. I could make every video track blend with a hidden black background track, but then there will be less performance with a single video track even if there is no transparency. In some cases where images and filters result in RGBA, there is a check to see if there is any non-opaque pixel before taking the performanc…

[rorycl]

Thank you very much for that clarification.

Some half-remembered workflow from Final Cut has resulted in a lot of wasted hours!

Anyhow, it is working well now, thanks a lot.