With GPU effects enabled, (CPU) Video Fade Out to transparent doesn't work

Bug
1

Windows 10 Pro 22H2, Shotcut 23.06.14, Core i7 9700K using integrated HD 630 GPU

Steps to demonstrate:

  1. With GPU effects enabled, create a new project.
  2. Add V1 and V2 to timeline
  3. Open Other - Color - pick a colour (I used red), drag to 00:00 of V1, stretch duration to significantly longer than your default (my default is 4s)
  4. Open Other - Color - pick a different colour (I used blue), drag to 00:00 of V2, leave duration at 4s
  5. Add Fade Out Video (CPU, not GPU) to V2, duration 2s, enable “adjust opacity” checkbox
  6. Preview video

Expected result: 2s of blue, 2s smooth transition to red, then red
Actual result: 4s of blue, sudden change to red

Replace the CPU fade to transparent with a GPU fade to transparent, move GPU fade to bottom of filter list, and it works. Disable GPU effects, restart Shotcut, do exactly the same steps as above, and it works. I also get the same result if V1 is a video and V2 is a transparent image with some Text: Simple on it (which is how I use it in real life - see below). It seems CPU fade to black works; it’s just fade to transparent that’s broken.

I discovered this when trying to make the following video - it’s a short, simple video, so I figured I’d try it with GPU effects enabled (intending to use GPU effects for fade in/out and blur on V1 and CPU effects for opening and closing titles on V2) and if it went boom, no biggie, just turn off GPU effects and start again, which is what I ended up having to do. You’ll see where the opening titles fade; that’s where I saw the problem.

2

Something similar was already reported. It is what it is (no “fix” planned). You have to work with the different options to make it work as desired.

3

GPU mode is clearly labelled as a “here be dragons” thing, so breakage is fair game, as is a response like what you wrote. No problem. If a user wants to use this for anything important and it doesn’t work, that’s on them; they should either have tested first, or heeded the warnings and turned this feature off.

If the eventual goal is for GPU mode to become stable enough to lose the “experimental” tag, though, I would suggest that this does need to be fixed at some point. Users are likely to be less accepting of, um, unanticipated results if it’s supposedly ready for prime time than if it has great big warnings saying it may go boom. And a filter that (with exactly the same settings) works with GPU effects disabled but fails to do anything at all with GPU effects enabled is indeed an unanticipated result.