Occasional instability with hardware decoding enabled

Bug
1

Windows 10 / 26.1.30

!! Update: see post 2 for replication!

Nvidia 4070 GPU (I think nvidia matters as this is only happening on my desktop pc, not on my laptop which only has integrated intel gpu)

I hesitated a bit to write a bug about this as it happens randomly with no clear steps but maybe other people have this same issue and we can figure out simmilarities.

I have found 3 separate bugs that I believe are related (all are happening with hardware decoder enabled):

  1. app crash or long freeze when changing rotation for a video (in properties panel)
  2. app crash or long freeze when disabling hardware decoder (also long freeze when simply closing Shotcut)
  3. bad mapping of a 4x3 video in a 16x9 project (“sometimes”)
  4. ** if 3 is triggered then some random actions take longer than usual (for example adding a filter has a 2-3 second delay where nothing happens)

The 3rd one is easiest to explain with screenshots, there are no filters here, preview scaling is set to off, the only difference is to enable and disable the Use hardware decoder option. This is just a screenshot of the file already dragged into the source player:

hw decoder on
hw decoder on1421Ă—865 219 KB

no hw decoder
no hw decoder1421Ă—883 215 KB

It seems the mapping of the video (which is 1920x1440, project is 1920x1080) is suddenly ruined, the image is stretched shorter than the viewport area. Note this is not visible in the export result. And after it’s triggered, this affects all videos either in project timeline or source player.

This “bad mapping” survives shotcut close and reopen, this is why I think it’s related to the display [nvidia] driver. It also happened for the entire day yesterday (I tried toggle but it always remained in the bugged state when hardware decoding was on) and it continued after PC shutdown overnight (I do have fast startup enabled, didn’t think to do a full restart), but right now when I’m writing this report it suddenly “fixed” itself after toggling Use hardware decoder for the above screenshots - I can now change rotation of the video with no issues (and it “finishes” rotating immediately, instead of ~5s of freeze that happened before).

I initially thought that rotating the 4x3 video was the trigger but it is not reproducing now so I am very confused what is triggering it.

Stuttering of imported video in preview mode in Shotcut
2

I found a way to reproduce 100% of the time: hardware decoding enabled + any option of Preview scaling (other than none). Drag a 4x3 video into the source player (project mode is 16x9). Instant replication.

To “fix the issue”, set preview scaling to None and drag that same video to the player.

Note, I tried this on my laptop and I still can’t replicate it there, it must be linked to nvidia, or windows10 I guess?

3

Toggling hardware decoder with a project opened requires reloading the project. I reproduced it on Windows 11 with a NVIDIA 4070 when using HEVC with preview scaling and hardware decoder, but not limited to 4:3 or a mismatching aspect ratio. I have not seen it with H.264 sources.

4

The problem is not limited to NVIDIA; it also happens with HEVC only on my Qualcomm Arm64 PC. I found that it happens sometimes when the HEVC hardware decoder pads the output Direct3D texture sent to the D3D scaler but there is no D3D crop filter in FFmpeg. I made a workaround for the next version to detect the situation and disable the D3D scaler in that case, and it seems to also mitigate the crash when rotating in my testing so far. I also added a busy dialog when reloading after toggling the hardware decoder when it takes longer than 2 seconds.

5
6

This fixed it for me, thanks!

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