[Bug] Bundled LAVFilters Ignore Imported QLV.reg Settings Due to Registry Path Mismatch

[WHYBBE]

Bug Summary

QuickLook VideoViewer imports LAV configuration into HKCU\Software\QLV, but the bundled QuickLook.LAVFilters package appears to read its settings from HKCU\Software\LAV. As a result, the imported QLV.reg settings, including hardware decoding options, are not applied by LAV Video.

Observed Behavior

QuickLook imports Resources\QLV.reg during plugin initialization:

public void Init()
{
    QLVRegistry.Register();
}

QLVRegistry.Register() writes the embedded .reg values directly to the registry:

var obj = new RegFileObject(Resource.QLV);
obj.RegValues.ForEach(k => k.Value.ForEach(k2 => ImportKey(k2.Value)));

The embedded registry file writes settings under:
HKEY_CURRENT_USER\Software\QLV
HKEY_CURRENT_USER\Software\QLV\Video
HKEY_CURRENT_USER\Software\QLV\Video\HWAccel
HKEY_CURRENT_USER\Software\QLV\Video\Output
HKEY_CURRENT_USER\Software\QLV\Splitter
HKEY_CURRENT_USER\Software\QLV\Audio

For example:
[HKEY_CURRENT_USER\Software\QLV\Video\HWAccel]
"HWAccel"=dword:00000004
"h264"=dword:00000001
"hevc"=dword:00000001
"vp9"=dword:00000001

However, runtime inspection of the DirectShow graph showed that LAV Video was still using software decoding:
ActiveDecoder=avcodec
ActiveHWDevice=(none)

Expected Behavior

The hardware acceleration settings imported from QLV.reg should be picked up by LAV Video, so playback should use hardware decoding when supported.

For example:
ActiveDecoder=dxva2n / d3d11va / similar
ActiveHWDevice=

Investigation

The QuickLook C# code does not appear to read these registry values directly or configure LAV via ILAVVideoSettings. It only imports the .reg file.
Relevant files:
QuickLook.Plugin.VideoViewer\Plugin.cs
QuickLook.Plugin.VideoViewer\QLVRegistry.cs
QuickLook.Plugin.VideoViewer\Resources\QLV.reg
ViewerPanel.xaml.cs only sets the LAV filter directory:

mediaElement.MediaUriPlayer.LAVFilterDirectory =
    IntPtr.Size == 8 ? @"LAVFilters-x64\" : @"LAVFilters-x86\";

No usage was found for:
ILAVVideoSettings
SetRuntimeConfig
SetHWAccel
SetHWAccelCodec

The Nevcairiel/LAVFilters source in the repository defines the video registry path as:

#define LAVC_VIDEO_REGISTRY_KEY L"Software\\LAV\\Video"
#define LAVC_VIDEO_REGISTRY_KEY_FORMATS L"Software\\LAV\\Video\\Formats"
#define LAVC_VIDEO_REGISTRY_KEY_OUTPUT L"Software\\LAV\\Video\\Output"
#define LAVC_VIDEO_REGISTRY_KEY_HWACCEL L"Software\\LAV\\Video\\HWAccel"

Source file:
LAVFilters\decoder\LAVVideo\LAVVideo.h
The splitter and audio filters also use Software\LAV, not Software\QLV:

#define LAVF_REGISTRY_KEY L"Software\\LAV\\Splitter"
#define LAVC_AUDIO_REGISTRY_KEY L"Software\\LAV\\Audio"

Verification

Copying the same settings from:
HKCU\Software\QLV
or from an isolated test path into:
HKCU\Software\LAV
caused LAV Video to pick up hardware decoding correctly.

Before copying settings to Software\LAV:
LAV runtime: ActiveDecoder=avcodec; ActiveHWDevice=(none)
After copying equivalent settings to Software\LAV:
LAV runtime: ActiveDecoder=dxva2n; ActiveHWDevice=NVIDIA GeForce RTX 3050 OEM
This strongly suggests that the imported QLV.reg settings are currently written to a registry path that the bundled LAV filters do not read.

Impact

The video plugin may still play media correctly, but LAV-specific configuration from QLV.reg may be ignored, including:
Hardware acceleration settings
Enabled video/audio formats
Output pixel formats
Splitter settings
Audio settings
The most visible impact is that hardware decoding may remain disabled even though QLV.reg configures it.

Conclusion

The current QLV.reg registry path appears inconsistent with the registry path used by the bundled LAV filters. This makes the imported settings ineffective. Updating the registry target path to HKCU\Software\LAV or configuring LAV via ILAVVideoSettings should fix the issue.

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Some of the content was generated with AI, so it may be a bit verbose, but I have verified both the phenomena and the conclusions.

I want to know how QuickLook.LAVFilters is compiled. Are there detailed steps? There don’t seem to be detailed steps in QL-Win/QuickLook.LAVFilters.

Maybe it is related to #1863.

The simplest verification method(Task Manager):

H/W on

H/W off

[emako]

Thank you for such sufficient feedback on the problem.
Now we have indeed located the problem.

We plan to add the SW/HW switching function in the next version (the default will still be SW).

[emako]

Done in preview version.

https://github.com/QL-Win/QuickLook/releases/tag/latest

Priority: CUDA first -> Auto GPU using DXVA Native

[WHYBBE]

Looks great!

[WHYBBE]

I took a look at the code implementation, and there seems to be a small issue.

If EnableLAVHardwareAcceleration is false, then SetRuntimeConfig won’t be set.

But in reality, the priority is: default value < HKLM registry < HKCU registry < current instance COM settings < actual runtime hardware availability/blacklist/failure fallback.

So it will fall back to the registry settings. If the registry is already set to hardware decoding, then setting software decoding won’t have any effect.

I’m not sure if this is the setting you want.

I wonder if it would be better to also set SetRuntimeConfig when it’s false.

[WHYBBE]

Solved!