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Assuming Avidemux has been compiled with support for it and your PC supports it, you should enable both the HW decoding and matching display driver
Both happens in preference
For example, for windows we'll want to use dxva2
The left one is the decoder used The right one is the display used
If the hardware accelerated fails, it should revert automatically to software
In the above example we are using software for both
If you see VDPAU/VDPAU or LIBVA/LIBVA or DXVA2/DXVA2 it means it's fully operationnal