Hmmm, I'm more confused now. If I disable Overlay or if I choose an alternate video output module, the picture is always "dark".
To summarise:
Overlay on, Default (DirectX) video output module, image fits on screen => "bright" picture
Overlay on, Default (DirectX) video output module, image doesn't fit on screen => "dark" picture
Overlay on, Windows GDI output module => "dark" picture
Overlay on, OpenGL output module => "dark" picture
Overlay on, DirectX 3D output module => "dark" picture
Overlay off, Any output module => "dark" picture
It sounds like maybe my complaint should be that the picture is too bright sometimes...!
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Unless you advise otherwise, I'll change to DirectX 3D and manually increase the brightness.
Thanks VLC_help!