Thanks!
I did have a look, but I think that's not what I asked about.
The video_shader_dir option in the retroarchcustom.cfg file shows to the shader folder in the share_init location. That's where all the glsp files are located and that's alright this way.
The two .cfg files in the shaderset folder though - they determine which shaders are used in the first place for each system when either scanlines or retro are selected through the GUI.
I successfully edited the scanlines.cfg and retro.cfg located in share_init/system/configs/shaderset, but because of the read_only file system that's a real nuissance if I want to experiment.
I'm just wondering what the two files in share/system/configs/shaderset are doing, if editing them has no effect whatsoever. Maybe there's a cfg entry somewhere which I can edit, so recalbox uses the these two files instead of their share_init incarnations.
And another related question: is there an easy way to add a third or fourth shader preset, so I'm not limited to two shaders available from recalbox's GUI ?
Again, thanks for any advice!