Firstly - I’m new to Picade, Retropie, Retroarch, Raspberry Pi’s.
I built myself a Picade this weekend and have been setting it up. While experimenting with crt_pi shaders, I generated a bunch of them for 1024x768 resolution. On using them, all my Mame games displayed a scaled up portion of the playfield, rather than the whole image.
I had also noticed that some internal setting somewhere wasn’t showing this as the selected resolution, which set me off googling.
Came across this on the wiki, talking about forcing video resolutions.
Added the example from the bottom of this page, which happens to match the resolution of the new Picade screen, to my config.txt in the boot partition.
Fixed everything. RetroPie UI looked cleaner, and crt_pi shaders for 1024x768 now rendered perfectly.
Thought this information might be useful for new builders. Of course if you think this shouldn’t be necessary do let me know, I might have something wrong somewhere else.