I was trying some vertical games with FBNeo, which rotates the screen automatically for vertical games. My favorite shader is crt-royale, which always looks great at 4k.
Surprisingly, with vertical games on FBNeo, crt-royale looks bad, seems like it’s broken. The scanlines are not synced with pixels. It seems like it’s trying to apply the wrong resolution for both horizontal and vertical pixels (it does rotate the scanlines, but image seems rescaled at wrong resolution). I have a 4K display at 2160p resolution, and I’ve even tested it with integer scaling enabled. Results are the same. Looks like crt-royale does not handle rotated games correctly.
Other shaders, like crt-apperture or crt-geom look perfect, otherwise.
I’ve tested this with 19xx, 1943, and several other games. It does not happen with mame core because mame core does the rotation in a different way and shaders are not rotated with content (scanlines are horizontal on vertical games on mame core), but it happens on FBNeo and any other core, like Flycast. On flycast is less noticieable because Naomi games render at higer resolution, but it has the same problem.