Shaders not working in hybrid mode

[reyaz006]

The same issue apparently exists(ed) in desmume core.
libretro/desmume#22
https://forums.libretro.com/t/shader-for-use-with-desmume-hybrid-layout/15105

Many existing shaders don't work on hybrid display (the zoomed in one), or crash RetroArch. I tried both software and OpenGL renders and it doesn't seem to make a difference. No problem in other modes. Seems weird to see how a single screen mode has no issues with shaders, but the hybrid mode, having only 2 extra small screens at the side (otherwise identical to single), works differently.

[Papermanzero]

The shaders work. However they are applied on the small screen and this screen is upscaled. therefore you do not see the effect.

I had the same issue with desmume. The only way is to use the hybrid shader on top of your shaders.

Another possibility would be if libretro adds a second framebuffer

[liandris]

I found that actually most shader presets just fail on melonds even in default non-hybrid modes.
Example log file attached. retroarch__2021_10_02__20_06_00.log

Also some other presets makes the game lag very hard. Example of this is scalefx+rAA+aa.

Are there any good presets that work with hybrid shader? I tried adding it on top of some others and didn't get good results - they either crash or not doing anything to the main screen. Got a nice looking one for mGBA based on scalefx9-aa-blur-hazy-ntsc-sh1nra358 but on this core it gives literally 0 fps, and doesn't seem to work properly since I'm still seeing upscaled pixels.