Since you’ll need to mess with .slangp files using a text editor, I recommend Notepad++.
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Start by loading one of the default koko-aio presets located in shaders_slang/bezel/koko-aio.
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Adjust parameters to your liking, but make sure that “background image” is enabled and that “image over content” is disabled (you’ll need the bezel enabled too, obviously).
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Now go back, choose save shader, make sure simple presets is on, then save shader as “name of your choice”.
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Now in your file explorer, navigate to RetroArch/shaders and open your newly created shader preset.slangp with Notepad++.
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Here we want to override the default bg_under without deleting or replacing it, so at the bottom type the following:
bg_under = “path/to/your_custom_image.png”
Choosing the path and filename accordingly. Note that this method is most useful with those solid background overlays that have no hole in the middle.
While some other overlays won’t look right this way, and require to be used as bg_over instead, in that case you’ll simply need to use bg_over = instead of bg_under = and you’ll need to enable bg over content (BG_IMAGE_OVER = “1.0”).
