Just updating the original post with a simple (to write) guide on how to setup the shader “image-adjustment”.
GPU Screenshot in RetroArch must be set to on. Take a screenshot and open the image-file in any photo-editor. Cut away borders and divide width with height.
Open up the game .cfg file and add if needed.
aspect_ratio_index = "20"
video_shader_enable = "true"
video_aspect_ratio = "1.355427"
Where 1.355427 is from earlier made division.
Load up the game again and load the image-adjustment shader from the games quick menu. Start messing with the Shader Parameters.
=================================================================== A lot of NEC PC Engine, Sony PlayStation and Sega Saturn games have black borders on top and/or bottom. Here is a somewhat easy way to cut them away while keeping the aspect ratio… In this example I will be using R-Type Complete for the PC Engine CD.
(Please let me know if there is an easier way and I will update this guide).
Start up you game. Make sure Settings / Video / GPU Screenshot Enable is set to on.
Wait until gameplay so you can properly see the black borders and then take a screenshot. Open your screenshot in a photo editor and measure the width (with borders, if any) and height (without borders). Then divide the two to get the aspect ratio. In this case 1296/924=1.402597.
Back to RertoArch. Go to “Options” and start cutting away the borders with “Initial scanline” and “Last scanline”. Remember to save “Game-options file” when done! (Beetle PSX needs to be restarted for you to see the changes made.)
Go to Settings/Video and change “Aspect Ratio” to config and “Config Aspect Ratio” to 1.40 (taken from your division in #2). Back to your game and “Configuration Override options” and “Save Game Overrides”. Done!