RetroArch Mirror Mode

Sega recently updated their Mega Drive & Genesis Classics emulator collecdtion on Steam to include a fun little feature known as "mirror mode," which flips the game image and controls to be a mirror image of what we're used to. This adds a really surprising challenge, since it breaks all of our muscle memory (including the deeply ingrained "run to the right!" imperative).

This has been possible in RetroArch for a very long time, but it requires twiddling a few esoteric options that may not be familiar to a majority of users, so I figured I'd run through the process briefly.

Step 1: Flip the screen

As with most of the fun video tweaks in RetroArch, we accomplish this through a shader. So, with some content loaded, bring up the 'quick menu' and scroll down to 'shader settings'. On that screen, increment the number of shader passes from 0 to 1 and where it says 'shader #0 N/A,' navigate to the image-adjustment shader located in the 'misc' subdirectory of your shader flavor of choice. I'm using slang shaders here, which are compatible with the Vulkan and D3D11/12 video backends. You can add additional shader passes after the image-adjustment shader, if you like.

Once the shader is selected, go up to the top of the shader menu and hit 'apply changes', then skip down to the 'shader parameters' submenu and you should see a ton of options for tweaking the image. Down at the very bottom, you should see 'flip horiz axis' as the second-to-last option. Set it '1.0' and you should see the screen mirror.

Step 2: Flip the controls

From the shader menu, hit backspace or 'cancel' to return to the 'quick menu' and look for the 'controls' submenu. Here, you can change which of your physical or retropad buttons trigger which core functions. So, scroll down until you see entries for the D-pad. In my case, we have 'Left D-pad (key: left)', to which is assigned D-pad Left, and 'Right D-pad (key: right)', to which is assigned D-pad Right. Simply swap those (as pictured) and you're all set.

Now, we could save a remap file to be automatically loaded whenever this core or game is loaded, but I don't want my controls to be mirrored indefinitely, so I'm not going to do that, and this setting will simply get wiped out when I 'close content' or 'quit RetroArch'.

Bonus Step 3: Flip the audio channels

This is not vital, but if you want to get the full mirror effect, we'll need to swap the audio channels so that the sound effects follow our mirrored video and controls. This is achieved using RetroArch's fun, powerful and oft-overlooked audio DSP system. The pre-installed 'panning' DSP has the ability to swap the channels, we just need to create a preset file that tells it how to do it. So, open up a text editor and paste in these contents:

filters = 1
filter0 = panning

# Swap left and right channels:
 panning_left_mix = "0.0 1.0"
 panning_right_mix = "1.0 0.0"

Save this file as something descriptive and change the file extension to 'dsp' (mine is named Mirror.dsp) and then move it into your RetroArch installation directory, under [RA dir]/filters/audio. Then, back out of the 'quick menu' and go to 'settings' -> 'audio' and scroll down to the 'Audio DSP Plugin' option, where you can select your newly made DSP preset:

 

Getting back to normal

Ok, that was fun, but I don't want to play like that forever, so it's time to put things back to normal. For the shader, go back to the shader options menu and navigate to the number of shader passes. Decrement it down to 0 (or hit the spacebar on your keyboard or 'start' on your gamepad) and then go up and 'apply changes' and you should be un-mirrored. Now, for the audio, go back into 'settings' -> 'audio' and navigate to the 'Audio DSP Plugin' option and press spacebar on your keyboard or 'start' on your gamepad to remove the plugin. As long as you didn't save the input remapping in the 'controls' menu, they should return to normal as soon as you 'close content' or 'quit RetroArch'.