mpv script which modifies rotates the current video file based on player rotation. Uses ffmpeg to update rotation metadata to avoid re-encoding the video.

input.conf

# This is an example of what you could add to your ~/.config/mpv/input.conf file to make use of this script.
# ctrl+r will cycle through rotations for the playback of the video. Rotate the video to the desired orientation.
# shift+r will invoke the 'ffmpeg-rotate' function from lua script to update the playback rotation of the file permanently.

ctrl+r cycle_values video-rotate 90 180 270 0
R script-message ffmpeg-rotate

mpv-ffmpeg-rotate.lua

-- This file should go in ~/.config/mpv/scripts/

local function run_command_sync(command_args)
    -- Run a subprocess synchronously, and report any unexpected errors
    local command = {
        name = "subprocess",
        args = command_args,
        capture_stdout = true,
        capture_stderr = true,
        playback_only = false,
    }

    local result = mp.command_native(command, 0)

    if not (result.error == nil or result.error == "success") then
        mp.msg.error("Subprocess failed: " .. tostring(result.error))
        if result.stderr then
            mp.msg.error("Stderr: " .. result.stderr)
        end
    end
end

local function ffmpeg_rotate()
    local rotation = mp.get_property_number("video-params/rotate")
  
    -- convert video rotation (clockwise) to counter-clockwise for ffmpeg
    if rotation == 270  
        then rotation=90
    elseif rotation == 90
        then rotation=270
    end
    
    local file_path = mp.get_property("path")
    local file_name = mp.get_property("filename")
    local tmp_file = "/tmp/"..rotation.."_"..file_name

    local ffmpeg_args = {
        "ffmpeg",
        "-y",
        "-loglevel", "fatal",
        "-display_rotation", tostring(rotation),  --this flag does the magic, only in somewhat recent versions
        "-i", file_path,
        "-codec", "copy",  --avoid re-encoding to make the rotation fast
        tmp_file
    }

    --replace the original file with the rotated file
    local mv_args = {"mv", tmp_file, file_path}

    run_command_sync(ffmpeg_args)
    run_command_sync(mv_args)
    mp.set_property("video-rotate", 0)
    mp.commandv("video-reload")
    mp.commandv("playlist-play-index", "current")
    mp.osd_message("Set "..file_name.." rotation to "..rotation)
end

mp.register_script_message("ffmpeg-rotate", ffmpeg_rotate)