Implementing playing against a "ghost" of a replay

[yoyyoy]

The other day I was playing osu! and there was this one map where I had a score that I just barely wasn't able the beat, even after trying again and again. During that, I thought about how cool it would be if I could see a ghost of the replay. I checked the discussions here and I've seen the idea brought up several times.

It's already been an accepted idea with plans to eventually implement it. However, many of these discussions took place years ago. It seems that it's not a high priority feature that's being actively worked on by the main osu! team. In which case I would like to start working on this myself.

I have not contributed to osu! before and actually this would be my first open source code contribution. I am familiar with coding and believe I have the skills to implement this while meeting code quality and style standards.

in accordance with CONTRIBUTING.md, I am asking here before starting any real work.

CONTRIBUTING.md
"In the case of simple issues, a direct PR is okay. However, if you decide to work on an existing issue which doesn't seem trivial, please ask us first."

I've done a little bit of planning as to how it should be implemented

• add a new option to the context menu when right clicking a score "Play against ghost"
  • additionally, add a new button on the score results screen next to the view replay button to play against the ghost.
• during gameplay, a 50% transparent cursor of the replay will be shown.
• leaderboard will only show the score of the replay, and the current play. the replay's score updates in real time with the ghost
• when the ghost gets 'Great', 'Good', etc, the ghost cursor turns blue, green, etc for a short moment. Alternatively a Great, Good, etc graphic could appear on the leaderboard beside the ghost replay's score.

for mania and taiko rulesets, just the score on the leaderboard following the ghost would be enough as there's nothing else visually to add. for catch the beat, a transparent catcher would act as the ghost.

In terms of code changes that I think will be needed, though I haven't taken a super extensive look at it yet

• Create a new Class in osu.Game.Screens.Play called GhostPlayer which extends SubmittingPlayer.
• This also requires a new Class GhostPlayerLoader which extends PlayerLoader
• Modify ReplayInputHandler to allow control of the ghost player while the human player still has control.
• Have an extra set of hit objects invisible to the human that only the ghost can interact to track its score. This is likely easier to program but I suspect will have a performance penalty.
  • The alternative would be to make normal hit objects aware of ghosts and keep track of if both current and ghost player have hit the object. the hitbox for the object would remain active as long as one or the other have not yet received a judgement on it yet, be it Great, good, miss, whatever.

Finally, I'd like to respond to this part of CONTRIBUTING.md

Our team believes in human contributions. Any contribution – be it an issue report or a pull request – which is created by, documented by, or aided by AI/LLM usage will typically be closed and locked without further discussion.

I do not use any agentic AI, nor do I copy paste code from any LLM output. However, I do use the auto-complete function of github copilot. In my opinion it's ok to use because it tends to match what I intended to write anyways, and also matches the code style that already exists in the project. But, if this is not considered acceptable then I will turn it off.

[peppy]

Have an extra set of hit objects invisible to the human that only the ghost can interact to track its score. This is likely easier to program but I suspect will have a performance penalty.

No.

You'd have two instances of gameplay running. None of this complex stuff.

From what I can tell everything written above is written by an LLM. It shows lacking/naive knowledge of the codebase and incorrect implementation direction. Please don't attempt this if so.

[yoyyoy]

Instead of just assuming I'm using AI how about you just check? This is what an online AI checker says about my text. I've cut out the parts where i quoted CONTRIBUTING.md as that's obviously human written and we wouldn't want the results skewed. You can try it yourself if you believe I've altered the image. https://gptzero.me/

In fact I tried many more checkers and they all agree that's it's human written. Of course, I knew that because I wrote it.

I admit I'm lacking knowledge, that was the point of me asking here. As per CONTRIBUTING.md

This way we can try to estimate if it is a good fit for you and provide the correct direction on how to address it.

The whole reason you've encouraged me to post here before working is for you to help supplement my lack of knowledge so that I may take the correct direction in my implementation

I understand that as the owner of this repo you get tons of people submitting AI slop and that you absolutely need to filter those out, as they're not worth your attention. You already have so many responsibilities that it's hard to ask you to do anything more. I just ask you take a couple more seconds to quickly check if your intuition is right. This isn't the first time I've seen you accuse someone of using AI only for them to say no they weren't.

Please understand that it can feel incredibly disrespectful to spend time carefully checking every rule and writing a post as instructed only for it to be largely dismissed and written off as AI.

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Back to the topic at hand. I was unaware that having an additional gameplay instance was a viable option. I felt like that would be doing lots of things under the hood that wouldn't be strictly necessary to get ghost replays working. However, if that's not an issue then I'll gladly go that route. The way you put it, it sounds like support for additional instances is already in the game, which would make this much easier.

If you have any other directions please let me know. Your initial feedback was a bit short, likely due to the misunderstanding around LLM usage