How does the V-Slice input system work?

[CollectorDev09]

I'm trying to wrap my head around the whole input system thing because I wanna make an fnf engine (just for learning purposes). Could someone explain to me how it works? I know someone asked the same question in 2022, but the game (as well as the input system) has changed since then so that's why I'm asking.

[VirtuGuy]

Yeah I'll try to explain it as best as I can. I'll assume you have basic programming knowledge, whether it's Haxe or something else. No I didn't AI generate this stuff. This is using my knowledge of the game.

Hit Windows

FNF loops through each note and determines whether it can be pressed at that time using hit windows. I believe the number the game uses for this is 160, but you can adjust that how you like it. We'll also call this number hitWindowMS.

I'll try my best to explain a very basic hit window system.

var hitStart:Float = noteTime - hitWindowMS;
var hitEnd:Float = noteTime + hitWindowMS;

// Hell yeah Jigsaw you the goat!
// Early enough where the note can be hit
if (songPosition >= hitStart) note.mayHit = true;

// Oh hell nah Jigsaw you tweakin'
// Late enough where the note cannot be hit (it's too late)
if (songPosition >= hitEnd) note.tooLate = true;

Input

Notes are checked by direction, so the notes are checked using an array that looks like this [[], [], [], []]. You can call this directionNotes or notesByDirection.

The game loops through each note that can be hit during that time and stores them within directionNotes.

Internally, it would something like [[left note, left note again], [down note], [up note, up note], [right note]]. Hopefully you understand that.

Then the game loops through those directions and checks the very first note for each of them (this is why sorting the notes is important).

for (direction in directionNotes)
{
    // Direction lacks a note
    if (direction[0] == null)
    {
        // Ghost missing
        // Probably not necessarily how FNF handles ghost missing, but it works
        // Try and figure out how YOU can retrieve the note direction
        if (pressedDirection)
        {
            ghostMiss(noteDirection);
        }
    }

    // Skip the note if its direction key wasn't pressed
    if (!pressedDirection) continue;
    
    // This can be named anything really (goodNoteHit, playerNoteHit, etc.)
    hitNote(direction[0]);
}

Well I tried.

[CollectorDev09]

I think I get how it works now. Thanks a lot!! Just one question, when you say ghost miss, are you referring to a miss made by pressing a note when you're not supposed to?

[VirtuGuy]

Yes. A ghost miss is when you press a key when there's no notes to press.

[CollectorDev09]

Okay. Thanks!