LlamaCasty/Services/TransitionService.cs

using ytLive.Models;

namespace ytLive.Services;

/// <summary>
/// The three v1 transition types. Clicking a scene thumbnail triggers the selected
/// transition; Go Live always uses Cut for a clean start.
/// </summary>
public enum TransitionType
{
    Cut,
    Fade,
    Move
}

/// <summary>
/// Manages the transition state between StagedScene and LiveScene. When a
/// transition is active, FramePump calls <see cref="BlendFrame"/> each tick
/// to get the blended output. When the transition completes, <see cref="Active"/>
/// becomes false and FramePump reverts to a direct LiveScene render.
/// </summary>
public sealed class TransitionService
{
    private const double DefaultDurationMs = 300;

    /// <summary>True while a transition is in progress.</summary>
    public bool Active { get; private set; }

    /// <summary>The transition type for the current in-progress transition.</summary>
    public TransitionType CurrentType { get; private set; } = TransitionType.Cut;

    /// <summary>Duration of the current transition in milliseconds.</summary>
    public double DurationMs { get; private set; } = DefaultDurationMs;

    /// <summary>Elapsed time in milliseconds (updated by caller).</summary>
    public double ElapsedMs { get; private set; }

    /// <summary>0.0 = fully "from" scene, 1.0 = fully "to" scene.</summary>
    public float Progress => DurationMs <= 0 ? 1f
        : Math.Clamp((float)(ElapsedMs / DurationMs), 0f, 1f);

    /// <summary>
    /// The "from" frame (captured at transition start). Populated by the caller
    /// before starting the transition.
    /// </summary>
    public VideoFrame? FromFrame { get; private set; }

    /// <summary>Begin a transition. Caller should capture FromFrame from the current LiveScene render.</summary>
    public void Start(TransitionType type, double durationMs, VideoFrame? fromFrame)
    {
        CurrentType = type;
        DurationMs = durationMs > 0 ? durationMs : DefaultDurationMs;
        FromFrame = fromFrame;
        ElapsedMs = 0;
        Active = true;

        if (type == TransitionType.Cut)
        {
            Active = false;
            ElapsedMs = DurationMs;
        }
    }

    /// <summary>
    /// Advance the transition clock. Returns true while the transition is still active.
    /// </summary>
    public bool Tick(double elapsedMs)
    {
        if (!Active) return false;
        ElapsedMs += elapsedMs;
        if (ElapsedMs >= DurationMs)
        {
            ElapsedMs = DurationMs;
            Active = false;
            return false;
        }
        return true;
    }

    /// <summary>
    /// Blend the from-frame and the to-frame based on the current progress.
    /// Caller passes the freshly-rendered "to" scene; we return the blended output.
    /// </summary>
    public VideoFrame BlendFrame(VideoFrame toFrame)
    {
        if (FromFrame == null || CurrentType == TransitionType.Cut)
            return toFrame;

        var t = Progress;
        return CurrentType switch
        {
            TransitionType.Fade => BlendFade(FromFrame, toFrame, t),
            TransitionType.Move => BlendMove(FromFrame, toFrame, t),
            _ => toFrame
        };
    }

    private static VideoFrame BlendFade(VideoFrame from, VideoFrame to, float t)
    {
        var w = from.Width;
        var h = from.Height;
        var src = from.BgraPixels;
        var dst = to.BgraPixels;
        var outPixels = new byte[src.Length];

        for (var i = 0; i < outPixels.Length; i += 4)
        {
            outPixels[i]     = (byte)(src[i]     * (1 - t) + dst[i]     * t);
            outPixels[i + 1] = (byte)(src[i + 1] * (1 - t) + dst[i + 1] * t);
            outPixels[i + 2] = (byte)(src[i + 2] * (1 - t) + dst[i + 2] * t);
            outPixels[i + 3] = 255;
        }

        return new VideoFrame(w, h, outPixels);
    }

    private static VideoFrame BlendMove(VideoFrame from, VideoFrame to, float t)
    {
        var w = from.Width;
        var h = from.Height;
        var src = from.BgraPixels;
        var dst = to.BgraPixels;
        var outPixels = new byte[src.Length];
        var offset = (int)(w * (1 - t));

        for (var y = 0; y < h; y++)
        {
            for (var x = 0; x < w; x++)
            {
                var di = (y * w + x) * 4;
                var fromX = x - offset;
                var toX = x + offset;

                var srcPixel = fromX >= 0 && fromX < w
                    ? (src[(y * w + fromX) * 4], src[(y * w + fromX) * 4 + 1], src[(y * w + fromX) * 4 + 2])
                    : ((byte)0, (byte)0, (byte)0);

                var dstPixel = toX >= 0 && toX < w
                    ? (dst[(y * w + toX) * 4], dst[(y * w + toX) * 4 + 1], dst[(y * w + toX) * 4 + 2])
                    : ((byte)0, (byte)0, (byte)0);

                var edgeFalloff = Math.Clamp((float)(offset) / (w * 0.3f), 0, 1);
                var lerpF = Math.Clamp(1f - edgeFalloff, 0, 1);

                outPixels[di]     = (byte)(srcPixel.Item1 * (1 - lerpF) + dstPixel.Item1 * lerpF);
                outPixels[di + 1] = (byte)(srcPixel.Item2 * (1 - lerpF) + dstPixel.Item2 * lerpF);
                outPixels[di + 2] = (byte)(srcPixel.Item3 * (1 - lerpF) + dstPixel.Item3 * lerpF);
                outPixels[di + 3] = 255;
            }
        }

        return new VideoFrame(w, h, outPixels);
    }
}