LlamaCasty/Services/BackgroundEffects/BackgroundEffectProcessor.cs

using ytLive.Helpers;
using ytLive.Models;

namespace ytLive.Services.BackgroundEffects;

/// <summary>
/// Applies a webcam's <see cref="BackgroundEffect"/> to one captured frame (TASK 51).
/// Pure pixel work over the <see cref="VideoFrame"/> seam — WPF-free and thread-agnostic,
/// so the camera worker thread can call it before the frame reaches the preview bitmap
/// and the compositor.
///
/// <list type="bullet">
/// <item><see cref="BackgroundEffect.None"/> returns the SAME frame instance — a true
/// passthrough, no copy.</item>
/// <item><see cref="BackgroundEffect.Remove"/> returns a copy whose alpha is the person
/// mask (bilinear-upscaled), RGB untouched, <c>IsOpaque = false</c> — the compositor's
/// straight-alpha blend then shows the layers below the webcam.</item>
/// <item><see cref="BackgroundEffect.Blur"/> is the next slice; until it lands it passes
/// through.</item>
/// </list>
///
/// A segmenter that returns no mask (or throws) never drops the layer: the frame passes
/// through and the failure is logged once.
/// </summary>
public sealed class BackgroundEffectProcessor
{
    private readonly IPersonSegmenter _segmenter;
    private bool _loggedFailure;

    public BackgroundEffectProcessor(IPersonSegmenter segmenter)
    {
        _segmenter = segmenter;
    }

    public VideoFrame Apply(VideoFrame frame, BackgroundEffect effect)
    {
        if (effect != BackgroundEffect.Remove) return frame;

        SegmentationMask? mask;
        try
        {
            mask = _segmenter.Segment(frame);
        }
        catch (Exception ex)
        {
            LogFailureOnce($"{ex.GetType().Name}: {ex.Message}");
            return frame;
        }

        if (mask == null || mask.Values.Length < mask.Width * mask.Height || mask.Width < 1 || mask.Height < 1)
        {
            LogFailureOnce("segmenter returned no usable mask");
            return frame;
        }

        return ApplyRemove(frame, mask);
    }

    internal static VideoFrame ApplyRemove(VideoFrame frame, SegmentationMask mask)
    {
        var src = frame.BgraPixels;
        var dst = new byte[src.Length];
        Buffer.BlockCopy(src, 0, dst, 0, src.Length);

        var w = frame.Width;
        var h = frame.Height;
        var stride = frame.Stride;
        for (var y = 0; y < h; y++)
        {
            var my = (y + 0.5f) * mask.Height / h - 0.5f;
            var row = y * stride;
            for (var x = 0; x < w; x++)
            {
                var mx = (x + 0.5f) * mask.Width / w - 0.5f;
                var a = SampleBilinear(mask, mx, my);
                var alpha = (int)(a * src[row + x * 4 + 3] + 0.5f);
                dst[row + x * 4 + 3] = (byte)Math.Clamp(alpha, 0, 255);
            }
        }

        return new VideoFrame(w, h, dst) { Epoch = frame.Epoch, IsOpaque = false };
    }

    private static float SampleBilinear(SegmentationMask mask, float x, float y)
    {
        var x0 = (int)MathF.Floor(x);
        var y0 = (int)MathF.Floor(y);
        var fx = x - x0;
        var fy = y - y0;
        var x1 = Math.Clamp(x0 + 1, 0, mask.Width - 1);
        var y1 = Math.Clamp(y0 + 1, 0, mask.Height - 1);
        x0 = Math.Clamp(x0, 0, mask.Width - 1);
        y0 = Math.Clamp(y0, 0, mask.Height - 1);
        var v = mask.Values;
        var top = v[y0 * mask.Width + x0] * (1 - fx) + v[y0 * mask.Width + x1] * fx;
        var bottom = v[y1 * mask.Width + x0] * (1 - fx) + v[y1 * mask.Width + x1] * fx;
        return Math.Clamp(top * (1 - fy) + bottom * fy, 0f, 1f);
    }

    private void LogFailureOnce(string reason)
    {
        if (_loggedFailure) return;
        _loggedFailure = true;
        AppLog.Write($"BackgroundEffectProcessor: passing webcam through — {reason}");
    }
}