LlamaCasty/Services/TextOverlayRenderer.cs

using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Globalization;
using System.Windows;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using ytLive.Models;

namespace ytLive.Services;

public sealed class TextOverlayRenderer
{
    private const int MaxRasterDimension = 4096;
    private readonly ConcurrentDictionary<Source, CacheEntry> _cache = new();

    public VideoFrame? RenderFrame(Source source)
        => _cache.TryGetValue(source, out var entry) ? entry.Frame : null;

    public void Remove(Source source) => _cache.TryRemove(source, out _);

    public void Retain(IEnumerable<Source> sources)
    {
        var active = new HashSet<Source>(sources);
        foreach (var source in _cache.Keys)
            if (!active.Contains(source))
                _cache.TryRemove(source, out _);
    }

    public void UpdatePreview(Source source)
    {
        var dispatcher = Application.Current?.Dispatcher;
        if (dispatcher != null && !dispatcher.CheckAccess())
        {
            dispatcher.Invoke(() => UpdatePreview(source));
            return;
        }

        var config = Capture(source);
        if (_cache.TryGetValue(source, out var cached) && cached.Config == config)
        {
            if (!ReferenceEquals(source.VideoImageSource, cached.Preview))
                source.VideoImageSource = cached.Preview;
            return;
        }

        var frame = Rasterize(config);
        var bitmap = new WriteableBitmap(frame.Width, frame.Height, 96, 96, PixelFormats.Bgra32, null);
        bitmap.WritePixels(new Int32Rect(0, 0, frame.Width, frame.Height), frame.BgraPixels, frame.Stride, 0);
        _cache[source] = new CacheEntry(config, frame, bitmap);
        source.VideoImageSource = bitmap;
    }

    private static RenderConfig Capture(Source source) => new(
        source.TextContent,
        source.TextFontFamily,
        source.TextFontSize,
        source.TextColor,
        source.TextBold,
        source.TextItalic,
        source.TextUnderline,
        source.TextAlignment,
        RasterDimension(source.Width),
        RasterDimension(source.Height));

    private static int RasterDimension(double value) =>
        double.IsFinite(value) ? (int)Math.Clamp(Math.Ceiling(value), 1, MaxRasterDimension) : 1;

    private static VideoFrame Rasterize(RenderConfig config)
    {
        var visual = new DrawingVisual();
        if (!string.IsNullOrEmpty(config.Content))
        {
            using var drawing = visual.RenderOpen();
            var family = new FontFamily(config.FontFamily);
            var typeface = new Typeface(
                family,
                config.Italic ? FontStyles.Italic : FontStyles.Normal,
                config.Bold ? FontWeights.Bold : FontWeights.Normal,
                FontStretches.Normal);
            var brush = new SolidColorBrush(ParseColor(config.Color));
            brush.Freeze();
            var formatted = new FormattedText(
                config.Content,
                CultureInfo.CurrentUICulture,
                FlowDirection.LeftToRight,
                typeface,
                config.FontSize,
                brush,
                1.0)
            {
                MaxTextWidth = config.Width,
                MaxTextHeight = config.Height,
                TextAlignment = config.Alignment switch
                {
                    TextLayerAlignment.Center => System.Windows.TextAlignment.Center,
                    TextLayerAlignment.Right => System.Windows.TextAlignment.Right,
                    _ => System.Windows.TextAlignment.Left,
                },
                Trimming = TextTrimming.CharacterEllipsis,
            };
            if (config.Underline)
                formatted.SetTextDecorations(TextDecorations.Underline, 0, config.Content.Length);
            drawing.DrawText(formatted, new Point(0, 0));
        }

        var bitmap = new RenderTargetBitmap(config.Width, config.Height, 96, 96, PixelFormats.Pbgra32);
        bitmap.Render(visual);
        var pixels = new byte[config.Width * config.Height * 4];
        bitmap.CopyPixels(pixels, config.Width * 4, 0);
        UnpremultiplyAlpha(pixels);
        return new VideoFrame(config.Width, config.Height, pixels);
    }

    private static Color ParseColor(string hex) =>
        (Color)ColorConverter.ConvertFromString(hex)!;

    private static void UnpremultiplyAlpha(byte[] pixels)
    {
        for (var i = 0; i < pixels.Length; i += 4)
        {
            var alpha = pixels[i + 3];
            if (alpha == 0)
            {
                pixels[i] = pixels[i + 1] = pixels[i + 2] = 0;
                continue;
            }
            if (alpha == 255) continue;
            pixels[i] = Unpremultiply(pixels[i], alpha);
            pixels[i + 1] = Unpremultiply(pixels[i + 1], alpha);
            pixels[i + 2] = Unpremultiply(pixels[i + 2], alpha);
        }
    }

    private static byte Unpremultiply(byte value, byte alpha) =>
        (byte)Math.Min(255, (value * 255 + alpha / 2) / alpha);

    private sealed record CacheEntry(RenderConfig Config, VideoFrame Frame, WriteableBitmap Preview);

    private readonly record struct RenderConfig(
        string Content,
        string FontFamily,
        double FontSize,
        string Color,
        bool Bold,
        bool Italic,
        bool Underline,
        TextLayerAlignment Alignment,
        int Width,
        int Height);
}