System.Drawing and SkiaSharp - fill blending via opacity channel for rgb (no alpha) image

Program.cs

using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Drawing.Processing;
using SixLabors.ImageSharp.Processing;
using SkiaSharp;
using System;
using System.IO;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            ImageSharp();
            CairoImage();
            SkiaSharp();
            SystemDrawing();
        }

        public static void ImageSharp()
        {
            Console.WriteLine("Hello World!");
            var image = new SixLabors.ImageSharp.Image<SixLabors.ImageSharp.PixelFormats.Rgb24>(SixLabors.ImageSharp.Configuration.Default, 100, 100, SixLabors.ImageSharp.Color.Black);

            image.Mutate(x => x.Fill(SixLabors.ImageSharp.Color.Red.WithAlpha(0.5f), new RectangleF(10, 10, 80, 80)));

            image.Save("rgb24demo_imagesharp.png");
        }

        public static void CairoImage()
        {
            var surface = new Cairo.ImageSurface(Cairo.Format.Rgb24, 100, 100);
            using (var cr = new Cairo.Context(surface))
            {
                cr.Color = new Cairo.Color(0, 0, 0);
                cr.Fill();

                cr.Color = new Cairo.Color(1, 0, 0, 0.5);
                cr.Rectangle(10, 10, 80, 80);
                cr.Fill();
            }

            surface.WriteToPng("rgb24demo_cairo.png");
        }

        public static void SkiaSharp()
        {
            using var image = new SKBitmap(new SKImageInfo(100, 100, SKColorType.Rgb888x));

            using (var canvas = new SKCanvas(image))
            {
                canvas.Clear(SKColors.Black);
                SKPaint redFill = new SKPaint
                {
                    Style = SKPaintStyle.Fill,
                    Color = SKColors.Red.WithAlpha(128),
                };
                canvas.DrawRect(10, 10, 80, 80, redFill);

                canvas.Flush();
            }

            using (var save = SKImage.FromBitmap(image))
            using (var data = save.Encode(SKEncodedImageFormat.Png, 100))
            {
                // save the data to a stream
                using (var stream = File.OpenWrite("rgb24demo_skia.png"))
                {
                    data.SaveTo(stream);
                }
            }
        }

        public static void SystemDrawing()
        {
            using var sdImage = new System.Drawing.Bitmap(100, 100, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
            using (var graphics = System.Drawing.Graphics.FromImage(sdImage))
            {
                graphics.Clear(System.Drawing.Color.Black);

                graphics.FillRectangle(new System.Drawing.SolidBrush(System.Drawing.Color.FromArgb(128, 255, 0, 0)), new System.Drawing.Rectangle(10, 10, 80, 80));
            }
            using (var stream = File.OpenWrite("rgb24demo_system.drawing.png"))
            {
                sdImage.Save(stream, System.Drawing.Imaging.ImageFormat.Png);
            }
        }
    }
}

sdTestBlending.png

skiaTestBlending.png

For future reference, this example was made for SixLabors/ImageSharp.Drawing#38