using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;

public static class Cutout
{
    // Remove a flat light-grey studio background by flood-filling from the image border,
    // soften the 1px edge, then scale down to size x size.
    public static void Run(string src, string dst, int bg, int tol, int size)
    {
        using (var b0 = new Bitmap(src))
        using (var b = b0.Clone(new Rectangle(0, 0, b0.Width, b0.Height), PixelFormat.Format32bppArgb))
        {
            int w = b.Width, h = b.Height;
            var data = b.LockBits(new Rectangle(0, 0, w, h), ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb);
            var px = new byte[w * h * 4];
            Marshal.Copy(data.Scan0, px, 0, px.Length);
            Func<int, bool> isBg = i =>
            {
                int B = px[i], G = px[i + 1], R = px[i + 2];
                int mx = Math.Max(R, Math.Max(G, B)), mn = Math.Min(R, Math.Min(G, B));
                return mx - mn <= 6 && Math.Abs(mx - bg) <= tol;
            };
            var seen = new bool[w * h];
            var q = new Queue<int>();
            for (int x = 0; x < w; x++) { q.Enqueue(x); q.Enqueue((h - 1) * w + x); }
            for (int y = 0; y < h; y++) { q.Enqueue(y * w); q.Enqueue(y * w + w - 1); }
            while (q.Count > 0)
            {
                int p = q.Dequeue();
                if (seen[p] || !isBg(p * 4)) continue;
                seen[p] = true;
                int x = p % w, y = p / w;
                if (x > 0) q.Enqueue(p - 1);
                if (x < w - 1) q.Enqueue(p + 1);
                if (y > 0) q.Enqueue(p - w);
                if (y < h - 1) q.Enqueue(p + w);
            }
            for (int p = 0; p < w * h; p++)
            {
                int i = p * 4;
                if (seen[p]) { px[i + 3] = 0; continue; }
                int x = p % w, y = p / w;
                bool edge = (x > 0 && seen[p - 1]) || (x < w - 1 && seen[p + 1]) || (y > 0 && seen[p - w]) || (y < h - 1 && seen[p + w]);
                if (!edge) continue;
                int d = Math.Max(Math.Abs(px[i + 2] - bg), Math.Max(Math.Abs(px[i + 1] - bg), Math.Abs(px[i] - bg)));
                double a = Math.Min(1.0, Math.Max(0.2, d / 50.0));
                for (int c = 0; c < 3; c++) px[i + c] = (byte)Math.Max(0, Math.Min(255, Math.Round((px[i + c] - (1 - a) * bg) / a)));
                px[i + 3] = (byte)(a * 255);
            }
            Marshal.Copy(px, 0, data.Scan0, px.Length);
            b.UnlockBits(data);
            using (var o = new Bitmap(size, size, PixelFormat.Format32bppArgb))
            using (var g = Graphics.FromImage(o))
            {
                g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                g.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
                g.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceCopy;
                g.DrawImage(b, new Rectangle(0, 0, size, size));
                o.Save(dst, ImageFormat.Png);
            }
        }
    }
}