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

public static class BoxCut
{
    // Cut the 3D gift box out of its lavender-pink gradient background. Background = bluish pink
    // (high blue, blue clearly above green); the box itself is warm pink, green ribbon, orange flowers.
    // Flood-fill from the crop border, feather the edge, trim to the content box, save PNG.
    public static void Run(string src, string dst, int cropH, int pad)
    {
        using (var b0 = new Bitmap(src))
        using (var b = b0.Clone(new Rectangle(0, 0, b0.Width, cropH), 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, double> bgScore = i => {
                int B = px[i], G = px[i + 1], R = px[i + 2];
                // 1 = surely background, 0 = surely box
                double s = (B - G - 4) / 10.0;
                if (R < 225 || B < 225) s = Math.Min(s, 0);
                return Math.Max(0, Math.Min(1, s));
            };
            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] || bgScore(p * 4) < 0.5) 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);
            }
            int minX = w, minY = h, maxX = 0, maxY = 0;
            for (int p = 0; p < w * h; p++)
            {
                int i = p * 4, x = p % w, y = p / w;
                if (seen[p]) { px[i + 3] = 0; continue; }
                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) px[i + 3] = (byte)(255 * (1 - 0.6 * bgScore(i)));
                if (x < minX) minX = x; if (x > maxX) maxX = x; if (y < minY) minY = y; if (y > maxY) maxY = y;
            }
            Marshal.Copy(px, 0, data.Scan0, px.Length);
            b.UnlockBits(data);
            int cw = maxX - minX + 1 + pad * 2, ch = maxY - minY + 1 + pad * 2;
            using (var o = new Bitmap(cw, ch, PixelFormat.Format32bppArgb))
            using (var g = Graphics.FromImage(o))
            {
                g.Clear(Color.Transparent);
                g.DrawImage(b, new Rectangle(pad, pad, maxX - minX + 1, maxY - minY + 1), new Rectangle(minX, minY, maxX - minX + 1, maxY - minY + 1), GraphicsUnit.Pixel);
                o.Save(dst, ImageFormat.Png);
            }
            Console.WriteLine("bbox " + minX + "," + minY + " " + (maxX - minX + 1) + "x" + (maxY - minY + 1));
        }
    }
}