// Draw the Buddhabrot until stopped or
// we plot the desired number of points.
private void DrawBrot()
{
    // Get parameters.
    int wid = int.Parse(txtWidth.Text);
    int hgt = int.Parse(txtHeight.Text);
    int cut_r = int.Parse(txtRedCutoff.Text);
    int cut_g = int.Parse(txtGreenCutoff.Text);
    int cut_b = int.Parse(txtBlueCutoff.Text);
    int stop_after = int.Parse(txtStopAfter.Text);
    int draw_every = int.Parse(txtDrawEvery.Text);

    if ((wid <= 0) || (hgt <= 0))
    {
        MessageBox.Show("Invalid parameter", "Error", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
        return;
    }

    // Make hit count arrays.
    int[,] hit_r = new int[wid, hgt];
    int[,] hit_g = new int[wid, hgt];
    int[,] hit_b = new int[wid, hgt];

    // Make the bitmap.
    m_Bitmap = new Bitmap(wid, hgt);
    picCanvas.Image = m_Bitmap;
    this.ClientSize = new Size(
        picCanvas.Left + picCanvas.Width + 8,
        System.Math.Max(
            btnDraw.Top + btnDraw.Height,
            picCanvas.Top + picCanvas.Height) + 8);
    mnuFileSave.Enabled = true;

    // Start drawing.
    DateTime start_time = DateTime.Now;
    DateTime stop_time;
    TimeSpan elapsed;
    m_Drawing = true;

    // Build the hit counts.
    double dx = (Wxmax - Wxmin) / hgt;
    double dy = (Wymax - Wymin) / wid;
    int max_r = 0, max_g = 0, max_b = 0, hits = 0, total_hits = 0;

    while (total_hits < stop_after)
    {
        double cx = Wxmin + m_Rand.NextDouble() * (Wxmax - Wxmin);
        double cy = Wymin + m_Rand.NextDouble() * (Wymax - Wymin);
        double dd = cx * cx + cy * cy;
        if (dd < 1)
        {
            DrawPoint(cx, cy, wid, hgt, dx, dy, ref max_r, hit_r, cut_r, ref hits);
        }
        else if (dd < 2)
        {
            DrawPoint(cx, cy, wid, hgt, dx, dy, ref max_g, hit_g, cut_g, ref hits);
        }
        else
        {
            DrawPoint(cx, cy, wid, hgt, dx, dy, ref max_b, hit_b, cut_b, ref hits);
        }

        if (hits >= draw_every)
        {
            total_hits += hits;
            hits = 0;
            DisplayBrot(wid, hgt, max_r, max_g, max_b, hit_r, hit_g, hit_b);

            stop_time = DateTime.Now;
            elapsed = stop_time.Subtract(start_time);
            this.Text = elapsed.TotalSeconds.ToString("0.00") + 
                " sec, " + total_hits.ToString() + " hits";

            Application.DoEvents();
            if (!m_Drawing) break;
        }
    }
}

// Plot one point.
private void DrawPoint(double cx, double cy , int wid, int hgt, double dx , double dy , ref int max_hits, int[,] hits, int cutoff, ref int num_hits)
{
    const double ESCAPING = 4;

    // Pick C.
    ////Dim cx As Double = Wxmin + m_Rand.NextDouble() * (Wxmax - Wxmin)
    ////Dim cy As Double = Wymin + m_Rand.NextDouble() * (Wymax - Wymin)

    // Zet Z0.
    double x, xx, y, yy;
    x = cx;
    y = cy;
    xx = x * x;
    yy = y * y;

    // Iterate.
    for (int i = 1; i <= cutoff; i++)
    {
        y = 2 * x * y + cy;
        x = xx - yy + cx;
        xx = x * x;
        yy = y * y;
        if (xx + yy >= ESCAPING) break;
    }

    // See if we escaped.
    if (xx + yy >= ESCAPING)
    {
        // Plot.
        x = cx;
        y = cy;
        xx = x * x;
        yy = y * y;

        // Iterate.
        for (int i = 1; i <= cutoff; i++)
        {
            int ix = (int)Math.Round((x - Wxmin) / dx);
            int iy = (int)Math.Round((y - Wymin) / dy);
            if ((ix >= 0) && (ix < hgt) && (iy >= 0) && (iy < wid))
            {
                hits[iy, ix] += 1;
                if (max_hits < hits[iy, ix]) max_hits = hits[iy, ix];
            }
            else
            {
                break;
            }

            y = 2 * x * y + cy;
            x = xx - yy + cx;
            xx = x * x;
            yy = y * y;
            if (xx + yy >= ESCAPING) break;
        }

        num_hits += 1;
    }
}

// Draw the current image.
private void DisplayBrot(int wid, int hgt, int max_r, int max_g, int max_b, int[,] hit_r, int[,] hit_g, int[,] hit_b)
{
    Graphics gr = Graphics.FromImage(m_Bitmap);
    gr.Clear(Color.Black);

    double scale_r = 255 * 2.5 / max_r;
    double scale_g = 255 * 2.5 / max_g;
    double scale_b = 255 * 2.5 / max_b;

    for (int y = 0; y < hgt; y++)
    {
        for (int x = 0; x < wid; x++)
        {
            int r  = (int)Math.Round(hit_r[x, y] * scale_r);
            if (r > 255) r = 255;
            int g  = (int)Math.Round(hit_g[x, y] * scale_g);
            if (g > 255) g = 255;
            int b  = (int)Math.Round(hit_b[x, y] * scale_b);
            if (b > 255) b = 255;

            m_Bitmap.SetPixel(x, y, Color.FromArgb(255, r, g, b));
        }
    }

    picCanvas.Refresh();
}