flow

        private void btnStep_Click(object sender, EventArgs e) {
            btnStep.Text = "Working...";
            Application.DoEvents();
            ovWater = Overlays[Overlays.Count - 1];
            Overlay ovHeight = Overlays[0];

            for (int idx = 0; idx < 100; idx++) {


                for (int x = 0; x < worldWidth; x++) {
                //Parallel.For(0, worldWidth, x => {
                    for (int y = 0; y < worldHeight; y++) {
                        slopeMap[x, y] = new FlowPoint();

                        if (x > 0) {
                            slopeMap[x, y].West = (ovHeight.Heightmap[x, y] + ovWater.Heightmap[x, y]) - (ovHeight.Heightmap[x - 1, y] + ovWater.Heightmap[x - 1, y]);
                        }
                        if (x < worldWidth - 1) {
                            slopeMap[x, y].East = (ovHeight.Heightmap[x, y] + ovWater.Heightmap[x, y]) - (ovHeight.Heightmap[x + 1, y] + ovWater.Heightmap[x + 1, y]);
                        }
                        if (y > 0) {
                            slopeMap[x, y].North = (ovHeight.Heightmap[x, y] + ovWater.Heightmap[x, y]) - (ovHeight.Heightmap[x, y - 1] + ovWater.Heightmap[x, y - 1]);
                        }
                        if (y < worldHeight - 1) {
                            slopeMap[x, y].South = (ovHeight.Heightmap[x, y] + ovWater.Heightmap[x, y]) - (ovHeight.Heightmap[x, y + 1] + ovWater.Heightmap[x, y + 1]);
                        }

                    }
                }//);

                double[,] tmp = new double[worldWidth, worldHeight];


                for (int x = 0; x < worldWidth; x++) {
                //Parallel.For(0, worldWidth, x => {
                    for (int y = 0; y < worldHeight; y++) {

                        bool wvalid = x > 0 && slopeMap[x, y].West > 0,
                             evalid = x < (worldWidth - 1) && slopeMap[x, y].East > 0,
                             nvalid = y > 0 && slopeMap[x, y].North > 0,
                             svalid = y < (worldHeight - 1) && slopeMap[x, y].South > 0;

                        int ct = (wvalid ? 1 : 0) +
                            (evalid ? 1 : 0) +
                            (nvalid ? 1 : 0) +
                            (svalid ? 1 : 0);

                        double flowCt =
                            (nvalid ? slopeMap[x, y].North : 0) +
                            (svalid ? slopeMap[x, y].South : 0) +
                            (evalid ? slopeMap[x, y].East : 0) +
                            (wvalid ? slopeMap[x, y].West : 0);

                        double flowAmt = ovWater.Heightmap[x, y];


                        //if (flowAmt > 0.01)
                        //  flowAmt = 0.01;

                        if (ct > 0) {
                            double flowTot = 0.0, amt = 0;

                            if (nvalid) {
                                amt = (flowAmt * (slopeMap[x, y].North / flowCt));
                                tmp[x, y - 1] += amt;
                                flowTot += amt;
                            }
                            if (svalid) {
                                amt = (flowAmt * (slopeMap[x, y].South / flowCt));
                                tmp[x, y + 1] += amt;
                                flowTot += amt;
                            }
                            if (evalid) {
                                amt = (flowAmt * (slopeMap[x, y].East / flowCt));
                                tmp[x + 1, y] += amt;
                                flowTot += amt;
                            }

                            if (wvalid) {
                                amt = (flowAmt * (slopeMap[x, y].West / flowCt));
                                tmp[x - 1, y] += amt;
                                flowTot += amt;
                            }

                            tmp[x, y] += (ovWater.Heightmap[x, y] - flowTot);
                        }
                        else {
                            tmp[x, y] = ovWater.Heightmap[x, y];
                        }
                    }
                }//);

                ovWater.Heightmap = tmp;
                Overlays[Overlays.Count - 1] = ovWater;

                if (idx % 10 == 0) {
                    FillDefaultOverlays();
                    Draw();
                }

                lblWater.Text =
                    idx.ToString() + "\n" +
                    (from double d in ovWater.Heightmap select d).Sum().ToString() + "\n" +
                    (from double d in ovWater.Heightmap select d).Min().ToString() + "\n" +
                    (from double d in ovWater.Heightmap select d).Max().ToString();
                Application.DoEvents();

            }

            FillDefaultOverlays();
            Draw();
            btnStep.Text = "Step water";
            Application.DoEvents();

        }