fixed4 frag(v2f i) : SV_Target
                {

                const int m = 5;
                float z[m], zed;
                float xs = 1/ 1024.0;
                float ys = 1 / 720.0;
                float r = _blurRadius;
                float2 pos = i.vertex;
                float2 p = 0.5 * (i.vertex + 1.0);
                z[0] = tex2D(_MainTex, i.uv).r;;       // oldest 2 frames
                z[1] = tex2D(_HistoryA, i.uv).r;
                if (abs(z[0] - z[1]) > _dataDelta)              // threshold depth change
                {
                    int index;
                    float x, y, xx, yy, rr, dx, dy, w, w0;

                    // 2D spatial gauss blur of z0
                    rr = r * r;
                    w0 = 0.3780 / pow(r, 1.975);
                    z[0] = 0.0;
                    for (dx = _MainTex_TexelSize.x, x = -r, p.x = 0.5 + (pos.x * 0.5) + (x * dx); x <= r; x++, p.x += dx) {
                        xx = x * x;
                        for (dy = _MainTex_TexelSize.y, y = -r, p.y = 0.5 + (pos.y * 0.5) + (y * dy); y <= r; y++, p.y += dy) {
                            yy = y * y;
                            if (xx + yy <= rr)
                            {
                                w = w0 * exp((-xx - yy) / (2.0 * rr));
                                z[0] += tex2D(_MainTex, p).r * w;
                            }
                        }
                    }