Compute first sample through a micro step from y0 to yi.

Instead of computing the precise X coordinate for the first row
directly, which involves a division, simply compute the error term at
y0, and then do a regular step from y0 to next_sample_y (y0).

1 files changed, 26 insertions, 2 deletions

diff --git a/dda.c b/dda.c
index 1b0cfa5..8e4d79e 100644
--- a/dda.c
+++ b/dda.c
@@ -73,8 +73,32 @@ dda (test_data_t *testdata)
     double dxdy = dx / dy;
     double yi = next_sample_y (y0);
     double x = (yi - y0) * dxdy + x0;
-    double xi = next_sample_x (x);
-    double e = (xi - x) * dy;
+    double xi = next_sample_x (x0);
+    double e = (xi - x0) * dy;
+
+    e -= (yi - y0) * dx;
+    if (dx >= 0)
+    {
+	while (e <= 0)
+	{
+	    double s = sample_step_x_forward (xi);
+
+	    e += s * dy;
+	    xi += s;
+	}
+    }
+    else
+    {
+	double s = sample_step_x_backward (xi);
+
+	while (e > s * dy)
+	{
+	    e -= s * dy;
+	    xi -= s;
+
+	    s = sample_step_x_backward (xi);
+	}
+    }
 
 #if 0
     printf ("= = = = %f %f %f %f = = = = \n", x0, y0, x1, y1);