Track error in e * dy

1 files changed, 8 insertions, 7 deletions

diff --git a/dda.c b/dda.c
index 6479c5a..2ef1209 100644
--- a/dda.c
+++ b/dda.c
@@ -68,11 +68,13 @@ dda (test_data_t *testdata)
     double y1 = testdata->segment.y1;
     int i = 0;
 
-    double dxdy = (x1 - x0) / (y1 - y0);
+    double dx = (x1 - x0);
+    double dy = (y1 - y0);
+    double dxdy = dx / dy;
     double y = next_sample_y (y0);
     double x = (y - y0) * dxdy + x0;
     double xi = next_sample_x (x);
-    double e = xi - x;
+    double e = (xi - x) * dy;
 
 #if 0
     printf ("= = = = %f %f %f %f = = = = \n", x0, y0, x1, y1);
@@ -87,8 +89,7 @@ dda (test_data_t *testdata)
 	    exit (-1);
 	}
 
-	e -= sample_step_y (y) * dxdy;
-	x += sample_step_y (y) * dxdy;
+	e -= sample_step_y (y) * dxdy * dy;
 
 	if (dxdy >= 0)
 	{
@@ -96,7 +97,7 @@ dda (test_data_t *testdata)
 	    {
 		double s = sample_step_x_forward (xi);
 
-		e += s;
+		e += s * dy;
 		xi += s;
 	    }
 	}
@@ -104,9 +105,9 @@ dda (test_data_t *testdata)
 	{
 	    double s = sample_step_x_backward (xi);
 
-	    while (e > s)
+	    while (e > s * dy)
 	    {
-		e -= s;
+		e -= s * dy;
 		xi -= s;
 
 		s = sample_step_x_backward (xi);