Fix: Change gradient math to avoid isqrt

isqrt adds some noticeable distortion artifacts to gradients (test case
I used is to draw a line from 10,10 to 20,15 with mindist = maxdist = 2
and see how the gradient is biased towards the upper right); changing
the distance calculations to use the square of the distance rather than
the raw distance avoids this. This makes radial gradients more radial,
and square gradients more square.

There still appears to be a bit of bias, but I think this is due to the
line algorithm not lining up perfectly with the tiles.

diff --git a/src/sp_lev.c b/src/sp_lev.c
index 7870d0b0a2d0ebf3890a4f98aedea42a8f10edaf..27655c47330c0b93185ec4974b10117fdda788b5 100644 (file)
--- a/src/sp_lev.c
+++ b/src/sp_lev.c
@@ -4961,18 +4961,18 @@ selection_do_ellipse(
     }
 }
 
-/* distance from line segment (x1,y1, x2,y2) to point (x3,y3) */
+/* square of distance from line segment (x1,y1, x2,y2) to point (x3,y3) */
 static long
 line_dist_coord(long x1, long y1, long x2, long y2, long x3, long y3)
 {
     long px = x2 - x1;
     long py = y2 - y1;
     long s = px * px + py * py;
-    long x, y, dx, dy, dist = 0;
+    long x, y, dx, dy, distsq = 0;
     float lu = 0;
 
     if (x1 == x2 && y1 == y2)
-        return isqrt(dist2(x1, y1, x3, y3));
+        return dist2(x1, y1, x3, y3);
 
     lu = ((x3 - x1) * px + (y3 - y1) * py) / (float) s;
     if (lu > 1)
@@ -4984,9 +4984,9 @@ line_dist_coord(long x1, long y1, long x2, long y2, long x3, long y3)
     y = y1 + lu * py;
     dx = x - x3;
     dy = y - y3;
-    dist = isqrt(dx * dx + dy * dy);
+    distsq = dx * dx + dy * dy;
 
-    return dist;
+    return distsq;
 }
 
 /* guts of l_selection_gradient */
@@ -5006,7 +5006,7 @@ selection_do_gradient(
         maxd = tmp;
     }
 
-    dofs = maxd - mind;
+    dofs = maxd * maxd - mind * mind;
     if (dofs < 1)
         dofs = 1;
 
@@ -5018,7 +5018,8 @@ selection_do_gradient(
         for (dx = 0; dx < COLNO; dx++)
             for (dy = 0; dy < ROWNO; dy++) {
                 long d0 = line_dist_coord(x, y, x2, y2, dx, dy);
-                if (d0 <= mind || (d0 <= maxd && d0 - mind < rn2(dofs)))
+                if (d0 <= mind * mind
+                    || (d0 <= maxd * maxd && d0 - mind * mind < rn2(dofs)))
                     selection_setpoint(dx, dy, ov, 1);
             }
         break;
@@ -5033,7 +5034,8 @@ selection_do_gradient(
                 long d5 = line_dist_coord(x, y, x2, y2, dx, dy);
                 long d0 = min(d5, min(max(d1, d2), max(d3, d4)));
 
-                if (d0 <= mind || (d0 <= maxd && d0 - mind < rn2(dofs)))
+                if (d0 <= mind * mind
+                    || (d0 <= maxd * maxd && d0 - mind * mind < rn2(dofs)))
                     selection_setpoint(dx, dy, ov, 1);
             }
         break;