[gimp] app: improve stability of Gaussian elimination in transform-utils

[N/A <ell src gnome org>]

commit e7b6103cc8eeb8ed4fced874f42f92c0fecfae96
Author: Ell <ell_se yahoo com>
Date:   Sat Jan 27 05:49:11 2018 -0500

    app: improve stability of Gaussian elimination in transform-utils
    
    When picking a row whose leading coefficient is nonzero, use fuzzy
    comparison against an epsilon, instead of exact comparison against
    zero, to minimize the effect of numerical errors.

 app/core/gimp-transform-utils.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/app/core/gimp-transform-utils.c b/app/core/gimp-transform-utils.c
index 4cd11db..93bc640 100644
--- a/app/core/gimp-transform-utils.c
+++ b/app/core/gimp-transform-utils.c
@@ -26,6 +26,9 @@
 #include "gimp-transform-utils.h"
 
 
+#define EPSILON 1e-6
+
+
 void
 gimp_transform_get_rotate_center (gint      x,
                                   gint      y,
@@ -363,7 +366,7 @@ mod_gauss (gdouble matrix[],
   for (r = 0; r < s; r++)
     {
       /* make sure that (r,r) is not 0 */
-      if (matrix[p[r] * t + r] == 0.0)
+      if (fabs (matrix[p[r] * t + r]) <= EPSILON)
         {
           /* we need to permutate rows */
           for (i = r + 1; i <= s; i++)
@@ -376,7 +379,7 @@ mod_gauss (gdouble matrix[],
                   return FALSE;
                 }
 
-              if (matrix[p[i] * t + r] != 0.0)
+              if (fabs (matrix[p[i] * t + r]) > EPSILON)
                 break;
             }