[gegl] lohalo: prevent floating point disaster

[Nicolas Robidoux <nrobidoux src gnome org>]

commit 4d73aadb5a681706941bd8209e00aca0ea28c5ac
Author: Nicolas Robidoux <nrobidoux git gnome org>
Date:   Tue Dec 11 19:29:07 2012 -0500

    lohalo: prevent floating point disaster

 gegl/buffer/gegl-sampler-lohalo.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)
---
diff --git a/gegl/buffer/gegl-sampler-lohalo.c b/gegl/buffer/gegl-sampler-lohalo.c
index 895ea47..46a7a98 100644
--- a/gegl/buffer/gegl-sampler-lohalo.c
+++ b/gegl/buffer/gegl-sampler-lohalo.c
@@ -1980,7 +1980,13 @@ gegl_sampler_lohalo_get (      GeglSampler*    restrict  self,
       const gdouble frobenius_squared = n11 + n22;
       const gdouble discriminant =
         ( frobenius_squared + twice_det ) * ( frobenius_squared - twice_det );
-      const gdouble sqrt_discriminant = sqrt( discriminant );
+      /*
+       * In exact arithmetic, the discriminant cannot be negative. In
+       * floating point, it can, leading a non-deterministic bug in
+       * ImageMagick (now fixed, thanks to Cristy).
+       */
+      const gdouble sqrt_discriminant =
+	sqrt (((double) discriminant > 0.) ? (double) discriminant : 0.);
 
       /*
        * Initially, we only compute the squares of the singular