[gegl] transform: make it transform pixel centers, not corners

[Nicolas Robidoux <nrobidoux src gnome org>]

commit 573c7642ad05f578218e518f122b3d2ff8047d8b
Author: Massimo Valentini <mvalentini src gnome org>
Date:   Sun Nov 18 17:27:14 2012 +0100

    transform: make it transform pixel centers, not corners
    
    https://bugzilla.gnome.org/show_bug.cgi?id=433436

 operations/transform/transform-core.c |   11 +++--------
 1 files changed, 3 insertions(+), 8 deletions(-)
---
diff --git a/operations/transform/transform-core.c b/operations/transform/transform-core.c
index 5918cc3..1713339 100644
--- a/operations/transform/transform-core.c
+++ b/operations/transform/transform-core.c
@@ -691,14 +691,9 @@ transform_affine (GeglBuffer  *dest,
       inverse_jacobian.coeff[1][0] = inverse.coeff[1][0];
       inverse_jacobian.coeff[1][1] = inverse.coeff[1][1];
 
-      u_start = inverse.coeff[0][0] * roi->x + inverse.coeff[0][1] * roi->y + inverse.coeff[0][2];
-      v_start = inverse.coeff[1][0] * roi->x + inverse.coeff[1][1] * roi->y + inverse.coeff[1][2];
-      w_start = inverse.coeff[2][0] * roi->x + inverse.coeff[2][1] * roi->y + inverse.coeff[2][2];
-
-      /* correct rounding on e.g. negative scaling (is this sound?) */
-      if (inverse.coeff [0][0] < 0.)  u_start -= .001;
-      if (inverse.coeff [1][1] < 0.)  v_start -= .001;
-      if (inverse.coeff [2][2] < 0.)  w_start -= .001;
+      u_start = inverse.coeff[0][0] * (roi->x + 0.5) + inverse.coeff[0][1] * (roi->y + 0.5) + inverse.coeff[0][2];
+      v_start = inverse.coeff[1][0] * (roi->x + 0.5) + inverse.coeff[1][1] * (roi->y + 0.5) + inverse.coeff[1][2];
+      w_start = inverse.coeff[2][0] * (roi->x + 0.5) + inverse.coeff[2][1] * (roi->y + 0.5) + inverse.coeff[2][2];
 
       for (dest_ptr = dest_buf, y = roi->height; y--;)
         {