Index: app/display/gimpdisplayshell.c
===================================================================
--- app/display/gimpdisplayshell.c	(revision 24445)
+++ app/display/gimpdisplayshell.c	(working copy)
@@ -1180,8 +1180,8 @@ gimp_display_shell_scale_changed (GimpDi
 
   shell->x_dest_inc = gimp_image_get_width  (image);
   shell->y_dest_inc = gimp_image_get_height (image);
-  shell->x_src_dec  = ceil (gimp_image_get_width  (image) * shell->scale_x);
-  shell->y_src_dec  = ceil (gimp_image_get_height (image) * shell->scale_y);
+  shell->x_src_dec  = SCALEX (shell, gimp_image_get_width  (image));
+  shell->y_src_dec  = SCALEY (shell, gimp_image_get_height (image));
 }
 
 void
Index: app/display/gimpdisplayshell-transform.c
===================================================================
--- app/display/gimpdisplayshell-transform.c	(revision 24445)
+++ app/display/gimpdisplayshell-transform.c	(working copy)
@@ -54,8 +54,8 @@ gimp_display_shell_transform_coordinate 
 
   *display_coords = *image_coords;
 
-  display_coords->x = shell->scale_x * image_coords->x;
-  display_coords->y = shell->scale_y * image_coords->y;
+  display_coords->x = SCALEX (shell, image_coords->x);
+  display_coords->y = SCALEY (shell, image_coords->y);
 
   display_coords->x += - shell->offset_x + shell->disp_xoffset;
   display_coords->y += - shell->offset_y + shell->disp_yoffset;
@@ -115,8 +115,8 @@ gimp_display_shell_transform_xy (GimpDis
       y += offset_y;
     }
 
-  tx = PROJ_ROUND64 (x * shell->x_src_dec) + (shell->x_dest_inc >> 1) - 1;
-  ty = PROJ_ROUND64 (y * shell->y_src_dec) + (shell->y_dest_inc >> 1) - 1;
+  tx = PROJ_ROUND64 (x * shell->x_src_dec);
+  ty = PROJ_ROUND64 (y * shell->y_src_dec);
 
   tx /= shell->x_dest_inc;
   ty /= shell->y_dest_inc;
@@ -223,8 +223,8 @@ gimp_display_shell_transform_xy_f  (Gimp
       gimp_item_offsets (item, &offset_x, &offset_y);
     }
 
-  *nx = shell->scale_x * (x + offset_x) - shell->offset_x;
-  *ny = shell->scale_y * (y + offset_y) - shell->offset_y;
+  *nx = SCALEX (shell, x + offset_x) - shell->offset_x;
+  *ny = SCALEY (shell, y + offset_y) - shell->offset_y;
 
   *nx += shell->disp_xoffset;
   *ny += shell->disp_yoffset;
@@ -314,10 +314,8 @@ gimp_display_shell_transform_points (Gim
       x = points[i*2]   + offset_x;
       y = points[i*2+1] + offset_y;
 
-      x = PROJ_ROUND64 (shell->x_src_dec * x +
-                        (shell->x_dest_inc >> 1) - 1) / shell->x_dest_inc;
-      y = PROJ_ROUND64 (shell->y_src_dec * y +
-                        (shell->y_dest_inc >> 1) - 1) / shell->y_dest_inc;
+      x = PROJ_ROUND64 (x * shell->x_src_dec) / shell->x_dest_inc;
+      y = PROJ_ROUND64 (y * shell->y_src_dec) / shell->y_dest_inc;
 
       coords[i].x = CLAMP (x + shell->disp_xoffset - shell->offset_x,
                            G_MININT, G_MAXINT);
@@ -366,10 +364,8 @@ gimp_display_shell_transform_coords (Gim
       x = image_coords[i].x + offset_x;
       y = image_coords[i].y + offset_y;
 
-      x = PROJ_ROUND64 (shell->x_src_dec * x +
-                        (shell->x_dest_inc >> 1) - 1) / shell->x_dest_inc;
-      y = PROJ_ROUND64 (shell->y_src_dec * y +
-                        (shell->y_dest_inc >> 1) - 1) / shell->y_dest_inc;
+      x = PROJ_ROUND64 (shell->x_src_dec * x) / shell->x_dest_inc;
+      y = PROJ_ROUND64 (shell->y_src_dec * y) / shell->y_dest_inc;
 
       disp_coords[i].x = CLAMP (x + shell->disp_xoffset - shell->offset_x,
                                 G_MININT, G_MAXINT);
@@ -421,14 +417,10 @@ gimp_display_shell_transform_segments (G
       y1 = src_segs[i].y1 + offset_y;
       y2 = src_segs[i].y2 + offset_y;
 
-      x1 = (x1 * shell->x_src_dec +
-            (shell->x_dest_inc >> 1) - 1) / shell->x_dest_inc;
-      x2 = (x2 * shell->x_src_dec +
-            (shell->x_dest_inc >> 1) - 1) / shell->x_dest_inc;
-      y1 = (y1 * shell->y_src_dec +
-            (shell->y_dest_inc >> 1) - 1) / shell->y_dest_inc;
-      y2 = (y2 * shell->y_src_dec +
-            (shell->y_dest_inc >> 1) - 1) / shell->y_dest_inc;
+      x1 = PROJ_ROUND64 (x1 * shell->x_src_dec) / shell->x_dest_inc;
+      x2 = PROJ_ROUND64 (x2 * shell->x_src_dec) / shell->x_dest_inc;
+      y1 = PROJ_ROUND64 (y1 * shell->y_src_dec) / shell->y_dest_inc;
+      y2 = PROJ_ROUND64 (y2 * shell->y_src_dec) / shell->y_dest_inc;
 
       dest_segs[i].x1 = CLAMP (x1 + shell->disp_xoffset - shell->offset_x,
                                G_MININT, G_MAXINT);
Index: app/display/gimpdisplayshell-selection.c
===================================================================
--- app/display/gimpdisplayshell-selection.c	(revision 24445)
+++ app/display/gimpdisplayshell-selection.c	(working copy)
@@ -740,6 +740,7 @@ selection_window_state_event (GtkWidget 
 
   return FALSE;
 }
+
 static gboolean
 selection_visibility_notify_event (GtkWidget          *shell,
                                    GdkEventVisibility *event,
Index: app/display/gimpdisplayshell-render.c
===================================================================
--- app/display/gimpdisplayshell-render.c	(revision 24445)
+++ app/display/gimpdisplayshell-render.c	(working copy)
@@ -591,13 +591,11 @@ gimp_display_shell_render_info_scale (Re
   info->scalex   = shell->scale_x * (1 << level);
   info->scaley   = shell->scale_y * (1 << level);
 
-  info->src_y    = (gdouble) info->y / info->scaley;
-
   /* use Bresenham like stepping */
   info->x_dest_inc = shell->x_dest_inc;
   info->x_src_dec  = shell->x_src_dec << level;
 
-  info->dx_start   = ((gint64) info->x_dest_inc) * info->x + info->x_dest_inc/2;
+  info->dx_start   = ((gint64) info->x_dest_inc) * info->x;
   info->src_x      = info->dx_start / info->x_src_dec;
   info->dx_start   = info->dx_start % info->x_src_dec;
 
@@ -605,7 +603,7 @@ gimp_display_shell_render_info_scale (Re
   info->y_dest_inc = shell->y_dest_inc;
   info->y_src_dec  = shell->y_src_dec << level;
 
-  info->dy_start   = ((gint64) info->y_dest_inc * info->y) + info->y_dest_inc/2;
+  info->dy_start   = ((gint64) info->y_dest_inc) * info->y;
   info->src_y      = info->dy_start / info->y_src_dec;
   info->dy_start   = info->dy_start % info->y_src_dec;