fix #515 axonometric grid snapping w/ y-axis down

1 files changed, 12 insertions, 5 deletions

diff --git a/src/display/canvas-axonomgrid.cpp b/src/display/canvas-axonomgrid.cpp
index 4699f0509..aedf90481 100644
--- a/src/display/canvas-axonomgrid.cpp
+++ b/src/display/canvas-axonomgrid.cpp
@@ -613,6 +613,14 @@ CanvasAxonomGridSnapper::_getSnapLines(Geom::Point const &p) const
         return s;
     }
 
+    SPDesktop const *dt = _snapmanager->getDesktop();
+    double ta_x = grid->tan_angle[X];
+    double ta_z = grid->tan_angle[Z];
+
+    if (dt && dt->is_yaxisdown()) {
+        std::swap(ta_x, ta_z);
+    }
+
     double spacing_h;
     double spacing_v;
 
@@ -622,7 +630,6 @@ CanvasAxonomGridSnapper::_getSnapLines(Geom::Point const &p) const
         spacing_v = grid->lyw; // vertical
         // convert screen pixels to px
         // FIXME: after we switch to snapping dist in screen pixels, this will be unnecessary
-        SPDesktop const *dt = _snapmanager->getDesktop();
         if (dt) {
             spacing_h /= dt->current_zoom();
             spacing_v /= dt->current_zoom();
@@ -644,16 +651,16 @@ CanvasAxonomGridSnapper::_getSnapLines(Geom::Point const &p) const
     Geom::Coord x_min = Inkscape::Util::round_to_lower_multiple_plus(p[Geom::X], spacing_h, grid->origin[Geom::X]);
 
     // Calculate the y coordinate of the intersection of the angled grid lines with the y-axis
-    double y_proj_along_z = p[Geom::Y] - grid->tan_angle[Z]*(p[Geom::X] - grid->origin[Geom::X]);
-    double y_proj_along_x = p[Geom::Y] + grid->tan_angle[X]*(p[Geom::X] - grid->origin[Geom::X]);
+    double y_proj_along_z = p[Geom::Y] - ta_z * (p[Geom::X] - grid->origin[Geom::X]);
+    double y_proj_along_x = p[Geom::Y] + ta_x * (p[Geom::X] - grid->origin[Geom::X]);
     double y_proj_along_z_max = Inkscape::Util::round_to_upper_multiple_plus(y_proj_along_z, spacing_v, grid->origin[Geom::Y]);
     double y_proj_along_z_min = Inkscape::Util::round_to_lower_multiple_plus(y_proj_along_z, spacing_v, grid->origin[Geom::Y]);
     double y_proj_along_x_max = Inkscape::Util::round_to_upper_multiple_plus(y_proj_along_x, spacing_v, grid->origin[Geom::Y]);
     double y_proj_along_x_min = Inkscape::Util::round_to_lower_multiple_plus(y_proj_along_x, spacing_v, grid->origin[Geom::Y]);
 
     // Calculate the versor for the angled grid lines
-    Geom::Point vers_x = Geom::Point(1, -grid->tan_angle[X]);
-    Geom::Point vers_z = Geom::Point(1, grid->tan_angle[Z]);
+    Geom::Point vers_x = Geom::Point(1, -ta_x);
+    Geom::Point vers_z = Geom::Point(1, ta_z);
 
     // Calculate the normal for the angled grid lines
     Geom::Point norm_x = Geom::rot90(vers_x);