for snapping to grids and guides, reduce numerical error for the case of uniform scaling of viewbox

(bzr r13881)

3 files changed, 19 insertions, 2 deletions

diff --git a/src/display/canvas-axonomgrid.cpp b/src/display/canvas-axonomgrid.cpp
index 05ba71a49..ac93f5c88 100644
--- a/src/display/canvas-axonomgrid.cpp
+++ b/src/display/canvas-axonomgrid.cpp
@@ -220,6 +220,11 @@ CanvasAxonomGrid::readRepr()
     if( root->viewBox_set ) {
         scale_x = root->width.computed  / root->viewBox.width();
         scale_y = root->height.computed / root->viewBox.height();
+        if (Geom::are_near(scale_x / scale_y, 1.0, Geom::EPSILON)) {
+            // scaling is uniform, try to reduce numerical error
+            scale_x = (scale_x + scale_y)/2.0;
+            scale_y = scale_x;
+        }
     }
 
     gchar const *value;
diff --git a/src/display/canvas-grid.cpp b/src/display/canvas-grid.cpp
index 1a284b46e..0a43ed8b7 100644
--- a/src/display/canvas-grid.cpp
+++ b/src/display/canvas-grid.cpp
@@ -544,6 +544,11 @@ CanvasXYGrid::readRepr()
     if( root->viewBox_set ) {
         scale_x = root->width.computed  / root->viewBox.width();
         scale_y = root->height.computed / root->viewBox.height();
+        if (Geom::are_near(scale_x / scale_y, 1.0, Geom::EPSILON)) {
+            // scaling is uniform, try to reduce numerical error
+            scale_x = (scale_x + scale_y)/2.0;
+            scale_y = scale_x;
+        }
     }
 
     gchar const *value;
diff --git a/src/sp-guide.cpp b/src/sp-guide.cpp
index 890c1f597..08b055508 100644
--- a/src/sp-guide.cpp
+++ b/src/sp-guide.cpp
@@ -156,8 +156,15 @@ void SPGuide::set(unsigned int key, const gchar *value) {
                 // If root viewBox set, interpret guides in terms of viewBox (90/96)
                 SPRoot *root = document->getRoot();
                 if( root->viewBox_set ) {
-                    newx = newx * root->width.computed  / root->viewBox.width();
-                    newy = newy * root->height.computed / root->viewBox.height();
+                    if(Geom::are_near((root->width.computed * root->viewBox.height()) / (root->viewBox.width() * root->height.computed), 1.0, Geom::EPSILON)) {
+                        // for uniform scaling, try to reduce numerical error
+                        double vbunit2px = (root->width.computed / root->viewBox.width() + root->height.computed / root->viewBox.height())/2.0;
+                        newx = newx * vbunit2px;
+                        newy = newy * vbunit2px;
+                    } else {
+                        newx = newx * root->width.computed  / root->viewBox.width();
+                        newy = newy * root->height.computed / root->viewBox.height();
+                    }
                 }
                 this->point_on_line = Geom::Point(newx, newy);
             } else if (success == 1) {