diff --git a/SketchWidget.py b/SketchWidget.py
index 1e738e6..22ee33f 100644
--- a/SketchWidget.py
+++ b/SketchWidget.py
@@ -84,7 +84,7 @@ class SketchWidget:
         glViewport(0, 0, width, height)
         glMatrixMode(GL_PROJECTION)
         glLoadIdentity()
-        glOrtho(0, width, 0, height, 1.0, -1.0)
+        glOrtho(0, width - 1, 0, height - 1, 1.0, -1.0)
 
         glMatrixMode(GL_MODELVIEW)
         glLoadIdentity()