Use column vector length to extract scale from matrices.

Closes #298.

canvas/src/lib.rs

@@ -33,6 +33,7 @@ use pathfinder_renderer::scene::{ClipPath, ClipPathId, DrawPath, RenderTarget, S
 use std::borrow::Cow;
 use std::default::Default;
 use std::f32::consts::PI;
+use std::f32;
 use std::fmt::{Debug, Error as FmtError, Formatter};
 use std::mem;
 use std::sync::Arc;
@@ -274,8 +275,9 @@ impl CanvasRenderingContext2D {
 
         // The smaller scale is relevant here, as we multiply by it and want to ensure it is always
         // bigger than `HAIRLINE_STROKE_WIDTH`.
-        let transform_scale = f32::min(self.current_state.transform.m11(),
-                                       self.current_state.transform.m22());
+        let transform_scales = self.current_state.transform.extract_scale();
+        let transform_scale = f32::min(transform_scales.x(), transform_scales.y());
+
         // Avoid the division in the normal case of sufficient thickness.
         if stroke_style.line_width * transform_scale < HAIRLINE_STROKE_WIDTH {
             stroke_style.line_width = HAIRLINE_STROKE_WIDTH / transform_scale;

geometry/src/transform2d.rs

@@ -76,6 +76,13 @@ impl Matrix2x2F {
         Matrix2x2F(self.0 * F32x4::splat(factor))
     }
 
+    /// Extracts the scale from this matrix.
+    #[inline]
+    pub fn extract_scale(&self) -> Vector2F {
+        let squared = self.0 * self.0;
+        Vector2F((squared.xy() + squared.zw()).sqrt())
+    }
+
     #[inline]
     pub fn m11(&self) -> f32 {
         self.0[0]
@@ -212,6 +219,12 @@ impl Transform2F {
         *self == Transform2F::default()
     }
 
+    /// Extracts the scale from this matrix.
+    #[inline]
+    pub fn extract_scale(&self) -> Vector2F {
+        self.matrix.extract_scale()
+    }
+
     #[inline]
     pub fn m11(&self) -> f32 {
         self.matrix.m11()