macOS: Use CGContextGetUserSpaceToDeviceSpaceTransform to get backing scale factor of graphics context to avoid unnecessary upscaling on retina displays on macOS 10.14+

modules/juce_graphics/native/juce_mac_CoreGraphicsContext.h

@@ -23,7 +23,7 @@ namespace juce
 class CoreGraphicsContext   : public LowLevelGraphicsContext
 {
 public:
-    CoreGraphicsContext (CGContextRef context, float flipHeight, float targetScale);
+    CoreGraphicsContext (CGContextRef context, float flipHeight);
     ~CoreGraphicsContext() override;
 
     //==============================================================================
@@ -69,7 +69,6 @@ public:
 private:
     CGContextRef context;
     const CGFloat flipHeight;
-    float targetScale;
     CGColorSpaceRef rgbColourSpace, greyColourSpace;
     mutable Rectangle<int> lastClipRect;
     mutable bool lastClipRectIsValid = false;

modules/juce_graphics/native/juce_mac_CoreGraphicsContext.mm

@@ -62,7 +62,7 @@ public:
     {
         freeCachedImageRef();
         sendDataChangeMessage();
-        return std::make_unique<CoreGraphicsContext> (context, height, 1.0f);
+        return std::make_unique<CoreGraphicsContext> (context, height);
     }
 
     void initialiseBitmapData (Image::BitmapData& bitmap, int x, int y, Image::BitmapData::ReadWriteMode mode) override
@@ -187,10 +187,9 @@ ImagePixelData::Ptr NativeImageType::create (Image::PixelFormat format, int widt
 }
 
 //==============================================================================
-CoreGraphicsContext::CoreGraphicsContext (CGContextRef c, float h, float scale)
+CoreGraphicsContext::CoreGraphicsContext (CGContextRef c, float h)
     : context (c),
       flipHeight (h),
-      targetScale (scale),
       state (new SavedState())
 {
     CGContextRetain (context);
@@ -238,14 +237,14 @@ void CoreGraphicsContext::addTransform (const AffineTransform& transform)
     lastClipRectIsValid = false;
 
     jassert (getPhysicalPixelScaleFactor() > 0.0f);
-    jassert (getPhysicalPixelScaleFactor() > 0.0f);
 }
 
 float CoreGraphicsContext::getPhysicalPixelScaleFactor()
 {
-    auto t = CGContextGetCTM (context);
+    auto t = CGContextGetUserSpaceToDeviceSpaceTransform (context);
+    auto determinant = (t.a * t.d) - (t.c * t.b);
 
-    return targetScale * (float) (juce_hypot (t.a, t.c) + juce_hypot (t.b, t.d)) / 2.0f;
+    return (float) std::sqrt (std::abs (determinant));
 }
 
 bool CoreGraphicsContext::clipToRectangle (const Rectangle<int>& r)

modules/juce_gui_basics/native/juce_ios_UIViewComponentPeer.mm

@@ -1087,7 +1087,7 @@ void UIViewComponentPeer::drawRect (CGRect r)
 
     // NB the CTM on iOS already includes a factor for the display scale, so
     // we'll tell the context that the scale is 1.0 to avoid it using it twice
-    CoreGraphicsContext g (cg, getComponent().getHeight(), 1.0f);
+    CoreGraphicsContext g (cg, getComponent().getHeight());
 
     insideDrawRect = true;
     handlePaint (g);

modules/juce_gui_basics/native/juce_mac_NSViewComponentPeer.mm

@@ -873,7 +873,7 @@ public:
        #if USE_COREGRAPHICS_RENDERING
         if (usingCoreGraphics)
         {
-            CoreGraphicsContext context (cg, (float) [view frame].size.height, displayScale);
+            CoreGraphicsContext context (cg, (float) [view frame].size.height);
             invokePaint (context);
         }
         else