Advanced Retina/HiDPI: BitmapForCaching and ScaleFactorChanged

The most direct way to support HiDPI* for custom controls is to draw into the Graphics object passed into the Paint event. That graphics object is already configured with the appropriate scale factor and double buffering- the entire control will be handled correctly by the framework if the DoubleBuffer property is set.

*As with other posts, we’ll use “HiDPI” to refer to both HiDPI on Windows and Retina on OS X.

If drawing directly into the Graphics object passed into the control’€™s Paint event isn’€™t feasible or you need to manage your own caching of costly drawing for some reason, the BitmapForCaching function can be used to create a mutable bitmap that’s configured appropriately for using as a cache. This bitmap has some screen-specific properties, like the scale factor, and should be thrown away in the ScaleFactorChanged event. For example:

Class Foo

 Inherits Canvas

 Sub Paint(g As Graphics) Handles Event

  CreateCacheIfNeeded()

  g.DrawPicture(mCachedContent, 0, 0)

 End Sub
 Sub ScaleFactorChanged() Handles Event

  mCachedContent = Nil

 End Sub
 Private Sub CreateCacheIfNeeded()

  If mCachedContent <> Nil Then Exit Sub
  // Pretend this is a costly operation

  mCachedContent = Self.TrueWindow.BitmapForCaching(100, 100)

  Dim g As Graphics = mCachedContent.Graphics

  g.FillRect(0, 0, g.Width, g.Height)

 End Sub
 Private Dim mCachedContent As Picture

End Class