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 thatâs 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:
Sub Paint(g As Graphics) Handles Event
g.DrawPicture(mCachedContent, 0, 0)
Sub ScaleFactorChanged() Handles Event
mCachedContent = Nil
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)
Private Dim mCachedContent As Picture