The preview canvas is WPF visuals; a blur layer contributes no visual of its own, so until now
the creator saw nothing where the blur was and could not tell what it was hiding.
Each visible blur now draws a rectangle (or ellipse) filled with a VisualBrush of the whole
BackdropLayer — the desktop capture plus the background art — sampled at the blur's own rect and
carrying a BlurEffect at the same strength the compositor uses. The host canvas sits after the
backdrop and before the element layers, which is the model's own z-rule, so the preview shows
exactly what the output will: desktop blurred, brand and everything above it untouched.
Rebuilt on scene change, on add/remove, and on any blur property change (position, size, shape,
strength) — the brush samples the live backdrop, so the picture keeps moving underneath.
gramps
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