BackdropFilter class

A widget that applies a filter to the existing painted content and then paints child.

The filter will be applied to all the area within its parent or ancestor widget's clip. If there's no clip, the filter will be applied to the full screen.

The results of the filter will be blended back into the background using the blendMode parameter. The only value for blendMode that is supported on all platforms is BlendMode.srcOver which works well for most scenes. But that value may produce surprising results when a parent of the BackdropFilter uses a temporary buffer, or save layer, as does an Opacity widget. In that situation, a value of BlendMode.src can produce more pleasing results, but at the cost of incompatibility with some platforms, most notably the html renderer for web applications.

Stack(
  fit: StackFit.expand,
  children: <Widget>[
    Text('0' * 10000),
    Center(
      child: ClipRect(  // <-- clips to the 200x200 [Container] below
        child: BackdropFilter(
          filter: ui.ImageFilter.blur(
            sigmaX: 5.0,
            sigmaY: 5.0,
          ),
          child: Container(
            alignment: Alignment.center,
            width: 200.0,
            height: 200.0,
            child: const Text('Hello World'),
          ),
        ),
      ),
    ),
  ],
)

This effect is relatively expensive, especially if the filter is non-local, such as a blur.

If all you want to do is apply an ImageFilter to a single widget (as opposed to applying the filter to everything beneath a widget), use ImageFiltered instead. For that scenario, ImageFiltered is both easier to use and less expensive than BackdropFilter.

 Widget buildBackdrop() {
   return Stack(
     children: <Widget>[
       Positioned.fill(child: Image.asset('image.png')),
       Positioned.fill(
         child: BackdropFilter(
           filter: ui.ImageFilter.blur(sigmaX: 6, sigmaY: 6),
         ),
       ),
     ],
   );
 }

 Widget buildFilter() {
   return ImageFiltered(
     imageFilter: ui.ImageFilter.blur(sigmaX: 6, sigmaY: 6),
     child: Image.asset('image.png'),
   );
 }

See also:

• ImageFiltered, which applies an ImageFilter to its child.
• DecoratedBox, which draws a background under (or over) a widget.
• Opacity, which changes the opacity of the widget itself.
• https://flutter.dev/go/ios-platformview-backdrop-filter-blur for details and restrictions when an iOS PlatformView needs to be blurred.