Use a widget to mask and blend another widget, for example to imprint text onto surfaces.

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Widgets that are used as children of WidgetMask or SaveLayer must not need compositing, or contain widgets which need compositing, such as RepaintBoundary.

This is because this package makes use of save layers, which cannot encompass compositing layers.

A mask is an image, which is positioned in front of another image and affects this images in some way, where it is not empty.

Blending is a process which takes two images and produces a new image by applying a mathematical function to each pair of pixels from the input images. For some of these functions, the order of the arguments matters. That's why the input images are labeled with src and dst. A mask is usually the image labeled with src.

The example below paints some text onto an image. The text is filled with the negative of the image.

WidgetMask(
  // `BlendMode.difference` results in the negative of `dst` where `src`
  // is fully white. That is why the text is white.
  blendMode: BlendMode.difference,
  mask: Center(
    child: Text(
      'Negative',
      style: TextStyle(
        fontSize: 50,
        color: Colors.white,
      ),
    ),
  ),
  child: Image.asset('images/my_image.jpg'),
);

WidgetMask delegates to child to lay itself out. It always has the same size as child. mask is forced to adopt the same size as child and positioned on top it.

During hit testing mask is positioned over child.

The different BlendModes use src and dst to describe how the colors of two images are blended with each other. In the context of WidgetMask mask is the src and child the dst.

The NegativeMaskedImageDemo widget, in the example app, implements the image at the top.

This widget paints its child into a save layer and allows you to fully specify the Paint to use with the save layer. If you need to use a custom Paint or require a different layout of the widgets, you can use SaveLayer.

WidgetMask is implement using two SaveLayers to blend the mask and child:

@override
Widget build(BuildContext context) {
  Widget child = Stack(
    textDirection: TextDirection.ltr,
    fit: StackFit.passthrough,
    children: [
      this.child,
      Positioned.fill(
        child: SaveLayer(
          paint: Paint()..blendMode = blendMode,
          child: mask,
        ),
      ),
    ],
  );

  if (childSaveLayer) {
    child = SaveLayer(
      child: child,
    );
  }

  return child;
}

Gabriel Terwesten • GitHub @blaugold • Twitter @GTerwesten • Medium @gabriel.terwesten