The <basic-shape> type can be specified using basic shape functions. When using this syntax to define shapes, the reference box is defined by each property that uses <basic-shape> values. The coordinate system for the shape has its origin on the top-left corner of the reference box with the x-axis running to the right and the y-axis running downwards. All the lengths expressed in percentages are resolved from the used dimensions of the reference box.
The shape-image-thresholdCSS property defines the alpha channel threshold used to extract the shape using an image as the value for shape-outside. A value of 0.5 means that the shape will enclose all the pixels that are more than 50% opaque.