Overlay simple polygons on a rastermap instance
mapOverlay.init takes a parameters object with the following properties:
- canvas: an HTML canvas element
- map: a rastermap instance (or equivalent). MUST have a method named xyToMapPixels, for converting x/y coordinates in the map's projection to pixel indices from the top left corner of the map
- width, height: the pixel dimensions of the rastermap drawingbuffer
Initialization returns an object with the following methods:
- draw(polygon, mapChanged): Draws polygon on the overlay canvas.
- polygong must be an array of points, where each point is a two-element array containing floating point numbers.
- mapChanged is a flag indicating whether the underlying map coordinates have changed since the last draw call.
- reset(): Clears the canvas and the stored polygon
Note that the .draw method will do nothing if all of the following conditions are true:
- polygon is the same as the last call
- overlay canvas has not changed size
- mapChanged is false
If the supplied polygon contains exactly two points, they will be treated as the top left and bottom right corners of a rectangle, and drawn using the Canvas2D strokeRect method.