An OpenSeadragon plugin that adds canvas overlay capability.

Compatible with OpenSeadragon 2.2.0 or greater.

To use, include the openseadragon-canvas-overlay.js file after openseadragon.js on your web page.

To add canvas overlay capability to your OpenSeadragon Viewer, call canvasOverlay(options) on it. It has two parameters: onRedraw - callback function that does the actual drawing, and clearBeforeRedraw (default as true): clear canvas before redrawing.

For example:

var overlay = this.viewer.canvasOverlay({
    onRedraw:function() {      
        overlay.context2d().fillStyle = "red";
        overlay.context2d().fillRect(0, 0, 500, 500);            
    },
    clearBeforeRedraw:true
});

This will return a new object with the following methods:

• canvas(): Returns canvas element.
• resize(): If your viewer changes size, you'll need to resize the canvas overlay by calling this method.
• clear(): Clears canvas.
• context2d(): Gives access to 2d context of canvas, to draw on it.

See online demo or demo.html for an example of it in use.