#### Layer spy example

View a portion of one layer over another

Layer rendering can be manipulated in `precompose` and `postcompose` event listeners. These listeners get an event with a reference to the Canvas rendering context. In this example, the `precompose` listener sets a clipping mask around the most recent mouse position, giving you a spyglass effect for viewing one layer over another.

Move around the map to see the effect. Use the ↑ up and ↓ down arrow keys to adjust the spyglass size.

See the layer-spy.js source to see how this is done.

spy, image manipulation