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.