Mastering OpenLayers 3

Blending layers

In the next three examples, we will fall back on the Canvas renderer. It is the most developed and stable one, providing a lot of perks by allowing canvas manipulation methods. The map and every layer is rendered on a different canvas, while in the end, they are aggregated into a single composition. This pattern, the existence of precompose, and the postcompose events (rendering hooks) enable us to manipulate the context of any layer or the map as a whole. We can basically use any canvas manipulation method as long as we can get the original context of the layers or map with these events.

In this example, called ch07_blend, we will discuss one of the most useful canvas manipulation methods, the globalCompositionContext. For this ...