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 example, we modify our layer tree's createRegistry method, and add some blending options to every registered layer:
var layerTree = function (options) {
[…]
this.createRegistry = function (layer, buffer) {
[…]
layerControls...