Glimmr Canvas-Based Drawing
version 2/101030 by Erik Temple
Section: Using canvases to manage display
At first glance, the canvas may almost look unnecessary: elements will be displayed in a graphics window, so why not assign the element directly to the window? Why do we need the canvas to mediate?
There are really two answers. First, canvases provide an easy way to change the display content of a graphics window at a stroke. We can, for example, set up two canvases with different elements. We first show Canvas A in our graphics window, and then, when we want to change the content of the window, we simply change the associated canvas of the window to Canvas B. The next time the window refreshes, it will display our second composition. Canvases can thus be used as discrete "pages" to be shown in a graphics window. See the "Two Canvases, One Window" example.
We can also display the same canvas in two or more windows at the same time, using different display parameters. So, if we have a detailed map that would benefit from being seen both in close-up and at long-range, we could show the map at one scale in one window, and at another in the other. See the "One Canvas, Two Windows" example.