The style property of a WebGL tile layer accepts a color expression that
can be used to modify pixel values before rendering. Here, RGB tiles representing elevation
data are loaded and rendered so that values at or below sea level are blue, and values
above sea level are transparent. The color expression operates on normalized pixel
values ranging from 0 to 1. The band operator is used to select normalized values
from a single band.
After converting the normalized RGB values to elevation, the case expression
is used to pick colors to apply at a given elevation. Instead of using constant
numeric values as the stops in the colors array, the var operator allows you to
use a value that can be modified by your application. When the user drags the
sea level slider, the layer.updateStyleVariables() function is called to update
the level style variable with the value from the slider.