function uniform( u ) {
	if ( isnan( u ) ) {
		return NaN;
	}
	if ( u < 0.0 || u > 1.0 ) {
		return 0.0;
	}
	return 0.5;
}
Computes the uniform kernel.

@private
@param {number} u - input value
@returns {number} kernel value at u

@example
var u = 0.2;
var out = uniform( u );
// returns 0.5