f 0 for <em class="parameter"><code>index</code></em> selects the first
	active attribute variable. Permissible values for
	<em class="parameter"><code>index</code></em> range from 0 to the number of
	active attribute variables minus 1.</p><p>Attribute variables have arbitrary names and obtain their values
	through numbered generic vertex attributes. An attribute
	variable is considered active
	if it is determined during the link operation that it may be
	accessed during program execution. Therefore,
	<em c