6.4.3 About refractions based on local cubemaps
Local cubemaps are an excellent technique for rendering reflections and developers have used static cubemaps to implement both reflections and refractions since they became available.
However, if you use static cubemaps to implement reflections or refractions in
a local environment, the results are incorrect if you do not apply a local
In the technique described here a local correction is applied to ensure
correct results. This technique is highly optimized. It is especially useful for mobile
devices where runtime resources are limited so must be carefully balanced.