There are two types of clipping you probably try to avoid introducing into your images during post-production: luminosity clipping (when the brightest areas of an image become white, or when the darkest areas become black), and channel clipping (when the data within an individual channel becomes compromised). Both forms – unless you've made a deliberate decision to clip your data – are something to avoid.
As you'll probably know, Photoshop doesn't make this easy. The only tool that provides direct feedback on any clipping within an image is the histogram, but it isn't completely accurate, nor is it dynamic: it updates after you implement a change rather that providing feedback during an adjustment. And while there are quite a few ways that allow you to accurately measure clipping – more of which below – none of the methods are either especially convenient or automatic.
Fortunately, there is a solution – a way to add a set of warnings that will provide real-time feedback on any shadow, highlight, and channel highlight clipping within an image – and I'll show you how to do so in the remainder of this article.
[watch the videos after the jump]