I notice lots of sentences use if directly followed by an adjective or an adjective phrase, for example:
If necessary, please import more of these types of goods.
If a little taller, he could reach to the book on the bookshelf.
This appears to be one of the places where English lets you omit words and consider them to be "understood". However, all grammars about if I googled are talking about conditional clauses; none is talking about if + phrase.
Can anyone explain the general rules of if + adjective or adjective phrases?