Error Message “Xcode alone is not sufficient on Sierra”

Question

I\'d like to install openCV to vectorize image, but there\'s a series error message regarding Xcode and Ruby.

First, I use terminal to install openCV, brew install

Answer 1

Let me explain this myself so people won't make the same mistakes.

When I saw the last line of the error message

Error: Xcode alone is not sufficient on Sierra.
Install the Command Line Tools:
xcode-select --install 

My thought was: I already have Xcode why the system ask me to "reinstall" it. However, thanks for @SamiKuhmonen @ Beartech @patrick kuang suggestion, I search a page (in Mandarin). xcode-select --install does not reinstall the whole Xcode. It means install some missing command line tools which is required by installing Ruby.