What does it mean to “preconcat” a matrix in Android?

In reviewing: http://developer.android.com/reference/android/graphics/Canvas.html

I’m wondering

translate(): “preconcat the current matrix with the specified translation” — what does this mean?

I can’t find a good definition of “preconcat” anywhere on the internet! The only place I can find it is in the Android Source – I’m starting to wonder if they made it up? 🙂

I’m familiar with “concat” or concatenate, which is to append to, so what is a pre-concat?

Answer

When working with matrices, the word concatenation refers to multiplication.

Since matrix multiplication is not commutative, there is a separate word for backwards multiplication.
Pre-concatenating a to b means setting a = b × a. (As opposed to a = a × b, which will give a different matrix)