What does it mean to "preconcat" a matrix in Android?

Question

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?

Solution

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)

Comments

[Brad Hein]

Thank you. My question was in the context of the Android Canvas however and I'm having a hard time making the connection between the canvas and the matrix?

[SLaks]

@Brad: The canvas maintains a transform matrix; translate manipulates it.

[Brad Hein]

The canvas maintains a matrix and the canvas is manipulated by methods which are well defined in OpenGL. A great source of relevant definitions is: gpwiki.org/index.php/OpenGL:Tutorials:Theory