Combine matrices

[raphaelmenges]

Hello 👋,

I looked through the documentation and search but could not find a good example how to combine multiple matrices or assign or replace a whole block of an existing matrix in nalgebra. E.g., I have a rotation matrix (3x3) and a translation vector (3x1) and I want to put them together as transformation matrix (3x4). What is the advisable way to achieve this?

[Andlon]

Hey @raphaelmenges. nalgebra 0.33 was released a few days ago, and it includes the new stack! macro, which hopefully offers you a convenient way to solve your problem.

[raphaelmenges]

Great, thanks for the reply! I am now actually trying to compose a 4x4 transformation matrix with homogeneous coordinates. I found two ways:

stack![rotation_and_scale, translation;
        matrix![0.,0.,0.], matrix![1.]]

or

stack![stack![rotation_and_scale, translation];
        matrix![0.,0.,0.,1.]]

Sadly, I cannot:

stack![rotation_and_scale, translation;
        matrix![0.,0.,0.,1.]]

Which way to prefer? I guess the second approach makes additional copies in-between 🤔

[Andlon]

I'd probably do something like this, personally:

let one = matrix![ 1.0 ];
let matrix = stack![ rotation_and_scale, translation;
                                      0,         one ];

This uses a nifty feature of stack!, in which 0 literals are interpreted as a zero matrix of the appropriate size (see the docs for more info).