Matrix3 and Matrix4 inconsistencies

Questions
1

When converting Matrix3 to Matrix4 I’m seeing inconsistencies that I wouldn’t expect. For example, converting a Matrix3 to a Matrix4 gives unexpected results, and is different to a Matrix4 created with the same transforms.

I’ve created a codepen at https://codepen.io/grokys/pen/eYVLBbK to demonstrate the difference (open the console at the bottom to see the output), but basically these two snippets produce different output:

var m3 = new THREE.Matrix3()
m3.scale(4, 5)
m3.translate(12, 14)
var m3to4 = new THREE.Matrix4().setFromMatrix3(m3)

// Result: 4,0,0,0,0,5,0,0,12,14,1,0,0,0,0,1

var m4 = new THREE.Matrix4()
m4.makeScale(4, 5, 1)
m4.multiply(new THREE.Matrix4().makeTranslation(12, 14, 0))

// Result: 4,0,0,0,0,5,0,0,0,0,1,0,48,70,0,1

Things to note:

  • In the Matrix4 the translation is multiplied by the scale, in the Matrix3 it’s not (Matrix3 is using pre-multiplication for translate? This is unexpected because multiply is specified as using post-multiplcation so I’d have thought this to be the default)
  • The translation values appear in different places
  • Decomposing the 3x3->4x4 matrix gives strange results

What I think is happening:

  • even though multiply on Matrix3 specifies post-multiplication, translate (and possibly scale and rotate) use pre-multiplication
  • converting a Matrix3 to a Matrix4 doesn’t work as I’d expect: it just does a row-wise copy, meaning that what should appear in row 4 stays in row3

Is this intentional? Is converting a 3x3 matrix to a 4x4 matrix not supposed to be done like this?

2

If I could start from scratch I would not implement a translate() method in Matrix3 at all since it only creates confusion.

You need a 4x4 affine transformation matrix to properly process translation, rotation and scale in 3D space. And only then a translate() method makes sense, IMO.

A 3x3 matrix can hold scaling but essentially you use it as an alternative format for representing rotation in 3D space (next to quaternions, euler angles or axis-angle). Translating quaternions or euler angles does also not make sense…

3

Well, I am using Matrix3 for 2D transforms. To give some background, the reason I was originally converting to a Matrix4 is that Matrix3 has no decompose so I figured I could convert from 3x3 to 4x4 and then decompose, which is when I noticed this.