Result difference between android.opengl.Matrix & com.google.android.filament.utils.Matrix

[Qheb]

Hello,

While struggling with some transformations in Filament, I tried the following code :

val mat4 = translation(Float3(1f, 2f, 3f))
Log.i("mat4", mat4.toFloatArray().contentToString())

val matrix = FloatArray(16)
Matrix.setIdentityM(matrix, 0)
Matrix.translateM(matrix, 0, 1f, 2f, 3f)
Log.i("matrix", matrix.contentToString())

Since, from what I understand, both APIs are handling matrices in column-major order, I would have expected the same result for mat4 and matrix. But it turns out it's not :

mat4      [1.0, 0.0, 0.0, 1.0, 0.0, 1.0, 0.0, 2.0, 0.0, 0.0, 1.0, 3.0, 0.0, 0.0, 0.0, 1.0]
matrix    [1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 1.0, 2.0, 3.0, 1.0]

Could someone please indicate me what I am missing ? :)
Thanks !

[romainguy]

Both matrices are in column-major order in how they store the data. However when calling toFloatArray(), Filament's Mat4 (which is from kotlin-math) returns data in a row-major format.

[Qheb]

Oh, thanks @romainguy , that explains quite a lot.

Is this the same for the Mat4.of() constructor method, does it take a row-major array as input ?

This means that there is an error in the article I used for getting started with Filament 😬

private fun Int.getTransform(): Mat4 {
   val tm = modelViewer.engine.transformManager
   return Mat4.of(*tm.getTransform(tm.getInstance(this), null))
}

private fun Int.setTransform(mat: Mat4) {
   val tm = modelViewer.engine.transformManager
   tm.setTransform(tm.getInstance(this), mat.toFloatArray())
}

[romainguy]

of() is row-major as well so you can write the matrix the "natural" way in your code:

of(
  column1_x, column2_x
  column1_y, column2_x
)

And yes, the article is incorrect. We could also change toFloatArray() to return column-major… Or provide a second variant for that.

[Qheb]

Ok, thanks for the clarifications !

In my opinion, yes a second variant might help understanding the specific aspects of the methods.
Even also another constructor method, for example fromArray(columnMajorArray: FloatArray), could indicate that of() is also row-major (even if after seeing the example of writing a matrix the "natural" way, it makes sense 🙂 ).