What does the different columns in transform in AR

问题:

An ARAnchor has 6 columns of which the last 3 represent the x, y, and z coordinates.

I was wondering what the other (first) 3 columns represent?

回答1:

If you're new to 3D then these transformation matrices will seem like magic. Basically, every "point" in ARKit space is represented by a 4x4 transform matrix. This matrix describes the distance from the ARKit origin (the point at which ARKit woke up to the world), commonly known as the translation, and the orientation of the device, aka pitch, roll, and yaw. A transform matrix can also describe scale, although typically you won't deal with scale until you render something.

What do the columns mean? That gets complicated but just remember that the first 3 elements of the 4th column are the x,y,z translation. That will come in handy. The rest holds scale and rotation information.

回答2:

ARKit and SceneKit frameworks use 4x4 Transformation Matrices to translate, rotate, scale and shear 3D objects. Let's see how these matrices look like.

In 3D Graphics we often use a 4x4 Matrix with 16 useful elements. The Identity 4x4 Matrix is as following:

Between those sixteen elements there are 6 different shearing coefficients:

shear XY
shear XZ
shear YX
shear YZ
shear ZX
shear ZY

In Shear Matrix they are as followings:

Because there are no Rotation coefficients at all in this Matrix, six Shear coefficients along with three Scale coefficients allow you rotate 3D objects about X, Y, and Z axis using magical trigonometry (sin and cos).

Here's an example how to rotate 3D object (CCW) about its Z axis using Shear and Scale elements:

Look at 3 different Rotation patterns using Shear and Scale elements:

And, of course, 3 elements for translation (tx,ty,tz) in the 4x4 Matrix are located on the last column:

 ┌               ┐
 |  1  0  0  tx  |
 |  0  1  0  ty  |
 |  0  0  1  tz  |
 |  0  0  0  1   |
 └               ┘

The columns' indices in ARKit and SceneKit are: 0, 1, 2 and 3.

Hope this helps.