iosswiftscenekitcmmotionmanager

Questions about how scene kit translations work, and accelerometer

  1. I have done a tiny bit of 3d graphics in the past. When you move or rotate a Scene Kit sprite does it automatically update its translation matrix, or do you have to make it yourself?
  2. Are "position" and "eulerAngles" both properties that are... absolute.
    • For example if I am in sprite kit and set the translation to (1, 0) it will be at that point relative to the origin.
    • And if I set the z rotation to 90 it will be rotated 90 degrees.
    • And if I incrament the translation (with +=) x it will start going in a line.
    • And same for zRotation if incremented it will rotate. In scene kit if I do similar things to the translation and euler angle values will they do the same thing?
  3. Also what exactly does the accelerometer think its measuring, it is like the amount of motion in a certain period? So basically is it the delta between the two simultaneous points that the device was in.

Yes, this question is definitely broad, however they are much better placed here, then scattered in three tiny posts.

Solution

  • Doe, let me see if I can help

    1. Translation matrix? It has a TRANSFORM matrix that includes translation, scale and rotation, and yes, it is automatically updated when you change one of these 3, and vice-versa.

    2. If I understood well, yes, just like in SpriteKit. They are related to their parent coordinates. The position (1,0,0) would mean the Node (its center, unless you change its pivot (anchorPoint in spriteKit)) will be at distance 1 along the X axis of its parent from its parent origin). The same works for the rotation, if a NodeA has 30 degrees rotation at axis X and you add a NodeB with 20 degrees rotation at X in NodeA, you would have the NodeA having visually a 50 degrees rotation at X.

    3. Accelerometer measures the acceleration forces given to the device in a specific moment, in the three axis of the device. Its unit is not [m^2/s] but [Gravity/s] (would be approximately [10m^2/s]). An important detail is that this measure includes the gravity acceleration as well. So, if you try to measure the acceleration with the device standing ortogonal to the ground, you would expect (0, 0, -1) (or 0,0,1, if upside down). Lying down the device on the ground it would be (0, 1or-1, 0) (depending if the screen is facing the ground or the ceiling)

    For for every tick (of update rate of the accelerometer) it calculates what was the acceleration imposed to the device at that moment. That's not the delta itself, but it can be easily calculated if you store the values.