Question

I’ve been playing around with an accelerometer with 3 axis: X, Y and Z. It says on the supplier’s site that it measures gravitational force.

I’m sending this data to the blender games engine where I am rotating a cube in real time depending on the data values coming from the accelerometer. However the values coming through don’t seem to match up.

On each axis the accelerometer spits out values from -700 to 700 on each axis and I need to convert these values to something I can use in Blender. My maths knowledge is not up to scratch so I don’t know where to start with this one.

If anybody could shed some light on this, that would be great.

Many thanks
Will

EDIT
Currently I’m using a bit of python code to convert the rotation values to a matrix:

def reorient(alpha, beta, gamma):
  a = math.cos(alpha) 
  b = math.sin(alpha) 
  c = math.cos(beta) 
  d = math.sin(beta) 
  e = math.cos(gamma) 
  f = math.sin(gamma)    
  ad = a*d 
  bd = b*d 
  matrix = [[c*e, -a*f+b*d*e, b*f+a*d*e], [c*f, a*e+b*d*f, -b*e+a*d*f], [-d, b*c, a*c]]
  return matrix 

I am then using setOrientation(matrix) to affect the rotation of the cube. However I am currently throwing the wrong values into the matrix reorient() function

Answer 1

I guess you are using the measured acceleration to find the direction of gravitational pull (ie, down). If you are moving the accelerometer, apart from just turning it, there will be some additional force; think of the accelerometer having a pendulum weight handing from it, as you move it the pendulum sways (although in this case it would be a very short, fast-reacting pendulum?). You could try doing some sort of movement compensation, but it might be simpler to just try to keep the sensor in a fixed location.

Edit: ok, it looks like I totally misread the question – you want to know how to do the rotation in a script?

It looks like each Blender object has three properties (.RotX, .RotY, .RotZ) which contain the current values (in radians) and a method (.rot(new_rotx, new_roty, new_rotz)) which performs a rotation (see documentation at http://www.blender.org/documentation/249PythonDoc/Object.Object-class.html). I am currently looking at how the rotations are applied; more shortly.

Edit2: it looks like the angles are specified as Euler angles (http://en.wikipedia.org/wiki/Euler_angles); they give some conversion matrices. It also looks like your accelerometer data is underconstrained (you need one more constraint, specifying rotation about the ‘down’ direction – maybe some sort of inertial ‘least distance from previous position’ calculation?)

Edit3: there is a sample script which may be helpful; on my machine it is at C:\Users\Me\AppData\Roaming\Blender Foundation\Blender.blender\scripts\object_random_loc_sz_rot.py It shows how to get the currently selected object and tweak its rotation. Hope that helps!

Edit4: for sake of discussion, here is some sample code; it may be a bit redundant (I haven’t worked in Blender before) and it doesn’t solve the problem, but it will at least give us a common basis for further discussion 😉

#!BPY

"""
Name: 'Set rotation by accelerometer'
Blender: 249
Group: 'Object'
Tooltip: 'Set the selected objects rotation by accelerometer'
"""

__bpydoc__=\
'''
This script sets the selected objects rotation by accelerometer.
'''

from Blender import Draw, Scene
import math

def reorient(alpha, beta, gamma):
    a = math.cos(alpha)
    b = math.sin(alpha)
    c = math.cos(beta)
    d = math.sin(beta)
    e = math.cos(gamma)
    f = math.sin(gamma)

    ad = a*d
    bd = b*d

    return = [
        [c*e, -a*f+b*d*e, b*f+a*d*e],
        [c*f, a*e+b*d*f, -b*e+a*d*f],
        [-d,  b*c,        a*c      ]
    ]

def getAccel():
    # test stub -
    # need to get actual values from accelerometer here
    dx = -700
    dy = 100
    dz = 250
    return (dx,dy,dz)

def normalize(vec):
    "Return scaled unit vector"
    x,y,z = vec
    mag = (x*x + y*y + z*z)**0.5
    return (x/mag, y/mag, z/mag)

def main():
    scn = Scene.GetCurrent()
    try:
        obj = scn.objects.context
        euler = (obj.RotX, obj.RotY, obj.RotZ)
    except AttributeError:
        return

    down = normalize(getAccel())
    matrix = None
    # do something here to find new rotation-matrix
    #   based on euler and down
    # then

    if matrix:
        obj.setOrientation(matrix)
    else:
        # test value:
        # if reorient() is working properly, the
        # object's rotation should not change!
        obj.setOrientation(reorient(*euler))

if __name__=="__main__":
    main()