Get orientation device in the iPhone for Opengl Es - iphone

Question

I'm trying to convert the geomagnetic and accelerometer to rotate the camera in opengl ES1, I found some code from android and changed this code for iPhone, actually it is working more or less, but there are some mistakes, I´m not able to find this mistake, I put the code, also the call to Opengl Es1: glLoadMatrixf((GLfloat*)matrix);
- (void) GetAccelerometerMatrix:(GLfloat *) matrix headingX: (float)hx headingY:(float)hy headingZ:(float)hz;
{
_geomagnetic[0] = hx * (FILTERINGFACTOR-0.05) + _geomagnetic[0] * (1.0 - FILTERINGFACTOR-0.5)+ _geomagnetic[3] * (0.55);
_geomagnetic[1] = hy * (FILTERINGFACTOR-0.05) + _geomagnetic[1] * (1.0 - FILTERINGFACTOR-0.5)+ _geomagnetic[4] * (0.55);
_geomagnetic[2] = hz * (FILTERINGFACTOR-0.05) + _geomagnetic[2] * (1.0 - FILTERINGFACTOR-0.5)+ _geomagnetic[5] * (0.55);
_geomagnetic[3]=_geomagnetic[0] ;
_geomagnetic[4]=_geomagnetic[1];
_geomagnetic[5]=_geomagnetic[2];
//Clear matrix to be used to rotate from the current referential to one based on the gravity vector
bzero(matrix, sizeof(matrix));
//MAGNETIC
float Ex = -_geomagnetic[1];
float Ey =_geomagnetic[0];
float Ez =_geomagnetic[2];
//ACCELEROMETER
float Ax= -_accelerometer[0];
float Ay= _accelerometer[1] ;
float Az= _accelerometer[2] ;
float Hx = Ey*Az - Ez*Ay;
float Hy= Ez*Ax - Ex*Az;
float Hz = Ex*Ay - Ey*Ax;
float normH = (float)sqrt(Hx*Hx + Hy*Hy + Hz*Hz);
float invH = 1.0f / normH;
Hx *= invH;
Hy *= invH;
Hz *= invH;
float invA = 1.0f / (float)sqrt(Ax*Ax + Ay*Ay + Az*Az);
Ax *= invA;
Ay *= invA;
Az *= invA;
float Mx = Ay*Hz - Az*Hy;
float My = Az*Hx - Ax*Hz;
float Mz = Ax*Hy - Ay*Hx;
// if (mOut.f != null) {
matrix[0] = Hx; matrix[1] = Hy; matrix[2] = Hz; matrix[3] = 0;
matrix[4] = Mx; matrix[5] = My; matrix[6] = Mz; matrix[7] = 0;
matrix[8] = Ax; matrix[9] = Ay; matrix[10] = Az; matrix[11] = 0;
matrix[12] = 0; matrix[13] = 0; matrix[14] = 0; matrix[15] = 1;
}
Thank you very much for the help.
Edit: The iPhone it is permantly in landscape orientation and I know that something is wrong because the object painted in Opengl Es appears two times.

Answer 1

Have you looked at Apple's GLGravity sample code? It does something very similar to what you want here, by manipulating the model view matrix in response to changes in the accelerometer input.

Answer 2

I'm unable to find any problems with the code posted, and would suggest the problem is elsewhere. If it helps, my analysis of the code posted is that:
The first six lines, dealing with _geomagnetic 0–5, effect a very simple low frequency filter, which assumes you call the method at regular intervals. So you end up with a version of the magnetometer vector, hopefully with high frequency jitter removed.
The bzero zeroes the result, ready for accumulation.
The lines down to the declaration and assignment to Hz take the magnetometer and accelerometer vectors and perform the cross product. So H(x, y, z) is now a vector at right angles to both the accelerometer (which is presumed to be 'down') and the magnetometer (which will be forward + some up). Call that the side vector.
The invH and invA stuff, down to the multiplication of Az by invA ensure that the side and accelerometer/down vectors are of unit length.
M(x, y, z) is then created, as the cross product of the side and down vectors (ie, a vector at right angles to both of those). So it gives the front vector.
Finally, the three vectors are used to populate the matrix, taking advantage of the fact that the inverse of an orthonormal 3x3 matrix is its transpose (though that's sort of hidden by the way things are laid out — pay attention to the array indices). You actually set everything in the matrix directly, so the bzero wasn't necessary in pure outcome terms.
glLoadMatrixf is then the correct thing to use because that's how you multiply by an arbitrary column-major matrix in OpenGL ES 1.x.