imu - Magnetometer ∞ shaped calibration

In mobile phones and other devices using a 3-axis electronic compass, a ∞/8/S shaped movement is used to calibrate the magnetometer as shown in these videos.

Why is this movement performed, what is the theory, and can anyone give some example C code to implement it?

You must have to go through my another similar question containing more info.

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Some additional info for this particular question: The platform is 8-bit AtMega32, using AVR Studio 5.

Till now I've tried: I tried dividing the average by 2 of vector values of the Magnetometer making the shape. Thinking might help in calculating offsets. I think some how the two identical parts/sides of the shape is cancelling the earth's magnetic field and giving out the offset values. I might be wrong. But particularly for the shape based calibration this is where I am currently. I think the calibration works out this way. The idea is to find out does that work out this way?

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Ok the code by which I can calculate the offsets and later simply subtract those from the Raw magnetic 3D vector. I might be totally wrong and have no explanation how it works. Seeing after the video and the data plotted on the sphere, somehow has accelerated my thought and I used that thought on form of equation. B)

Code:

The Read_accl(); and Read_magnato(1); functions are reading the sensor data. I hope the code is self explanatory. Hoping wise ppl will surely be using this in much better ways. :\

void InfinityShapedCallibration()

{
    unsigned char ProcessStarted = 0;
    unsigned long cnt = 0; 

    while (1)
    {

            Read_accl();

            // Keep reading Acc data

            // Detect Horizontal position
            // Detect Upside down position
            // Then detect the Horizontal position again.
            // Meanwhile an infinity shaped movement will be created.
            // Sum up all the data, divide by the count, divide by 2 .
            // !We've offsets.          

                if (ProcessStarted!=3)
                {
                //

                    //USART_Transmit_String("\r");
                    //rprintfFloat(4, g_structAccelerometerData.accx_RAW);
                    //USART_Transmit_String(",");
                    //rprintfFloat(4, g_structAccelerometerData.accy_RAW);
                    //USART_Transmit_String(",");
                    //rprintfFloat(4, g_structAccelerometerData.accz_RAW);

                }



            if (
             abs( g_structAccelerometerData.accx_RAW) < 100 
            && abs(g_structAccelerometerData.accy_RAW) < 100 
            && g_structAccelerometerData.accz_RAW < -350 
            && ProcessStarted != 2 && ProcessStarted != 3 && ProcessStarted != 1 )
            {
                ProcessStarted = 1; 
            }   

            if (ProcessStarted==1)

            { 

            Read_magnato(1);

                structMagnetometerOffsetDataToEEPROM.Off_X += g_structMegnetometerData.magx_RAW;
                structMagnetometerOffsetDataToEEPROM.Off_Y += g_structMegnetometerData.magy_RAW;
                structMagnetometerOffsetDataToEEPROM.Off_Z += g_structMegnetometerData.magz_RAW;

                cnt++;


            }               
                if ( g_structAccelerometerData.accz_RAW > 350 
                && ProcessStarted==1)
                {
                    ProcessStarted = 2; 
                }

                if ( g_structAccelerometerData.accz_RAW < -350 
                && ProcessStarted == 2 )
                {

                    ProcessStarted=3; 
                    structMagnetometerOffsetDataToEEPROM.Off_X /= cnt;
                    structMagnetometerOffsetDataToEEPROM.Off_X /= 2;

                    structMagnetometerOffsetDataToEEPROM.Off_Y /= cnt;
                    structMagnetometerOffsetDataToEEPROM.Off_Y /= 2;

                    structMagnetometerOffsetDataToEEPROM.Off_Z /= cnt;
                    structMagnetometerOffsetDataToEEPROM.Off_Z /= 2;  


                    UpdateOFFSETDATAinEEPROM();  

                    break;

                } 
    }   
} 

After getting these offsets I used them as follows:

void main()

{
...

Read_magnato(1);
        g_structMegnetometerData.magx_RAW -= structMagnetometerOffsetDataToEEPROM.Off_X ;
        g_structMegnetometerData.magy_RAW -= structMagnetometerOffsetDataToEEPROM.Off_Y ;
        g_structMegnetometerData.magz_RAW -= structMagnetometerOffsetDataToEEPROM.Off_Z ;
...
}

As I mentioned.