Calculation error with Optical Encoder I have corrected my code for reading the optical encoder index but I am now getting a different calculation error.  At the index the encoder reset's to zero but now my readouts are as follows:
Index point - resets to zero.
- At the 90 degree rotation mark I have a readout of 180 degrees.
- At the 180 degree rotation mark I have a readout of 0 because the code has it reset to zero after 359 degrees.
- At the 270 degree rotation mark I have a readout of 180 degrees.
- At the 0 degree mark it is zero again and that's what I would expect from the index.  If I restart the Arduino Mega at the 270 degree mark, the index resets itself to zero like it should.  Being new to this, before I had the index working it was working correctly minus the index function, now it seems to be double counting.  Any ideas?

// Wiring connections for my encoder:
// Brown : VCC = 5V
// Blue + Shield : 0V = GND
// Black : ENCODER0PINA, Pin 20
// White : ENCODER0PINB, Pin 17
// Orange : ENCODER0PINI, Pin 19

#define ENCODER0PINA         20      // this pin needs to support interrupts
#define ENCODER0PINB         17      // no interrupt required
#define ENCODER0PINI         19      // this pin needs to support interrupts

// variables modified by interrupt handler must be declared as volatile
volatile long encoder0Position = 0;
volatile long interruptsReceived = 0;
volatile byte INTFLAG1 = 0;

// track last position so we know whether it's worth printing new output
long previousPosition = 0;

void setup()
{

  // inputs
  pinMode(ENCODER0PINA, INPUT);
  digitalWrite(ENCODER0PINA, HIGH);  // turn on pullup resistor
  pinMode(ENCODER0PINB, INPUT);
  digitalWrite(ENCODER0PINB, HIGH);  // turn on pullup resistor
  pinMode(ENCODER0PINI, INPUT);
  digitalWrite(ENCODER0PINI, HIGH);  // turn on pullup resistor

  // interrupts
  attachInterrupt(3, onInterrupt, CHANGE);  // encoder track A on interrupt 3 - pin 20
  attachInterrupt(4, onReset, CHANGE);  // encoder Index track on interrupt 4 - pin 19

  // enable diagnostic output
  Serial.begin (115200);
  Serial.println("Ready.");
}

void loop()
{
  // only display position info if has changed
  if (encoder0Position != previousPosition )
  {
    Serial.println(encoder0Position, DEC);

    previousPosition = encoder0Position;
  }
}

// interrupt function needs to do as little as possible

void onReset()
{
  // read Index input
  int i = digitalRead(ENCODER0PINI);
  if (i == 1)
  {
  encoder0Position = INTFLAG1;
  }
}

void onInterrupt()
{
  // read both directional inputs
  int a = digitalRead(ENCODER0PINA);
  int b = digitalRead(ENCODER0PINB);

  if (a == b)
  {
    // b is leading a (counter-clockwise)
    encoder0Position--;
    if (encoder0Position &lt; 0)
    {
      encoder0Position = (encoder0Position + 360);
    }
  }
  else
  {
    // a is leading b (clockwise)
    encoder0Position++;
    if (encoder0Position &gt;= 360)
    {
      encoder0Position = (encoder0Position - 360);
    }
  }
}
I have figured out my issue, at least I am getting good results. With this code I had to change interrupts from CHANGE to RISING. I want to thank everyone that has made comments on this question. –