double mouseActiveForCycles = 0;
float carriedOverX = 0;
float carriedOverY = 0;

double mouse_accel (double cycles)
{
  double accel = atan((cycles / 50) - 5);
  accel += 1.5707963267944; // we want the whole s curve, not just the bit that's usually above the x and y axes;
  accel = accel * 0.85;
  if (accel < 0.25) {
    accel = 0.25;
  }
  return accel;
}

void handle_mouse_movement( char x, char y)
{

  if (x != 0 || y != 0) {
    mouseActiveForCycles++;
    double accel = (double) mouse_accel(mouseActiveForCycles);
    float moveX = 0;
    float moveY = 0;
    if (x > 0) {
      moveX = (x * accel) + carriedOverX;
      carriedOverX = moveX - floor(moveX);
    } else if (x < 0) {
      moveX = (x * accel) - carriedOverX;
      carriedOverX = ceil(moveX) - moveX;
    }

    if (y > 0) {
      moveY = (y * accel) + carriedOverY;
      carriedOverY = moveY - floor(moveY);
    } else if (y < 0) {
      moveY = (y * accel) - carriedOverY;
      carriedOverY = ceil(moveY) - moveY;
    }
#ifdef DEBUG_SERIAL
    Serial.println();
    Serial.print("cycles: ");
    Serial.println(mouseActiveForCycles);
    Serial.print("Accel: ");
    Serial.print(accel);
    Serial.print("	moveX is ");
    Serial.print(moveX);
    Serial.print(" moveY is ");
    Serial.print(moveY);
    Serial.print(" carriedoverx is ");
    Serial.print(carriedOverX);
    Serial.print(" carriedOverY is ");
    Serial.println(carriedOverY);
#endif
    Mouse.move(moveX, moveY, 0);
  } else {
    mouseActiveForCycles = 0;
  }

}