how to draw circle using Multitouch

问题

i had searched about Multitouch & i got lots of example related to

pich zoom in & zoom out &

getting multiple touch coordinates but

i want to use multitouch to draw a circle on a view.. i tried this but when i draw it the coordinates changes the circle didn't draw at the exact place where i touch the screen..

here is the CODE....

public class TestMultiTouchActivity extends Activity {
public class MultiTouchView extends View {

    // In this test, handle maximum of 2 pointer
    final int MAX_POINT_CNT = 2;

    private Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    float[] x = new float[MAX_POINT_CNT];
    float[] y = new float[MAX_POINT_CNT];
    boolean[] isTouch = new boolean[MAX_POINT_CNT];

    public MultiTouchView(Context context) {
        super(context);
        // TODO Auto-generated constructor stub
    }

    @Override
    protected void onDraw(Canvas canvas) {
        // TODO Auto-generated method stub
        if (isTouch[0]) {
            paint.setStyle(Paint.Style.STROKE);
            paint.setStrokeWidth(1);
            paint.setColor(Color.RED);
            // canvas.drawCircle(x[0], y[0], 50f, paint);
        }
        if (isTouch[1]) {
            paint.setStyle(Paint.Style.STROKE);
            paint.setStrokeWidth(1);
            paint.setColor(Color.BLUE);
            // canvas.drawCircle(x[1], y[1], 50f, paint);
        }
        float x1 = (Math.abs((x[0] - x[1]))) / 2;
        float y1 = (Math.abs((y[0] - y[1]))) / 2;

        float r = Math.abs(x1 - x[0]);
        canvas.drawCircle(x1, y1, r, paint);

    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // TODO Auto-generated method stub
        setMeasuredDimension(MeasureSpec.getSize(widthMeasureSpec),
                MeasureSpec.getSize(heightMeasureSpec));
    }

    @Override
    public boolean onTouchEvent(MotionEvent motionEvent) {
        // TODO Auto-generated method stub

        int pointerIndex = ((motionEvent.getAction() & MotionEvent.ACTION_POINTER_ID_MASK) >> MotionEvent.ACTION_POINTER_ID_SHIFT);
        int pointerId = motionEvent.getPointerId(pointerIndex);
        int action = (motionEvent.getAction() & MotionEvent.ACTION_MASK);
        int pointCnt = motionEvent.getPointerCount();

        if (pointCnt <= MAX_POINT_CNT) {
            if (pointerIndex <= MAX_POINT_CNT - 1) {

                for (int i = 0; i < pointCnt; i++) {
                    int id = motionEvent.getPointerId(i);
                    x[id] = (int) motionEvent.getX(i);
                    y[id] = (int) motionEvent.getY(i);
                }

                switch (action) {
                case MotionEvent.ACTION_DOWN:
                    isTouch[pointerId] = true;
                    break;
                case MotionEvent.ACTION_POINTER_DOWN:
                    isTouch[pointerId] = true;
                    break;
                case MotionEvent.ACTION_MOVE:
                    isTouch[pointerId] = true;
                    break;
                case MotionEvent.ACTION_UP:
                    isTouch[pointerId] = false;
                    break;
                case MotionEvent.ACTION_POINTER_UP:
                    isTouch[pointerId] = false;
                    break;
                case MotionEvent.ACTION_CANCEL:
                    isTouch[pointerId] = false;
                    break;
                default:
                    isTouch[pointerId] = false;
                }
            }
        }

        invalidate();
        return true;
    }
}

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(new MultiTouchView(this));
}

回答1:

Try this for calculating the size/position of the circle:

// Calculate average x and y.
float x1 = (x[0] + x[1]) / 2.0f;
float y1 = (y[0] + y[1]) / 2.0f;

// Calculate the distance between the points
float r = Math.sqrt(Math.pow(x[1] - x[0], 2.0f) + Math.pow(y[1] - y[0], 2.0f) / 2.0f;

canvas.drawCircle(x1, y1, r, paint);

来源：https://stackoverflow.com/questions/10409476/how-to-draw-circle-using-multitouch