Android: How to get the string of points from the line being drawn?

Question

This is actually our Thesis, we are required to use the Ramer-Douglas-Peucker Algorithm in simplifying lines, can anyboy help me how to implement this in an Android App.

I just want to know how to get the string of points from the line I've drawn and simplify the line by reducing the total no. points based on the given code below?

This is the main class.

public class SketchTimeNewActivity extends GraphicsView implements ColorOption.OnColorChangedListener {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(new MyView(this));

    myPaint = new Paint();
    myPaint.setAntiAlias(true);
    myPaint.setDither(true);
    myPaint.setColor(Color.CYAN);
    myPaint.setStyle(Paint.Style.STROKE);
    myPaint.setStrokeJoin(Paint.Join.ROUND);
    myPaint.setStrokeCap(Paint.Cap.ROUND);
    myPaint.setStrokeWidth(12); 
}

private Paint       myPaint;    

    public void colorChanged(int color) {
    myPaint.setColor(color);
}


public class MyView extends View {

    private static final float MINP = 0.25f;
    private static final float MAXP = 0.75f;

    private Bitmap  mBitmap;
    private Canvas  mCanvas;
    private Path    mPath;
    private Paint   mBitmapPaint;

    public MyView(Context c) {
        super(c);

        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);
    }

    @Override
    protected void onSizeChanged(int width, int height, int oldwidth, int oldheight) {
        super.onSizeChanged(width, height, oldwidth, oldheight);
        mBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(color.black);

        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

        canvas.drawPath(mPath, myPaint);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }
    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
            mX = x;
            mY = y;
        }
    }

    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, myPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
        }
        return true;
    }
}

private static final int COLOR_MENU_ID = Menu.FIRST;
private static final int EXISTING_MENU_ID = Menu.FIRST + 2;
private static final int ENHANCED_MENU_ID = Menu.FIRST + 3;
private static final int ERASE_MENU_ID = Menu.FIRST + 1;


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    super.onCreateOptionsMenu(menu);

    menu.add(0, COLOR_MENU_ID, 0, "Color").setShortcut('1', 'c');
    menu.add(0, EXISTING_MENU_ID, 0, "Enhanced").setShortcut('2', 's');
    menu.add(0, ENHANCED_MENU_ID, 0, "Existing").setShortcut('3', 'z');
    menu.add(0, ERASE_MENU_ID, 0, "Erase").setShortcut('4', 'z');

    return true;
}



@Override
public boolean onPrepareOptionsMenu(Menu menu) {
    super.onPrepareOptionsMenu(menu);
    return true;
}



@Override
public boolean onOptionsItemSelected(MenuItem item) {
    myPaint.setXfermode(null);
    myPaint.setAlpha(0xFFAAAAAA);

When the EXISTING MENU is clicked, it will simplify the line being drawn and display a line that has lesser points or a line that is already simplified. I'm planning to create a new class for it but I don't know how to get the string of points from the line being drawn in the canvas.

    switch (item.getItemId()) {

        case COLOR_MENU_ID:
            new ColorOption(this, this, myPaint.getColor()).show();
            return true;

  /**      case EXISTING_MENU_ID:

            return true;

        case ENHANCED_MENU_ID:

            return true;*/

        case ERASE_MENU_ID:{

                  myPaint.setColor(color.black);
                  myPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
                  return true;
              }

    }

    return super.onOptionsItemSelected(item);
}

}

Answer 1

Copy paste from my original comment for some context:

You are generating the points on the line from your onTouchEvent, so in stead of trying to query the Canvas afterwards, why not simply keep a list of these created points? You can add a point as you draw each new line segment.

In terms of code, your most basic example will look something like this:

List<Point> mPoints = new ArrayList<Point>();

private void touch_up() {
    // save this point on the line
    mPoints.add(new Point(mX, mY);
    // add point to path
    mPath.lineTo(mX, mY);
    // commit the path to our offscreen
    mCanvas.drawPath(mPath, myPaint);
    // kill this so we don't double draw
    mPath.reset();
}

I'm assuming here that touch_up() is where you add each line segment's start or endpoint to the path.

//Edit: Upon reading your problem once more, I feel like you might be asking for all points on the line you've drawn - since your code snippet includes curves? My guess is that the only way to realise this is by evaluating every (x,y) using the different underlying mathematical equations and store the result for each point. In other words: by writing your own lineTo and quadTo functions.

For straight lines this is relatively trivial, but curves will increase the level of difficulty. You might want to take a look at the Path's source code, which internally delegates most of its work to a java.awt.geom.GeneralPath object.