Bizarre touch coordinates

[Jeet Dholakia]

Im making an application for painting on a bitmap, and its been asked several times before on SO like here and here

I used them inside my app and none seemed to work , here's the code i have used:

public class EditActivity extends Activity implements OnTouchListener,
        OnClickListener {
    GestureDetector gd;
    View.OnTouchListener gl;
    RelativeLayout parent;
    ImageView im;
    Bitmap bmp;
    Bitmap alteredBitmap;
    Canvas canvas;
    Paint paint;
    Matrix matrix;
    Rect imageBounds;
    Path mPath;
    int dw;
    int dh;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_edit);
        parent = (RelativeLayout) findViewById(R.id.rl);


        matrix = new Matrix();
        parent.setBackgroundColor(Color.BLACK);
        gd = new GestureDetector(this, new MyGestureDetector());
        gl = new View.OnTouchListener() {
            public boolean onTouch(View v, MotionEvent event) {

                float x = event.getX();
                float y = event.getY();

                int action = event.getAction();

                    switch (action) {

                    case MotionEvent.ACTION_DOWN:

                        mPath.moveTo(x,y);
                        im.invalidate();
                        break;
                    case MotionEvent.ACTION_MOVE:
                        mPath.lineTo(x,y);
                        im.invalidate();
                        break;
                    case MotionEvent.ACTION_UP:
                        // canvas.drawPath(mPath, paint);
                        mPath.reset();
                        im.invalidate();
                        break;
                    case MotionEvent.ACTION_CANCEL:
                        break;
                    default:
                        break;
                    }
                    return gd.onTouchEvent(event);
                } else {
                    return gd.onTouchEvent(event);
                }
            }
        };

        im = (ImageView) findViewById(R.id.ImageView);

        Uri imageFileUri = Uri.parse(getIntent().getStringExtra("uri"));
        Display currentDisplay = getWindowManager().getDefaultDisplay();

        dw = currentDisplay.getWidth();

        dh = currentDisplay.getHeight();

        try {
            // Load up the image's dimensions not the image itself
            BitmapFactory.Options bmpFactoryOptions = new BitmapFactory.Options();

            bmpFactoryOptions.inJustDecodeBounds = false;
            bmp = BitmapFactory.decodeStream(getContentResolver()
                    .openInputStream(imageFileUri), null, bmpFactoryOptions);

            alteredBitmap = Bitmap.createBitmap(bmp.getWidth(),
                    bmp.getHeight(), bmp.getConfig());

            mPath = new Path();
            canvas = new Canvas(alteredBitmap);
            paint = new Paint(Paint.SUBPIXEL_TEXT_FLAG);
            paint.setFlags(Paint.DEV_KERN_TEXT_FLAG);
            paint.setStyle(Paint.Style.FILL);
            paint.setColor(Color.WHITE);
            paint.setFilterBitmap(true);
            paint.setDither(true);
            paint.setAntiAlias(true);
            paint.setTextSize(40f);

            paint.setTypeface(getTypeFaceFromFile("Roboto-Light"));

            canvas.drawBitmap(bmp, matrix, paint);
            canvas.save();
            im.setImageBitmap(alteredBitmap);
            im.setOnClickListener(this);
            im.setOnTouchListener(gl);

        } catch (FileNotFoundException e) {
            Log.v("ERROR", e.toString());
        }

    }

    class MyGestureDetector extends SimpleOnGestureListener {

        @Override
        public boolean onDown(MotionEvent event) {

            return super.onDown(event);
        }

        @Override
        public boolean onSingleTapConfirmed(MotionEvent e) {

            return super.onSingleTapConfirmed(e);
        }

        @Override
        public boolean onDoubleTap(MotionEvent e) {

            return super.onDoubleTap(e);
        }

        @Override
        public void onLongPress(MotionEvent e) {

            super.onLongPress(e);
        }

        @Override
        public boolean onDoubleTapEvent(MotionEvent e) {

            return super.onDoubleTapEvent(e);
        }

    }

}

Now, as i draw the path by touching on the screen, it just draws itself elsewhere, also it only happens in the images with have been scaled by android itself, not on images which are small enough to fit without scaling , please look into it and help me out

Thanks, Jeet

[Steve Luke]

The coordinates you get from the event are screen coordinates. The coordinates you are painting on are image coordinates. Since the image is scaled, you should not expect those coordinates to be the same, so you can't just use the event's X and Y coordinates to paint with. Since the image is scaled, you will need to scale the coordinates the same way. That is usually done by getting the width and height of the original image and comparing them to the width and height of the image view to get a scaling factor. Then multiplying the coordinates appropriately.

[Jeet Dholakia]

Steve Luke wrote:The coordinates you get from the event are screen coordinates. The coordinates you are painting on are image coordinates. Since the image is scaled, you should not expect those coordinates to be the same, so you can't just use the event's X and Y coordinates to paint with. Since the image is scaled, you will need to scale the coordinates the same way. That is usually done by getting the width and height of the original image and comparing them to the width and height of the image view to get a scaling factor. Then multiplying the coordinates appropriately.

Thanks for pointing me in the correct direction,

i did some tinkering by myself, and I hope its useful to others

This are the steps i followed

1) Calculate original width and scaled width
2)find the scale ratio
3)multiply the scaled ratio to the events' x and y coordinate, and you can get the desired co-ordinates...