如何放大/缩放图像的一部分

Question

我正在创建一个应用，用户可以点击部分图片并在WebView的角落获得放大版本。我设法制作了一个可以制作缩放版本的Paint，但它显示的位置错误，就像有一些偏移。

我知道这个问题已被问过很多次并且已经得到了回答，但似乎并没有帮助解决这些问题。

这是我用过的代码：

  @Override
        public boolean onTouchEvent(@NonNull MotionEvent event) {
            zoomPos = new PointF();
            zoomPos.x = event.getX();
            zoomPos.y = event.getY();


        matrix = new Matrix();
        mShader = new BitmapShader(MainActivity.mutableBitmap, TileMode.CLAMP, TileMode.CLAMP);
        mPaint = new Paint();
        mPaint.setShader(mShader);
        outlinePaint = new Paint(Color.BLACK);
        outlinePaint.setStyle(Paint.Style.STROKE);

        int action = event.getAction(); 

        switch (action) { 
        case MotionEvent.ACTION_DOWN:
        case MotionEvent.ACTION_MOVE:
            zooming = true;
            this.invalidate();
            break; 
        case MotionEvent.ACTION_UP: 
            Point1 = true;
            zooming = false;
            this.invalidate();
            break;
        case MotionEvent.ACTION_CANCEL:
            zooming = false;
            this.invalidate();
            break; 

        default: 
            break; 
        }



     return true;
    }

    @Override
    protected void onDraw(@NonNull Canvas canvas) {
        super.onDraw(canvas);
        if (zooming) {
            matrix.reset();
            matrix.postScale(2f, 2f, zoomPos.x, zoomPos.y);
            mPaint.getShader().setLocalMatrix(matrix);


            canvas.drawCircle(100, 100, 100, mPaint);

        }

    }

从技术上讲，它应该在左上角绘制一个圆圈，并显示我的手指所在区域的缩放图像，它绘制一个圆圈，但同样，缩放也会移动。

最终结果应如下所示：

MainActivity.java

public class MainActivity extends Activity {
static ImageView takenPhoto;
static PointF zoomPos;
Paint shaderPaint;
static BitmapShader mShader;
BitmapShader shader;
Bitmap bmp;
static Bitmap mutableBitmap;
static Matrix matrix;
Canvas canvas;
static Paint mPaint;
static Paint Paint;
static boolean zooming;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        File file = new File(Environment.getExternalStorageDirectory() + "/Pictures/boxes.jpg");

        String fileString = file.getPath();
        takenPhoto = (ZoomView) findViewById(R.id.imageView1);
        bmp = BitmapFactory.decodeFile(fileString);
        mutableBitmap = bmp.copy(Bitmap.Config.ARGB_8888, true);
        takenPhoto.setImageBitmap(mutableBitmap);    
        matrix = new Matrix();
        mShader = new BitmapShader(mutableBitmap, TileMode.CLAMP, TileMode.CLAMP); 
        mPaint = new Paint();
        mPaint.setShader(mShader);
        zoomPos = new PointF();
        Paint = new Paint(Color.RED);  
    }
}

ZoomView.java

public class ZoomView extends ImageView {

    private PointF zoomPos;
    PointF fingerPos;
    private Paint paint = new Paint(Color.BLACK);
    boolean zooming;
    Matrix matrix;
    BitmapShader mShader;
    Paint mPaint;
    Paint outlinePaint;
    boolean Point1;

    public ZoomView(Context context) {
        super(context);
    }

    public ZoomView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public ZoomView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean onTouchEvent(@NonNull MotionEvent event) {
        zoomPos = new PointF();
        zoomPos.x = event.getX();
        zoomPos.y = event.getY();


        matrix = new Matrix();
        mShader = new BitmapShader(MainActivity.mutableBitmap, TileMode.CLAMP, TileMode.CLAMP);
        mPaint = new Paint();
        mPaint.setShader(mShader);
        outlinePaint = new Paint(Color.BLACK);
        outlinePaint.setStyle(Paint.Style.STROKE);

        int action = event.getAction(); 

        switch (action) { 
        case MotionEvent.ACTION_DOWN:
        case MotionEvent.ACTION_MOVE:
            zooming = true;
            this.invalidate();
            break; 
        case MotionEvent.ACTION_UP: 
            Point1 = true;
            zooming = false;
            this.invalidate();
            break;
        case MotionEvent.ACTION_CANCEL:
            zooming = false;
            this.invalidate();
            break; 

        default: 
            break; 
        }



     return true;
    }

    @Override
    protected void onDraw(@NonNull Canvas canvas) {
        super.onDraw(canvas);
        if (zooming) {
            matrix.reset();
        matrix.postScale(2f, 2f, zoomPos.x, zoomPos.y);
        mPaint.getShader().setLocalMatrix(matrix);
        RectF src = new RectF(zoomPos.x-50, zoomPos.y-50, zoomPos.x+50, zoomPos.y+50);
        RectF dst = new RectF(0, 0, 100, 100);
        matrix.setRectToRect(src, dst, Matrix.ScaleToFit.CENTER);
        matrix.postScale(2f, 2f);
        mPaint.getShader().setLocalMatrix(matrix);


        canvas.drawCircle(100, 100, 100, mPaint);
        canvas.drawCircle(zoomPos.x, zoomPos.y, 100, mPaint);
        canvas.drawCircle(zoomPos.x-110, zoomPos.y-110, 10, outlinePaint);

        }
        if(Point1){
            canvas.drawCircle(zoomPos.x, zoomPos.y, 10, paint);
        }
    }
}

修改

正如你所看到的新代码更好，仍然有一些偏移 - 黑点 - 手指的位置。