绘制比屏幕大的位图

Question

我想绘制一个大于我的屏幕的位图，然后保存它。

图像尺寸（横向）为1080 x 1776（Nexus 5），而位图图像为3264 x 2448。

使用我的活动xml文件正确缩放图像以适应屏幕：

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:scaleType="fitCenter"
    android:id="@+id/ivPicture"
    android:src="@mipmap/ic_launcher"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true" />

如果我在屏幕中心画一个点，绘制的点不在我的手指下，但它是一些像素左/上。

绘制一个矩形（从屏幕的边缘到边缘）会产生缩放的矩形，如图像附加：

似乎有一些比例问题，但我无法弄明白，这是代码的相关部分。

public void loadImage(Uri imageFileUri) {

    /*

      ivPicture = (ImageView) findViewById(R.id.ivPicture);
      Display display = getWindowManager().getDefaultDisplay();
      Point size = new Point();
      display.getSize(size);
      displayWidth = size.x;
      displayHeight = size.y;

      screen size: 1794 x 1080
      bitmap size: 3264 x 2448
    * */
    try {
        BitmapFactory.Options bmpFactoryOptions = new BitmapFactory.Options();
        bmpFactoryOptions.inJustDecodeBounds = true;

        ContentResolver cr = getContentResolver();
        InputStream is = cr.openInputStream(imageFileUri);

        bmp = BitmapFactory.decodeStream(is, null, bmpFactoryOptions);

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

        alteredBitmap = Bitmap.createBitmap(bmp.getWidth(), bmp.getHeight(), bmp.getConfig());
        Log.d(TAG, "screen size: " + displayWidth + " x " + displayHeight);
        Log.d(TAG, "bitmap size: " + bmp.getWidth() + " x " + bmp.getHeight());

        canvas = new Canvas(alteredBitmap);
        paint = new Paint();
        paint.setColor(Color.GREEN);
        paint.setStrokeWidth(15);
        matrix = new Matrix();

        canvas.drawBitmap(bmp, matrix, paint);

        ivPicture.setImageBitmap(alteredBitmap);
        ivPicture.setOnTouchListener(this);

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


public boolean onTouch(View v, MotionEvent event) {
    int action = event.getAction();
    switch (action) {
        case MotionEvent.ACTION_DOWN:
            downx = event.getX();
            downy = event.getY();
            break;
        case MotionEvent.ACTION_MOVE:
            upx = event.getX();
            upy = event.getY();
            canvas.drawLine(downx, downy, upx, upy, paint);
            ivPicture.invalidate();
            downx = upx;
            downy = upy;
            break;
        case MotionEvent.ACTION_UP:
            upx = event.getX();
            upy = event.getY();
            canvas.drawLine(downx, downy, upx, upy, paint);
            ivPicture.invalidate();
            break;
        case MotionEvent.ACTION_CANCEL:
            break;
        default:
            break;
    }
    return true;
}

我想我遇到了数学问题，如何在图像上精确绘制？