Cropping a Perspective Transformation of Image on Android

I'm trying to do a perspective transform on a bitmap that I capture via the camera. The user adjusts a bounding quad (as depicted by the white box) around a rectangular object. I then attempt to transform this to a rectangular image using the code below:

public static Bitmap perspectiveTransformation(Bitmap bitmap,BoundingQuad boundingQuad)
{
    Matrix matrix = new Matrix();
    float[] dst = new float[] {
            0,
            0,
            bitmap.getWidth(),
            0,
            bitmap.getWidth(),
            bitmap.getHeight(),
            0,
            bitmap.getHeight()
    };
    float[] src = new float[] {
            boundingQuad.topLeft.x,
            boundingQuad.topLeft.y,
            boundingQuad.topRight.x,
            boundingQuad.topRight.y,
            boundingQuad.bottomRight.x,
            boundingQuad.bottomRight.y,
            boundingQuad.bottomLeft.x,
            boundingQuad.bottomLeft.y
    };
    matrix.setPolyToPoly(src, 0, dst, 0, src.length >> 1);
    return Bitmap.createBitmap(bitmap, 0, 0, bitmap.getWidth(), bitmap.getHeight(), matrix, true);
}

However my resulting image contains image data that is outside the boundary of my quad. This would be acceptable if I could figure out what the coordinates of the quad are after transformation so I could crop the result but I have absolutely no idea how to do that.

Any help would be greatly appreciated in either finding the coordinates of the quad after transformation or ideally finding a way to prevent this situation from occurring in the first place.

Input:

https://i.sstatic.net/33RfN.png

Output:

https://i.sstatic.net/zWFvA.png