Using an instance of the Bitmap class, you can use the Bitmap.creatBitmap(); method passing the original image x y (for the top left corner) and then width and height.
see documentation here.
in your original example it would be:
Bitmap newBitmap=Bitmap.createBitmap(oldBitmap,10,20,70,80);
Edit
The Bitmap class also allows you to access an array of pixel int's representing color. if you know the shape you want to crop in terms of co-ordinates of each point. you could iterate through the array and set alpha to full on the ones that are outside your shape.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…