当允许用户擦除时，在用户绘制的图像中裁剪出空白

I have an app where the user draws pictures and then these pictures are converted to pdf. I need to be able to crop out the whitespace before conversion. Originally I kept track of the highest and lowest x and y values (https://stackoverflow.com/questions/13462088/cropping-out-whitespace-from-a-user-drawn-image). This worked for a while, but now I want to give the user the ability to erase. This is a problem because if for example the user erases the topmost point the bounding box would change, but I wouldn't the new dimensions of the box. I have an app where the user draws pictures and