Crop image after selecting region using edge detection in android

I want to automatically select the page area to crop later. I thought edge detection could be useful and use edge detection to find the edges of the image. Now I have this image! but I don't have a clear idea to select the page area as a rectangle. Can anyone suggest a method or implementation for this problem? I really want to do this by selecting the page area as follows. Is there any other way to do this? I also saw border detection in the introduction of digital image processing using Matlab. But I am not familiar with this. Can I use this for this purpose?