It is similar to the aspectRatio option on cropperjs https://github.com/fengyuanchen/cropperjs#aspectrati I'm using aspectRatio: 3 / 4 and it is great because it is the size of a full Brazilian license, very helpful.
So, if I could enable the idcrop with a pre-selected area, plus using it with #21 then I can use idcrop instead of cropperjs.