The transformation that we are tackling next is to place one picture above an other. although in our example both pictures evenly divide the box between them, we are going to create a more versatile primitive and than use it to create the above function.

We are going to create a split_box_horizontally that takes a factor and a box and splits the box in two. So it return a pair of boxes. The factor determines how much the top box gets to occupy of the original box. The bottom box then will occupy 1.0 - factor.

Head over to src/canvas/mod.rs and implement split_box_horizontally. It helped my to introduce helper functions scale_box_horizontally and move_box_horizontally.

With this primitive open src/picture/mod.rs and define function above_ratio. This function accepts two pictures and two non-negative integers m and n. These integers are used to calculate how much of the pictures get to occupy of the original box. I.e. the top picture should occupy \(\frac{m}{m+n}\) of the original box.

Go on and define above in terms of above_ratio. Then use it in geometry.rs. For above we need to provide two pictures. Remember that we can clone a picture to use it more than once. Find the correct combination of primitives to recreate the picture at the start of this chapter.