Stacker

The stacker tooltip tutorial as seen in the game.

The stacker is a building that takes shapes and joins them together, for the last shape mechanic to be introduced in the story mode. When supplied with a shape on each of the two inputs, the stacker takes the shape from the right input and "drops" it upon the left shape. Due to the top-down perspective of the game, this is visually represented as shrinking the shapes from the upper shape.

Mechanic

The stacking operation takes a top shape and lowers it onto a bottom shape until a layer above where they would begin to intersect. Therefore, if no corners would intersect, then the two shapes would ultimately be joined side-by-side on the base layer. After joining the shapes, if the result is any higher than 4 layers, the top layers will be cut off. Even though floating layers cannot be created by stacking, at no point will either input shape independently be collapsed, leaving any existing floating layers intact.

Internally, this process is optimized — the height difference between the highest corner of the bottom layer and the lowest corner of the top layer is first computed for each of the 4 quadrants, then the top shape is simply shifted up by the greatest of the 4.