The term "Solidity" is used to refer to a set of rules that determines which shapes are applied to which tiles. Each sprite can have its own solidity setting, referring to a particular Solidity object defined in the project. This will affect whether the sprite can walk straight through a tile, whether it walks up it like a hill or whether it hits it and stops dead in its tracks.

Solidity is based on tile categories, so tile categories must be defined before any useful solidity information can be created. The function of this window is to associate tile categories with tile shapes to form a single named "Solidity" object that sprites can refer to as a single set of rules about what is solid and what is not. Note that assigning a tile category to a shape does not apply globally, but only within the scope of a Solidity definition. Therefore, it's possible to have one solidity object that defines tiles in a category named "water" as solid, and a different solidity object that defines the same tiles as empty. This would allow, for example, a "fish" sprite to use one solidity object to be able to travel freely through water, and a "player" sprite to be blocked by water. It's even possible to change this solidity during the game so that, for example, if the player receives an item to allow it to walk on water, it can switch to the other solidity definition.

To create a new solidity definition, right-click on the "Solidity" folder in the project tree and select "New", or select the folder, and then choose "New Object" from the file menu. To edit an existing solidity object, double-click it in the project tree, or select it and choose "Properties" from the view menu or "Edit" from the context (right-mouse) menu.