CONCEPT
Janus is a layer-type render pass system and is based around two entities: groups and commands.

Groups are a collection of Layout items, which may include cameras, meshes, and lights. A group entity also consists of property overrides, which affect things like item-centric render properties (i.e. UnseenByCamera, Clip Maps), and plugin handlers (i.e. displacement plugins). Applying overrides to a group affects all items assigned to that group.

Groups also have a type. A type is, in essence, a pre-set override. Instead of setting oft-used parameters by hand, a preset can be made by the user and saved as a group type, and thence cast a group onto that type. Two 'built-in' type exists in Janus: NOM (nominal) and NULL and is explained in the Groups in Detail section.

Commands are strings (e.g. text) that changes scene settings and item parameters. Commands are essentially synonymous with 'render pass'. They instruct Janus how to apply item-related settings onto groups, and how to change scene settings based on the pass.

Every command (e.g. render pass) must have an associated group or assocgroup. Associated groups are groups that are specifically assigned to a command so that a command knows which groups to apply its settings to.

Just like groups, commands also have a type, and in the same sense they are pre-set overrides. Whatever you can define in a command, it can be recreated through user-defined types.

Commands have syntax rules. In its basic form, it must contain at least three elements separated by a period (.): command type, layer type, and target group. A command may look like this:

The command type is always the first element. In the example above, NOM is the command type. Command types are defined by the user-definable (although Janus comes with several pre-defined ones).

So what exactly is a command type? Simply, a command type is a preset that controls various parameters. A command type may change visibility settings of items, or certain specific parameters, like light intensity, or even change scene setting like backdrop color.

The layer type is always the second element. The layer type can only be one of two choices: BG or FG, which stands for background and foreground, respectively. This parameter serves as a switch so that the command type's functionality is modified and given new meaning depending on the switch. This switch was designed and named after compositing principles of background and foreground elements. The BG and FG switches are of a slightly advanced nature, and as such, users do not need to depend nor utilise them. But as you will see in the tutorials, they are very efficient and speeds up the set-up process.

The target group is the last element in a command. Note last, not third. Scene commands (e.g. subcommands) may be inserted after the layer type. The target group is always the last element. The target group is significant because when the command is run it will consider the target group to be the active group, which may have different settings from other groups. This depends on the command type.

So, let's back up and see what happens when a command is run. When the command is executed, it will retrieve all pre-set parameters for its command type. It keeps notice if it is switched for BG or FG operation, and the command type carries instructions to the associated groups. First it will go through all the groups except the active group. Then it will apply all parameters that are meant for the other groups, after which it will it will process the active group, if there happens to be any. The command applies the parameters that are particular to the active group.

Commands, in general, will have its own section here in the documentation due to its complex nature. Its details are listed in the Commands in Detail section.

Groups and commands are intrinsically related and are connected through the concept of associated groups. The Group - Command Relationship section explains this.