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@@ -0,0 +1,104 @@ +-*- org -*- + +#+TITLE The Catbird Game Engine + +* About + +Catbird is a game engine written in Guile Scheme that is built on top +of the Chickadee game toolkit. It uses an object-oriented, scene tree +architecture similar to Godot. Catbird encourages a live coding +approach to game development, allowing for almost all aspects of the +game, and even the engine itself, to be modified at runtime. + +* Features + +** Scene node tree +** State machine abstraction +** Input bindings +** General purpose 2D game objects (sprites, text, etc.) +** Automatic asset reloading +** In-engine REPL and command console + +* Key Abstractions + +** Nodes + +Nodes encapsulate the state of an object in the game world. Nodes can +be rendered, updated, and run asynchronous scripts. Complex game +behavior is created by composing simpler nodes. For example, a text +box could be implemented as the composition of a background sprite +with a text label over it. Nodes can have up to one parent node and +zero or more child nodes, forming a tree structure. + +** Modes + +Modes encapsulate pieces of a game's state machine and serve as the +input handling layer. There are 2 types of modes: major and minor. +Major modes are considered mutually incompatible with each other and +are used to represent different states within a game scene. For +example, map traversal in an RPG could be represented by a major mode +for moving the player character around the map and another major mode +for interacting with objects/talking to NPCs. Minor modes implement +smaller, composable pieces of game logic. For example, text field +editing controls could be contained within a minor mode and composed +with every major mode that has the player type something. All modes +have an input map, which translates keyboard/mouse/controller input +events to calls to their respective event handlers. + +** Scenes + +Scenes are a special type of node that encapsulate a coarse chunk of a +game's state machine. For example, an RPG could be divided into +several scenes: world map, inventory, and battle. Modes are attached +to scenes to form a playable game. Scenes always have one active +major mode and zero or more minor modes. + +** Cameras + +Cameras provide a view into a scene. They have a projection, +position, and orientation. A 2D camera uses an orthographic +projection. A 3D camera uses a perspective projection. The same +scene can be rendered using multiple cameras, if desired, such as in a +split-screen multiplayer game. + +** Regions + +Regions represent a sub-section of the game window, creating a +viewport to which a game scene can be rendered. Regions can be +associated with one scene and one camera. When both a scene and +camera are present, the scene is rendered to the region's viewport +from the perspective of the camera. In a typical case, one region +that covers the entire window is all that's needed. A split-screen +multiplayer game, however, could divide the window into two windows +and render the same scene using different cameras. + +** Assets + +Assets are containers for non-Scheme data that is loaded from the file +system, such as an images or audio files. Assets are lazy loaded when +they are first dereferenced or can be manually eager loaded. When in +development mode, the files associated with assets are watched for +changes and automatically reloaded when they change. + +* Building + +If using Guix, life is easy. Run =guix shell= to launch a shell that +has all the dependencies needed to build Catbird. + +If not using Guix, install these dependencies some other way: + +- Guile >= 3.0 +- Chickadee >= 0.9.0 + +To build: + +#+BEGIN_SRC sh + ./bootstrap + ./configure + make -j${nproc} +#+END_SRC + +* License + +Catbird is available under the terms of the GNU General Public +License, version 3 or later. |