1	A Data-Driven Game Object System GDC 2002
2	Introduction Me Scott Bilas, scott@gaspowered.com Background You System architect types Tired of fighting with statically typed systems for game code The Test Subject Dungeon Siege >7300 unique object types (i.e. can be placed in the editor) >100000 objects placed in our two maps Continuous world means anything can load at any time
3	Cell Phones?
4	Definitions Data-Driven Meaning: “No engineer required” Engineers are slow Causes designers to hack around missing functionality Goal: remove C/C++ from game Line between engine and content is always moving
5	Definitions (Cont.) Game Object (Go) Piece of logical interactive content Perform tasks like rendering, path finding, path following, speaking, animating, persisting Examples are trees, bushes, monsters, levers, waypoint markers, doors, heroes, inventory items Many are “pure logic”, never see them (triggers, elevator movers, camera sequences) Every game has these in some form
6	Definitions (Cont.) Game Object System Constructs and manages Go’s Maps ID’s to object pointers Routes messages Build from many things, but for this talk GoDb: Go database ContentDb: Static content database Every game has this in some form
7	Example Class Tree Vintage
8	Example Class Tree Newfangled
9	It Won’t Work There are hundreds of ways to decompose the Go system problem into classes They are all wrong They don’t start out wrong, of course… Games constantly change Designer makes decisions independently of engineering type structures They will ask for things that cut right across engineering concerns
10	Just Give In To Change Requirements get fuzzier the closer your code gets to the content Will end up regularly refactoring Do not resist, will cause worse problems! However: C++ does not support this very well!!
11	C++: Not Flexible Enough Code has a tendency to “harden” Resists change over time Rearranging class tree requires lots of work Needing to change it causes engineering frustration, which leads to… Class merging/hoisting (fights clean OOP) Virtual override madness Increased complexity Ü increasing resistance Doc rot, editor out of sync
12	Reexamine The Problem This is a database (a very well understood problem) “The data is important, nothing else matters” …and we’re hard coding it every time To meet changing design needs, can’t just data-drive the object properties, must data-drive structure (schema) of the objects
13	Solution: Component System Each component is a self-contained piece of game logic Assemble components into Go’s to build complete objects Specification for assembly driven by data Lay out data in a C++-style specialization tree to promote reuse and reduce memory usage Include and enforce an external schema
14	Two-Part Implementation
15	Dynamic Content Layout
16	Extension: Skrit (DS Scripting Language) Obvious requirement: build components out of skrit Leave high performance components in C++ Permits extremely fast prototyping No rebuilds required Don’t even have to restart game (reload on the fly) Schema is internal
17	Extension: Skrit (Cont.) Simple implementation (assuming you already have event-driven scripting language ready) GoSkritComponent derivative owns a skrit Override all virtuals and pass as events to skrit Game and editor don’t know/care difference between C++ and skrit components (Neither do the designers)
18	21 C++ Components actor, aspect, attack, body, common, conversation, defend, edit, fader, follower, gizmo, gold, gui, inventory, magic, mind, party, physics, placement, potion, store
19	148 Skrit components base_chest, cmd_actor_stats, cmd_ai_dojob, cmd_animation_command, cmd_auto_save, cmd_camera_command, cmd_camera_move, cmd_camera_waypoint, cmd_delete_object, cmd_dumb_guy, cmd_enter_nis, cmd_inv_changer, cmd_leave_nis, cmd_party, cmd_party_wrangler, cmd_report_gameplay_screen_player, cmd_selection_toggle, cmd_send_world_message, cmd_steam_puzzle, cmd_texture, dev_console, dev_path_point, door_basic, elevator_2s_1c_1n, elevator_2s_1c_1n_act_deact, elevator_2s_1c_2n, elevator_2s_2c_1n, elevator_2s_2c_2n, elevator_2s_3c_1n, elevator_2s_4c_2n, elevator_3s_1c_1n, elevator_3s_2c_1n, elevator_hidden_stairwell, elevator_hidden_stairwell_act_deact, elevator_instant_1c, elevator_instant_4s_1c, fireball_emitter, fire_emitter, fire_emitter_act, generic_emitter, generic_emitter_act, glow_emitter, glow_emitter_act, go_emitter, particle_emitter, particle_emitter_act, sound_emitter, sound_emitter_act, spark_emitter, animate_object, camera_quake, camera_stomp, decal_fade, effect_manager, effect_manager_server, gom_effects, guts_manager, light_colorwave, light_enable, light_flicker, light_flicker_lightweight, nodal_tex_anim, nodal_tex_swap, tsd_manager, water_effects, generator_advanced_a2, generator_auto_object_exploding, generator_basic, generator_breakable, generator_cage, generator_dumb_guy, generator_in_object, generator_object_exploding, generator_object_pcontent, generator_random, activate_chapter, alignment_switcher, attach_robo, breaking_object, check_bool, check_level, check_quest, chipper, clone_preloader, enchantment_manager, experience_award, fountain, freeze_manager, generic_accumtrigger, generic_objblock, hidden_reveal, interface_fade, msg_switch, object_selection_toggle, on_client, play_chapter_sound, point_snapper, position_sync, respawn_shrine, screen_report, self_destruct, set_bool, tip, vis_toggle, locked, on_off_lever, gremal_reward, spell, spell_area_effect, spell_balance, spell_body_bomb, spell_chain_attack, spell_charm, spell_damage_volume, spell_deathrain, spell_death_explosion, spell_default, spell_fire, spell_freak, spell_freeze, spell_instant_hit, spell_launch, spell_lightning, spell_mass_control, spell_mass_enchant, spell_multiple_hit, spell_penalty, spell_polymorph, spell_reactive_armor, spell_resurrect, spell_return_summoned, spell_status_effect, spell_summon, spell_summon_multiple, spell_summon_random, spell_switch_alignment, spell_transmute, spell_turret, test_marker, test_timer, trapped, trp_explosion, trp_firetrap, trp_launch, trp_lightning, trp_particle, trp_trackball, minigun_magic
20	Alert! Before Moving On Generic datastore required to continue INI file, config file, XML store, RIFF, all the same Permits generic data retrieval/storage DS has “gas”, think “INI with nesting + goodies^2” Not difficult to roll your own Many books/articles on this Probably need one for other parts of the game anyway (i.e. you’ll find uses for it no problem)
21	Static Content Layout (Code)
22	Schema Layout (Code)
23	Compile ContentDb Part 1: Build Schema Process components.gas (C++ table specs) Build table specs directly from .gas spec Recursively scan components base directory for all skrit components Compile each skrit Build table specs from metadata in …now we’ve got the schema constructed.
24	C++ Component Schema (Data)
25	Skrit Component Schema (Data) (Concept adapted from UnrealScript)
26	Compile ContentDb Part 2: Build Templates (This is just prep work) Recursively scan .gas template tree Note: doesn’t need to be a physical tree Open data handles to each template Keep track of root nodes, build specialization tree
27	Template Forest (Data)
28	Template Specification (Data)
29	Compile ContentDb Part 3: Compile Templates Recursively compile templates root-down Add data components on demand Read in values, override base template fields This is all similar to C++ base-first member initialization in ctors.
30	Compile ContentDb Special notes We want a flat tree for performance reasons Depends on how frequently you construct objects and how fast your data override system is Also permits special const-read optimization that can eliminate memory usage and CPU for variables that are never changed Copy data components on write to avoid unnecessary memory usage If have many templates, will need to JIT compile leaf templates to save memory
31	Editor Integration This is almost trivial Editor should have a property sheet type thing This is a one-entry view into the db Map types and names onto fields using schema Can un-override easily by querying template Be sure to add a column or tooltip for docs!
32	Editor Integration (Cont.) For DS all editing support done through a special “GoEdit” component Transforms data between game object and editor Supports cheap rollback (undo) by double buffering Does not exist in game, only needed in editor Automates saving all game object instances – just compare vs. the const data and write out if different Not recommended: permitting forced overrides of duplicate data
33	Instance Specification (Data)
34	Loading Objects In DS, objects are referenced by content ID Look up instance block to get template to use Instantiate Go by that template For each block in instance, create a new data component Specialize that data component from base in template Finally iterate through GoComponents and xfer in data to set initial values
35	New C++ Components Can be done with little regard for other components (just add it) Derive from GoComponent only Specializing an existing class just asking for trouble Add new block to C++ components schema (DOC IT) Use a factory method Simple LUT mapping name Ü ‘new GoJooky’ Wait a second, wouldn’t it be better to write using the scripting language? (Probably…)
36	New Skrit Components Same as C++, just stick it in there Everything should be autodetect here Extend the scripting language with metadata Pass it straight through to schema query Can implement flags, docs, and custom game features like “server only” components etc.
37	Managing the Template Tree Can be maintained by nearly anyone once it’s set up Should have multiple roots for broad types Try to avoid data duplication Reserve one branch for test templates Mark it dev-only (so is excluded for retail build) Prefix with test_ or dev_ to avoid namespace pollution DS ended up with 150 or so
38	Advantages I Forgot To Mention Direct and automatic editor support Designers can construct their own types to place in the editor (careful, monitor this!) By only saving out modified data in instances, can make global changes easily by modifying templates Reorganizing the template tree is easy If embed a sub-tree for designers to build custom views into the database
39	Some Pitfalls C++ components prone to becoming intertwined Operations can end up being order-dependent, though this is more easily controlled Nothing here is unique to components It’s a little too easy to add templates, perhaps DS has >7300 of them, many auto-generated System was designed for <100 Need to keep close eye on template complexity to avoid memory/CPU hog (i.e. unnecessary components or wacky specialization) “With power comes responsibility”
40	Future Schema extensible Add flags and constraints that editor can use Auto-detect when can use color chooser or slider or listbox or whatever Add defaults computed from script
41	Contact Info Scott Bilas scott@gaspowered.com http://www.drizzle.com/~scottb/gdc