Frame Order

This section is currently under construction. Please feel free to add any insights you may have into the structure of the Source engine that you would like to share with the community.

Introduction

This is the order in which systems are invoked for each engine frame.

Engine

The engine has two modes of execution, a threaded path and a non-threaded path. The host_thread_mode cvar determines which path is taken. Dedicated servers will ignore this cvar and always use the non-threaded path. XBox 360 runs in threaded mode by default, while PC runs in non-threaded mode by default. Threaded mode was introduced with the Orange Box version of the Source engine, and does not exist in Episode 1 or previous versions.

Listenservers (ie, singleplayer games, or multiplayer games hosted by one of the players) will run all sections of code, calling both client.dll and server.dll functions from the same thread and in the same engine loop. Clients connected to a remote server will run the italicized sections only, and dedicated servers will run the bold items only. Dedicated server builds (Linux or Windows SrcDs) will run only the underlined sections.

Legend:

• Bold - Server code, occurs on any server engine, server-only or listenserver
• Italics - Client code, occurs on any client engine, client-only or listenserver
• Underline - Code path for a dedicated server (Non underline items do not run on a dedicated server.)
• Green - Function call from the engine into game source

Non-threaded model

• FrameStageNotify: FRAME_START
• Console commands
• For each tick:
  • Networking
  • Input
  • Server game code
  • Client logic
  • Client prediction
• Rendering
• Sound
• Client HUD update

Threaded model

• FrameStageNotify: FRAME_START
• Console commands
• For each client tick:
  • Networking
  • Client logic
• Client prediction
• For each server tick:
  • Input
  • Networking
• Server game code (in a separate thread)
• Rendering
• Sound
• Client HUD update

Sections

Legend:

• Green - Function call from the engine into game source
• Purple - Code that exists inside the game sources available to modders
• Red - Special information

Console commands

This section evaluates and executes commands input into the game console. If the console command is a ConCommand created by the game code, it will execute a callback into the game code through ConCommand::Dispatch( const CCommand &command ) in convar.cpp.

Networking

This section sends and receives network messages. It runs on a localhost server through an internal backdoor loopback device. On server-only or client-only instances it runs over a UDP connection. No game code callbacks are made during this time.

Input

This section reads and processes input devices and messages. It runs only on client or localhost engines.

• CHLClient::HudProcessInput()
• Process console commands
• Process player movement commands
  • CHLClient::CreateMove()
  • Queue movement commands to network to the server

Server game code

This section calls Think functions for each entity and processes game logic.

• If this is the last tick in the frame: CServerGameDLL::Think()
• Read network input
• For each plugin: IServerPluginCallbacks::GameFrame()
• CServerGameDLL::GameFrame()
  • For each game system: FrameUpdatePreEntityThink()
  • GameRules::Think()
  • Physics simulations
    • For each player: CBasePlayer::PlayerRunCommand()
  • For each game system: FrameUpdatePostEntityThink()
• If this is the last tick in the frame: (This part of the tree occurs in a separate thread if the engine is threaded.)
  • CServerGameDLL::PreClientUpdate()
    • For each game system: PreClientUpdate()
  • Transmit queued network messages
  • For every client:
    • Set up network pack data
      • CServerGameClients::ClientSetupVisibility()
      • CServerGameEnts::CheckTransmit()
      • In a singleplayer game, Client logic is run through the local network backdoor
      • Changed entity data tables are packed for network transmission, and snapshots are sent to clients
  • CServerGameClients::PostClientMessagesSent()

Client logic

This section reads network packets and updates the entities on the client.

• Read packets
  • For every packet received:
    • If multiplayer or if there are extra commands:
      • Run prediction
    • CPrediction::PreEntityPacketReceived
    • CHLClient::FrameStageNotify(FRAME_NET_UPDATE_START)
      • Absolute origin/angle queries invalidated
    • Update baseline, update entity data tables
      • Calls to C_BaseEntity::PreDataUpdate as necessary.
    • CHLClient::FrameStageNotify(FRAME_NET_UPDATE_POSTDATAUPDATE_START)
    • For every updated entity: C_BaseEntity::PostDataUpdate()
    • CHLClient::FrameStageNotify(FRAME_NET_UPDATE_POSTDATAUPDATE_END)
      • CPrediction::PostEntityPacketReceived()
    • If an entity has entered or exited the client's PVS: C_BaseEntity::NotifyShouldTransmit()
    • CHLClient::FrameStageNotify(FRAME_NET_UPDATE_END)
      • Absolute origin/angle queries re-validated
    • CPrediction::PostNetworkDataReceived()

Client prediction

This section runs prediction code for the local client.

• CPrediction::Update()
  • Perform prediction. For every player movement command:
    • CPrediction::RunSimulation()
    • For every predictable entity:
      • If data table has changed: C_BaseEntity::OnPreDataChanged()
      • C_BaseEntity::PhysicsSimulate()
        • CPrediction::RunCommand()
        • If the entity is a player:
          • C_BasePlayer::PreThink()
          • CPrediction::SetupMove()
          • CGameMovement::ProcessMovement()
          • CPrediction::FinishMove()
          • C_BasePlayer::PostThink()
          • CMoveHelperClient::ProcessImpacts()
        • If the entity is not a player:
          • Run physics simulation and C_BaseEntity::Think()
  • CPrediction::Untouch()

• Update view angles with device information
  • CPrediction::SetLocalViewAngles()
• CHLClient::ExtraMouseSample()

Rendering

Rendering code sets up the view, updates all entities, and then renders the scene and user interface.

• CHLClient::FrameStageNotify(FRAME_RENDER_START)
  • CInput::CAM_Think()
  • CViewRender::OnRenderStart()
    • CViewRender::SetUpView()
      • CBasePlayer::CalcView()
    • For every player: CBasePlayer::UpdateClientSideAnimation()
      • CMultiPlayerAnimState::Update()
    • ProcessOnDataChangedEvents()
      • For every entity with data changed: C_BaseEntity::OnDataChanged()
    • Update entities, tempents, particle systems, simulate physics
• CHLClient::View_Render()
  • Draws world and then UI
• Incomplete!
• CHLClient::FrameStageNotify(FRAME_RENDER_END)