Viewer Architecture

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The Second Life client (also known as the "viewer") is a complex piece of software. It can be visualized as a streaming media client like RealPlayer, a game engine like Quake 3, or a web browser.

First you'll need to learn some Second Life Terminology, like "agent", "sim", and "region".

If you like to learn by example, you can learn about:

If you prefer a more hierarchical approach, the viewer is built from:

  • Message System - reliable and unreliable transport over UDP
  • Image System - prioritizes and decodes JPEG2000 images into OpenGL textures
  • Viewer Object System - objects in scene for rendering and editing
  • Rendering System - from viewer object to drawable to face to vertices to graphics card
  • Culling - octree and occlusion based culling to speed rendering
  • UI Widgets - cross platform buttons, scroll bars, etc.
  • UI Floaters - dialogs and windows built from XML files
  • Asset System - transport for animations, sounds, notecards, scripts, etc.
  • Inventory - server-side storage of assets for each user
  • Sound System - cross platform audio based on FMOD
  • Movie System - QuickTime-based video on object surfaces
  • VFS - cached data is held in two "virtual file systems" in large files on the client
  • Selection Manager - holds extended information for selected objects and allows editing
  • Avatar Appearance - appearance is constructed from a mesh, parameters to deform the mesh, and textures to describe clothing
  • Avatar Profiles
  • Groups
  • Money

Threads

The viewer is a single process with a few threads:

  • Main thread -- The input/output main program function.
  • VFS Thread -- Thread responsible for reading/writing to the virtual file system.
  • LFS Thread -- Thread responsible for some reading/writing to the local file system. Not sure if this is actually used anywhere.
  • Image Thread -- Currently unused.

Program Flow

  1. Initialize - newview/viewer.cpp :: main()
  2. Loop - newview/viewer.cpp :: main_loop()
    • Gathers input
    • Pumps the TCP i/o
    • Render the frame
    • idle()
      • everything
  3. Shutdown

Sources of Input

  • The Message System.
  • Files copied out of the VFS into local file store and read through c/c++/ll file API.
  • XMLRPC calls such as login and buying currency/land.
  • HTTP GET/POST to the Backbone.
  • Streaming music and videos.
  • The web browser.