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201909 - A look into threejs editor app

A look into threejs editor

My notes while reading the source code from threejs Editor app, as I've been curious about :

  • the editor architecture
  • the undo/redo system
  • the camera control behaviour & code
  • the object transform gizmos behaviours & code

TLDR

  • The code is simple, easy to follow and straight to the point
  • The editor components communicate through concepts of signals and command-objects
  • The command-objects are stored and replayed for undo/redo of operations
  • Camera controls are basic but effective, the code could be easier to follow
  • Object transforms gizmos code is from a threejs 'example', is complex (complicated?), and I run out of time for now - I'll try to back for it

Table of contents

I've been looking at the current master source code, not a specific version : github commit b3ce68b4 on sept. 2019.

Getting threejs source

The git repo is larger than I thought it would be, almost 800Mb.

A good point, IMHO : even if threejs uses nodejs for its build chain, it has no external npm dependencies for its sources - all used libraries are included in the git repo.

Once the mandatory npm install is done (using my probably outdated v3.10.5), the build goes fine, following the doc instructions.

Editor Javascript code structure

Located in editor/ folder, each 'class' is neatly stored in its source file.

The code do not use require or import statements :

  • all objects and modules are sourced as globals in index.html
  • makes less easy to find related code and definitions

The UI code often seem to :

  • simply build html strings to feed innerHtml in DOM
  • use an UI module that uses raw javascript DOM manipulation (without any lib)

Starting with index.html

The code is pretty clear, even if there's no comments.

The general operation sequence is :

  1. include all (many many) scripts from :

    • threejs itself
    • examples/ for controls, loaders, exporters, renderers
    • editor/js/libs for external libs
    • editor/js, for the actual editor code
  2. construct the editor objects, and add their dom elements to the html document

  3. initialize the storage

  4. hook some signals to a saveState function that can handle autosave on actions

    • editor exposes toJSON/ fromJSON methods to handle document serialization
  5. hook on some DOM events, such as drag, drop, windows resize

  6. load an optional initial 'document' given in the URL

  7. initialize ServiceWorker

And that's it.

The editor top-level objects are structured as follow :

  • Editor, which is the root object, holding the editor state and operations
  • Viewport, handles 3D rendering, interactions, reacting to change signals
  • SideBar, setup panels UI and handle its actions
    • e.g. SideBar.Scene.js handles scene properties
    • signals are dispatched on value changes
  • MenuBar, setup top menu UI and handle its actions
    • e.g. MenuBar.File.js handles UI & logic for file export actions
  • Toolbar, dispatches signals on UI actions, and listens to signals to update UI
  • Script, handles the script editor
  • Player, handles a threejs preview

I spotted a historyChanged signal that leads me to think that the undo/redo system is named History.

Editor state

The editor state is held in multiple objects, members of Editor object, such as :

  • all the Signal objects, held in a signals object
  • scene and selected for nodes storage and selection
  • history for undo/redo
  • camera, cameras, viewportCamera, addCamera will need some digging
  • many methods to manipulate the state :
    • Object related: add, remove, move, name
    • Geometry related
    • Material
    • Texture
    • Camera
    • Helper, used to represent cameras, lights and skeletons in 3D viewport
    • Script
    • Selection & focus
    • JSON serialization
    • undo/redo

Communication architecture

While browsing through the top-level objects, I found that the top-level objects in the app communicate with the editor only through 2 channels :

  • signals
  • commands

Signals

The editor.signals are all declared in the same spot, at Editor construction.

They are used in all other objects :

  • to dispatch change notifications (and values)
  • to react on change notifications through a callback function

Commands

Commands are dispatched by editor.execute, passing a command-object.

Commands are the key elements for undo/redo system, as described in Implementing additional commands for undo-redo.md doc.

Command-objects are responsible to directly call editor methods to change its state, and dispatch appropriate signals.

Viewport

The general behavious is :

  • helpers geometry are created/managed :
    • an inert grid mesh
    • a selection box that reacts to selection changes
  • object manipulation is done using the THREE.TransformControls 'example'
    • translation, rotation, manipulation, etc.
  • object selection is done using DOM events and THREE.Raycaster
  • camera manipulation is done using EditorControls
    • including focus on object to adjust camera to best fit object on screen
  • render is triggered only when needed (change signals, ...)

I saw a few things that are not too clear :

  • Viewport maintains, for some reason, its own list of active objects used for Raycaster intersection.

    • some special handling is done for Helpers, using special 'picker' Mesh
  • Some of the scene state seem to be updated in the viewport :

    • I saw a call to object.updateProjectionMatrix()

Camera control

The code is in EditorControls.js (link).

The viewport can only provide view from cameras in the scene - there is no default views such as 'perspective', 'bottom', 'top', etc.

Only the default Camera can be controlled, the additional ones have to be moved using the SideBar properties.

Maybe a bug ? as the default Camera moves when trying to control another another camera in viewport

The behaviour, seen from the user is :

  • left-button, orbits camera around some center

  • right-button, pans the camera in a left/right/bottom/up fashion

  • mousewheel & middle-button, zooms camera towards the center

  • doubleclick, zooms on target object

  • one-finger touch, orbits camera

  • two-finger touch, zooms and pans

The code in EditorControls is not that clear... Let's see the API :

  • THREE.EditorControls = function(object, domElement)
  • this.rotate(delta)
  • this.zoom(delta)
  • this.pan(delta)
  • this.focus(target)
  • rest of methods are handling DOM events (down, move, up events for mouse and touch)

Looking at the new THREE.EditorControls call, object is in fact a camera - that should clarify things.

Looking at the rotate, pan, zoom call sites, I see the delta values are expressed in pixels, from the previous DOM event, stored in proper component(s) of a THREE.Vector3.

rotate

  • considers a sphere from center to camera.position
  • computes spherical coordinates from the camera position
  • adds an offset to these angles using delta and a rotationSpeed
    • does not seem to account for framerate
  • derives a new camera position from new angles and center

pan

  • scales delta according to a panSpeed and distance from camera to center
    • not accounting for framerate or camera FOV
  • transforms delta from eye-space to world-space (I suppose)
    • using delta.applyMatrix3( normalMatrix.getNormalMatrix( camera.matrix ) );
    • from threejs docs : "normal matrix is the inverse transpose of the matrix"
  • offsets camera position and center

zoom

  • scales delta (which only have Z component set) much like in pan
  • ensure the offset will not move camera past center
    • not accounting for camera near clip plane
  • transforms delta much like in pan
    • the camera looks to center so delta is moving from/back this point
  • move the camera position only

focus

  • from the target object bounding box
  • set the new camera center to the box center
  • find a view distance, using bounding sphere radius and some constants
    • not accounting for camera FOV
  • find the camera orientation to preserve it, giving a delta look vector
  • place the camera position along delta vector from center

Object transform gizmos

The code is in threejs examples/controls/js/TransformControls.js.

As I expected, the code is quite big and hard to follow.

I run out of time, so I'll try to get back to it later.

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