Raymond
Demo: Green Dragon
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Demo: CharSim

version 001

INTRODUCTION

Demo: The "Green Dragon" character running on the CharSim 3D character simulation framework.

For an introduction to the demo's origin, see the following blog entry.

What follows is a technical description of CharSim's framework after completion of version 001 as well as a description of the example character created for the demo.

Links to my working journal are provided for more detailed information and links to the web and book resources which helped me considerably. I emphasise "working" because any code or method I have outlined in a blog entry may be changed or improved in the final code, so always look at the source code for the working (in the different sense) implementation.

With that said...

Where can I get it?

All source code for the framework and Green Dragon demo is provided under the MIT License and is distributed via the resources below.

Open source framework

The framework code is open source and can be found in my Github repository.

The resources in the dragon character demo are not included in the open source licence, but they can be downloaded from the complete demo source code and binaries bundle.

Complete demo Source code and binaries

The complete demo source code and binaries (64bit) can be found here:

CharSim 001 Makefile project

FRAMEWORK

Charsim Architecture (Generalised Layout)

The framework is designed to be a very basic 3D game engine in order to simulate a 3D environment for the purpose of animating and rendering 3D models and environments. The bias of my demo is to direct and simulate a character's neck and face, so the framework provides support for Inverse Kinematics and vertex morphing. The resource management system takes a 3D mesh which has been rigged and given UV mapping and bone animation and creates an internal data type which can be animated and rendered by the framework.

For these capabilities to be possible, the framework therefore supports a scene graph, a renderer, timing, animation support (ClipRunner) and resource management. Input is also necessary for interactive environments and characters. These are discussed in more detail below.

For flexibility in creating custom 3D environments and characters, the framework/GameWorld.h class is intended to be inherited by a custom game world in an object-oriented manner where the developer can create their own scene graph elements and add them to the scene graph. This is obviously very crude, hence the 'very basic' part. 'To make CharSim resemble something more akin to a game engine, a scripting language could be implemented in order to abstract away from the tedious scene graph maintainence and allow scenes to be loaded as data separate from the compiled code. A step further would be to create a 3D editing environment which generates ids for elements and allows easy model creation and placement.

The source code is well-documented - every class header contains a description of its purpose.

SCENE GRAPH

In version 001, "ModelNodes" inherit from framework/SceneNode.h and "ControllerNodes" inherit from framework/SceneGraph.h. SceneNode and SceneGraph inherit from the abstract framework/GraphNode.h. In a future version there will be "CameraNodes" and "LampNodes" which inherit from SceneNode and can be controlled by ControllerNodes just like ModelNodes.

ModelNodes and ControllerNodes are added to the scene graph. ControllerNodes inherit from framework/SceneGraph.h and can contain other ControllerNodes and ModelNodes. This allows collections of scene elements to be controllable as a group by a single ControllerNode, and the effects of multiple ControllerNodes can be combined down the hierarchy. This allows for useful control, for example: a character could enter the driving seat of a ship and become a child element of the ship's control node, so that all the movements of the ship translate to the occupying character. Collision detection can still be performed on the ship and the occupant if the game designer desires the occupant to have more granular interaction with the environment.

When CameraNodes and LampNodes are added they can too be made child elements of ControllerNodes which would allow cameras and lights to be attached to moving scene elements for mobile POVs and lighting. In version 001, however, the camera is hardcoded into Renderer and lights are hardcoded into the shader program. These will be made SceneGraph nodes. The CameraNode will define the Renderer's camera settings, and LampNodes will add themselves to the Renderer's list of lights.

RENDERER

The scene graph and rendering system use an object-oriented design. The world must declare a new renderer object based on a selection of customised renderers which inherit from framework/Renderer.h (in version 001 there is only the base class Renderer, and it is designed to suit my Green Dragon demo rather than be an abstract class).

The renderer object is composited with every ModelNode, which update the renderer's World matrix and provide the OpenGL vertex buffer object and shader program.

For skinned models, the renderer must accept an array of bone transformation matrixes which are sent to the shader. For vertex morphing models, it must accept an array of shape indices and their associated weightings (see MATERIALS & SHADING).

TIMING

Interactive motion, predefined animation and simulated physics require knowledge of the elapsed time of the game and the time taken for each frame to render (AKA the time delta or frame interval). framework/GameLoop creates these values which are provided to scene elements with an ITimer interface.

In version 001 the time delta is calculated using Euler integration which is accurate enough for my short demo character interaction, but if I want to extend the demo with gameplay and physics simulation, I would implement a better integrator.

CLIPRUNNER

Each animateable scene model provides its own animation clip functions which can be called by hosting controller nodes and contains an instance of the framework/ClipRunner.h class. When an animation clip is played, it is added to the ClipRunner's array of clips. Every time ClipRunner is called each frame, it progresses the model's animation data forward depending on the elapsed game time and time delta.

In version 001 it supports animation clips in the form of skeletal animation and vertx morph targets and provides basic looping ability. It allows the speed of any animation type to be changed via a "warp" factor.

A character can have an unlimited number of vertex morph targets, but in version 001 only two may be blended by the shader in any one frame. Future versions will allow up to five targets to be blended in each frame.

RESOURCE MANAGEMENT

In version 001, the transfer of data follows the strict path:

HDD storage -> Host computer memory -> GPU device memory.

The resource manager's job is to transfer data from the HDD to the host memory and provide a central repository for scene resources. It ensures that only one copy of a resource exists in memory and that all resource pointers are removed when unused.

The resource manager should never transfer anything to the GPU. GPU device memory in version 001 is managed within the scene nodes and resources themselves. The nature of the resource manager means that so long as a mesh is not loaded multiple times from the HDD under different names, there will only be one array of those mesh vertices in the GPU memory.

Scene resources in version 001 include Meshes and Materials. Textures should be a resource, but in version 001 they are loaded directly by the materials/DiffuseMaterial.h class.

UTILITIES & SOLVERS

Bézier-based Inverse Kinematics (IK) is what I chose to use for the Green Dragon character's neck because it was more simple to control artistically. More traditional IK (i.e. the skeleton takes the shortest path possible towards a target) is implemented and can be found in framework/InverseKinematics.cpp, but in version 001 a mechanism for constraining joint angles has not yet been implemented, which is necessary to create natural movement in humanoid and animal characters.

In version 001, the Bézier-based IK is contained in characters/dragon/DragonBustNode.cpp but this will be moved to the InverseKinematics class.

In version 001, the Bézier method uses framework/CollisionDetection.h to compute the intersection between a finite line and a sphere. This utility class will be expanded to support other sorts of intersection testing such as between Axis-Aligned Bounding Boxes (AABB) if Bounding Volume Hierarchies (BVH) are implemented for general collision detection in a game scenario.

MATERIALS & SHADING

Though the framework is mostly written in C++, a lot of heavy lifting is done on the GPU device where the problem is well-suited for parallel computing.

Skinning defines the position of each vertex in a mesh as being based on the rotation of every bone and a given weighting. The shader already multiplies each vertex with a WVP transformation matrix, so it makes sense for it to handle bone transformation matrix multiplication as well.

Vertex morphing defines the position of each vertex as being based on a number of alternate mesh shapes and a given weighting. These weights and alternate mesh vertices are given ther own vertex buffer objects (VBOs) and are processed along with the regular vertex attributes.

Otherwise, in version 001, the shaders use Lambert+Phong shading as per the standard lighting model. I would like to look at physically-based illumination methods and shadowing methods (such as shadow buffers and shadow volumes).

The shaders are written using GLSL. In version 001, GLSL v120 is used for compatibility with older graphics cards, though v330 versions are also provided).

INPUT

Mouse and keyboard input is supplied by the GLFW 3 mouse and keyboard callback functions. GLFW also provides functions to determine specific key presses which allow detection of combined presses.

Controls in version 001:
  • Moving the mouse cursor around the screen causes the dragon to follow it. This is the base of the interactive demo in version 001.
  • Up and Down cursor keys spin the Green Dragon model. This is for testing purposes and isn't part of the interactive demo.

TECHNOLOGY & APIS

I chose OpenGL as the graphics API specification for its multiplatform nature. In version 001 CharSim is only compatible for Linux, but lack of Windows compatibility is listed as a bug.

I am intrigued by the use of WebGL for mobile game development, which is another reason I sought to familiarise myself with the GL way of doing things. I have worked with Microsoft Direct3D 11 and Direct3D 9 in previous projects, and a Microsoft DirectX (i.e. Direct3D 12) alternate backend for CharSim isn't out of the question.

GLFW provides the OpenGL context with a window and input handling. I chose it for its cross-platform compatibility, its flexible input integration and its accessible documentation.

A C++ XML parser I chose to parse the COLLADA (see MODEL FORMATS) files for rigged models. I tried using ASSIMP but it was unable to parse the files exported by Blender 2.70a's COLLADA exporter.

A C JSON parser I used to parse JSON model files with exported by the Three.js exporter for Blender.

Instead of creating my own linear algebra utility for graphics-related mathematical computation on the CPU as I did in a previous framework, I went with a third-party library, GLM. I chose it because it aims to match the functionality of shader languages.

  • Others

I had a lot of help from a number of web resources. Code borrowed from these is identified and referenced in the source code and LICENSE.

MODEL FORMATS

Due to my interest in WebGL, I decided to use a rigged model format which used JSON, a data format popularly associated with Javascript apps.

I didn't find the transformation data for the bones to be suitable, so I switched to COLLADA part-way through development. Future versions will likely use only COLLADA and perhaps implement an internal data storage format for faster scene loading.

A 3D model data storage format which supports rigging armatures and armature animation.

PIPELINE TOOLS

Blender is a fantastic free and open source 3D modelling and animation suite. It also supports video editing and compositing. Needless to say it's what I used to

An invaluable free and Photoshop-like image editor. Equally good for quickly editing a texture, or for working on a large drawing or painting.

Its brush tools are quite limited, so for better painting support look at Krita.

More painting-oriented than GIMP. Has great textured brushes which I used to create the skin on the Green Dragon.

A tool for turning texture bitmaps into mipmap'd DDS textures.

COMPATIBILTY & SUPPORT

I developed the framework on a Linux system so it should compile as long as you have the following requirements:

  • GLFW 3.0.4 (static library, but you can modify the Makefile for a dynamic library)
  • GLM 3.5
  • JSON-C library >= 0.11
  • OpenGL implementation

The "Green Dragon" character resources (meshes and textures) are not provided in the GitHub repo, but can be downloaded from the project homepage. Binaries containing the demo are available there also.

I can't get JSON-C to work in Windows so I am switching to RapidJSON. I may even switch to an entirely Collada-based model format, removing the need for JSON parsing.

CHARACTERS

GREEN DRAGON

The demo portrays a green dragon from the shoulders-up. He/she has a rigged neck and interacts with the mouse cursor when the player moves it inside the window to demonstrate real-time interaction with the armature system and its IK solver.

At all times, though especially noticeable when the player moves the cursor outside of the window, the dragon displays a gentle swaying motion to make the character feel more natural. This is a demonstration of armature animation: the animation was created in Blender and exported in the chosen model format and is played by the animation subsystem (ClipRunner).

In future versions I want to explore procedural animation, which is so far uncovered by the demo. The idle swaying motion could be generated using procedural noise or a Markov chain, to give examples.

The character has two vertex morph targets in the form of facial expressions: "Blink" and "Snap". In version 001 these are simply placed on random timers (I suppose that counts as really basic procedural animation), but they and more expressions could be made to be trigged by interactive gameplay.

I am considering interactive gameplay elements to explore effects such as said procedural animation and as particle effects: flocks of enemies (perhaps fireflies) could swarm around the dragon's environment and the player clicks to cause the dragon to breath fire on or snap at an enemy in front of his/her snout.

Last updated: 2014-03-19 11:28:00