COLLADA：3D数字资产交换标准与场景描述

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"COLLADA是一种开放的3D数字资产交换标准，主要目的是允许不同的3D创作和处理软件之间无缝地交换数据。它是由SCEA（Sony Computer Entertainment Inc.）发起，并得到了Autodesk、XSI等多家知名厂商的支持。COLLADA的核心是基于XML的规范，名为Digital Asset Schema Release 1.5.0，该规范于2008年发布。此标准的版权由Khronos Group和Sony Computer Entertainment Inc.共同持有。 COLLADA不仅用于3D建模工具之间的数据交换，还能够作为一种场景描述语言，服务于实时渲染场景的构建。它包含了一个完整的视觉编译系统，支持OpenGL ES着色语言，允许开发者创建和封装着色器，从而在不同3D创作工具间协同制作OpenGL或OpenGL ES应用及资产。在COLLADA中，DOM（Document Object Model）库是一个关键部分，它提供了一种结构化的方式来表示和处理场景中的各种元素，如几何体、材质、光照、摄像机等。通过使用COLLADA DOM，开发者可以读取和操作场景文件，其编程接口设计类似于COM（Component Object Model）。DOM使得对3D场景的复杂操作变得更为简便。此规范强调了版权保护，未经许可，不得复制、分发或以任何形式利用其中的内容。然而，Khronos Group允许其当前的Promoter、Contributor或Adopter成员无修改地复制和重新分发此规范的副本，但不得收费。 COLLADA的使用对于3D内容创作者和开发者来说具有重要意义，因为它打破了不同3D软件间的兼容性障碍，提升了工作效率，促进了跨平台3D项目的协作。无论是在游戏开发、虚拟现实应用还是影视特效制作等领域，COLLADA都扮演着至关重要的角色，它提供了一种标准化的方法来管理、存储和共享复杂的3D数字资产。"

1-4 COLLADA – Digital Asset Schema Release 1.5.0

April 2008

Catalyze Digital Asset Schema Design

Goal: To be a catalyst for digital-asset schema design among developers and DCC, hardware, and

middleware vendors.

There is fierce competition among and within market segments: the DCC vendors, the hardware vendors,

and the middleware vendors. But all of them need to communicate to solve the digital-content problems for

developers. Not being able to collaborate on a common digital-asset language format has a direct impact

on the overall optimization of the market solutions:

• Hardware vendors are suffering from the lack of features exposed by DCC tools.

• Middleware vendors suffer because they lack compatibility among the tool chains.

• DCC vendors suffer from the amount of support and specific development required to make

developers happy.

• Developers suffer by the huge amount of investment necessary to create a working tool-chain.

None of these actors can lead the design of a common language format, without being seen by the others

as factoring a commercial or technical advantage into the design. No one can provide the goals that will

make everybody happy, but it is necessary that everybody accept the format. It is necessary for all major

actors to be happy with the design of this format for it to have wide adoption and be accepted.

Sony Computer Entertainment (SCE), because of its leadership in the videogame industry, was the right

catalyst to make this collaboration happen. SCE has a history of neutrality toward tool vendors and game

developers in its middleware and developer programs, and can bring this to the table, as well as its desire

to raise the bar of quality and quantity of content for the next-generation platforms.

The goal is not for SCE to always drive this effort, but to delegate completely this leadership role to the rest

of the actors and partners when the time becomes appropriate. Note that:

• Doing this too early will have the negative effect of partners who will feel that SCE is abandoning

COLLADA.

• Doing this too late will prevent more external involvement and long-term investment from

companies concerned that SCE has too much control over COLLADA.

April 2008

Chapter 2:

Tool Requirements and Options

Introduction

Any fully compliant COLLADA tool must support the entire specification of data represented in the schema.

What may not be so obvious is the need to require more than just adherence to the schema specification.

Some such additional needs are the uniform interpretation of values, the necessity of offering crucial user-

configurable options, and details on how to incorporate additional discretionary features into tools. The goal

of this chapter is to prioritize those issues.

Each “Requirements” section details options that must be implemented completely by every compliant tool.

One exception to this rule is when the specified information is not available within a particular application.

An example is a tool that does not support layers, so it would not be required to export layer information

(assuming that the export of such layer information is normally required); however, every tool that did

support layers would be required to export them properly.

The “Optional” section describes options and mechanisms for things that are not necessary to implement

but that probably would be valuable for some subset of anticipated users as advanced or esoteric options.

The requirements explored in this chapter are placed on tools to ensure quality and conformance to the

purpose of COLLADA. These critical data interpretations and options aim to satisfy interoperability and

configurability needs of cross-platform application-development pipelines. Ambiguity in interpretation or

omission of essential options could greatly limit the benefit and utility to be gained by using COLLADA. This

section has been written to minimize such shortcomings.

Each feature required in this section is tested by one or more test cases in the COLLADA Conformance

Test Suite. The COLLADA Conformance Test Suite, under development by The Khronos Group, is a set of

tools that automate the testing of exporters and importers for applications such as Maya

, XSI

, and 3ds

Max

. Each test case compares the native content against that content after it has gone through the tool’s

COLLADA import/export plug-in.

Exporters

Scope

The responsibility of a COLLADA exporter is to write all the specified data according to certain essential

options.

Requirements

Hierarchy and Transforms

Data Must be possible to export

Translation Translations

Scaling Scales

Rotation Rotations

Parenting Parent relationships

Static object instantiation Instances of static objects. Such an object can have multiple transforms

Animated object instantiation Instances of animated objects. Such an object can have multiple transforms

Skewing Skews

2-2 COLLADA – Digital Asset Schema Release 1.5.0

April 2008

Data Must be possible to export

Transparency / reflectivity Additional material parameters for transparency and reflectivity

Texture-mapping method A texture-mapping method (cylindrical, spherical, etc.)

Transform with no geometry It must be possible to transform something with no geometry (for example,

locator, NULL)

Materials and Textures

Data Must be possible to export

RGB textures An arbitrary number of RGB textures

RGBA textures An arbitrary number of RGBA textures

Baked procedural texture coordinates Baked procedural texture coordinates

Common profile material

A common profile material (PHONG, LAMBERT, etc.)

Per-face material Per-primitive materials

Vertex Attributes

Data Must be possible to export

Vertex texture coordinates An arbitrary number of Texture Coordinates per vertex

Vertex normals Vertex normals

Vertex binormals Vertex binormals

Vertex tangents Vertex tangents

Vertex UV coordinates Vertex UV coordinates (distinct from texture coordinates)

Vertex colors Vertex colors

Custom vertex attributes Custom vertex attributes

Animation

All of the following kinds of animations (that don’t specifically state otherwise) must be able to be exported

using samples or key frames (according to a user-specified option).

Animations are usually represented in an application by the use of sparse key frames and complex controls

and constraints. These are combined by the application when the animation is played, providing final

output. When parsing animation data, it is possible that an application will not be able to implement the full

set of constraints or controllers used by the tool that exported the data, and thus the resulting animation

will not be preserved. Therefore, it is necessary to provide an option to export fully resolved transformation

data at regularly defined intervals. The sample rate must be specifiable by the user when samples are

preferred to key frames.

Exporting all available animated parameters is necessary. This includes:

• Material parameters

• Texture parameters

• UV placement parameters

• Light parameters

• Camera parameters

• Shader parameters

• Global environment parameters

• Mesh-construction parameters

• Node parameters

• User parameters

Specification – Tool Requirements and Options 2-3

April 2008

Scene Data

Data Must be possible to export

Empty nodes Empty nodes

Cameras Cameras

Spotlights Spotlights

Directional lights Directional lights

Point lights Point lights

Ambient lights Ambient lights

Exporter User Interface Options

Data Must be possible to export

Export triangle list Triangle lists

Export polygon list Polygon lists

Bake matrices Baked matrices

Single <matrix> element

An instance document that contains only a single <matrix> elemen

t for

each node. (See the following “Single <matrix> Element Option” discu

ssion.)

Single <matrix> Element Option

COLLADA allows transforms to be represented by a stack of different transformation element types, which

must be composed in the specified order. This representation is useful for accurate storage and/or

interchange of transformations in the case where an application internally uses separate transformation

stages. However, if this is implemented by an application, it should be provided as a user option, retaining

the ability to store only a single baked <matrix>.

A side effect

of this requirement is that a

ny other data that target specific elements inside a transformation

stack (such as animation) must target the matrix instead.

Command-Line Operation

It must be possible to run the full-featured exporter entirely from a command-line interface. This

requirement’s purpose is to preclude exporters that demand user interaction. Of course, a helpful

interactive user interface is still desirable, but interactivity must be optional (as opposed to necessary).

Optional

An exporter may add any new data.

Shader Export

An exporter may export shaders (for example: Cg, GLSL, HLSL).

Importers

Scope

The responsibility of a COLLADA importer is to read all the specified data according to certain essential

options.

In general, importers should provide perfect inverse functions of everything that a corresponding exporter

does. Importers must provide the inverse function operation of every export option described in the

“Exporters” section where it is possible to do so. This sectio

n desc

ribes only issues where the requirements

placed on importers diverge or need clarification from the obvious inverse method of exporters.

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COLLADA：3D数字资产交换标准与场景描述

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