Using URIs in COLLADA
URIs are used extensively in COLLADA documents to reference other COLLADA elements or external resources such as texture files, shader source code, and so on. A URI must be formatted properly for the application to find the resource the URI identifies. This article describes how to use URIs in COLLADA.
URIs in COLLADA
The COLLADA schema uses the xs:anyURI type to represent URIs. xs:anyURI allows for URI references in addition to URIs, so you can use either a URI or a relative URI reference.
Some examples of URI usage in COLLADA include
- The target attribute of <instance_material> elements
- The url attribute on <instance_geometry>, <instance_node>, and <instance_effect> elements
- The <image>/<init_from> element
For URIs that reference COLLADA elements (like the url attribute on the <instance_geometry> element), the fragment portion identifies the element by its id attribute. An example of this:
<instance_geometry url="file:///models/car.dae#carGeometry />
This <instance_geometry> references the <geometry> element whose ID is
carGeometry in the document
<instance_geometry url="../car.dae#carGeometry />
In this example a relative reference is used. It'll need to be resolved to a URI before the application can obtain the referenced <geometry> element.
Base URIs in a COLLADA document
To resolve relative URI references, a base URI is needed. In COLLADA, the base URI can be specified in the root <COLLADA> element:
<COLLADA xmlns="http://www.collada.org/2005/11/COLLADASchema" version="1.4.1" xml:base="file:///home/sthomas/models/duck.dae">
If the xml:base attribute isn't specified, then the base URI is the document URI. So if the document URI is "file:///car.dae", then that'll be the base URI used to resolve any relative references in the document.
Paths versus URIs
A common mistake is to use Windows or Linux paths as URIs. A file path needs to be converted to a file scheme URI before being passed to the COLLADA DOM. ((EDITOR: This page needs the following improvement: This also applies to COLLADA, not just the DOM, right? ))
A Windows absolute path must be preceded by a forward slash character '/'. An example:
Note: Windows file paths are not proper URIs.
Only the slash (/) character is used as a path delimeter in URIs. Windows uses the backslash (\) to delimit path segments. Using the backslash can result in incorrect URI processing. "A\B\C" is considered one path segment. If using "file:///A\B\C" as a base URI and trying to resolve "../doc.dae" the result will be "file:///doc.dae" and not "file:///A\B\doc.dae" as one might have expected.
The following are some examples of converting a file path to a URI.
|Example Description||File Path||URI|
|Windows absolute path||
|Windows relative path||
|Linux absolute path||
|Linux relative path||