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
An ordinary Windows or Linux file path is not a valid URI. A file scheme URI is used to represent a file on the local machine. The file scheme is described at http://tools.ietf.org/html/rfc1738. File paths need to be converted to file scheme URIs before they can be used properly by COLLADA applications. If the base URI's scheme is file (which is usually the case), then you can use a relative reference to the file instead specifying a full file scheme URI. When the reference is resolved by the application it'll become a valid file scheme URI.
Below are some examples of specifying a file via a relative URI reference in COLLADA. These examples assume that the base URI's scheme is file, otherwise the reference won't resolve correctly. You might use these examples in the <image>/<init_from> element to refer to an image file on disk.
|Example Description||File Path||URI Reference|
|Windows absolute path||
|Windows relative path||
|Linux absolute path||
|Linux relative path||
Looking at the second example more closely, I could write
<image> <init_from>../folder/image.tga</init_from> </image>
in my COLLADA document. Suppose the base URI is
file:///C:/models/maya/car.dae. An application will resolve the relative reference against the base URI to get
file:///C:/models/folder/image.tga, which is a valid file scheme URI.
Note: Windows file paths are not proper URI references. Only the slash (/) character is used as a path delimeter in URIs or URI references. 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.