Platform/GFX/WebGL/Contribute/Extensions

Adapted from bug 728354.

Extension drafts for WebGL are found at the Khronos WebGL Extension Registry.

Good models for how to add support for new WebGL extensions are: bug 684853 and bug 728354

General Approach

For the purposes of this page, we will pretend there is a WebGL extension named WEBGL_foo_bar that we're trying to implement.

Adding the IDL

Add the IDL from the spec to mozilla-central/dom/interfaces/canvas/nsIDOMWebGLRenderingContext.idl, with a new UUID:

Let's call this interface, nsIWebGLExtensionFooBar. A C++ header will be auto-generated from that IDL, and will define a nsIWebGLExtensionFooBar class. But that's just a base class. Below we'll manually define a WebGLExtensionFooBar (notice: no nsI prefix) class, inheriting nsIWebGLExtensionFooBar.

The Goop

Edit these files, add goop similar to what there is for WebGLExtensionStandardDerivatives / nsIWebGLExtensionStandardDerivatives:

• mozilla-central/dom/base/nsDOMClassInfo.cpp
• mozilla-central/dom/base/nsDOMClassInfoClasses.h
• mozilla-central/js/src/xpconnect/src/dom_quickstubs.qsconf

The Extension Class

Edit mozilla-central/content/canvas/src/WebGLExtensions.h and define the WebGLExtensionFooBar class, inheriting nsIWebGLExtensionFooBar, similar to what's being done for WebGLExtensionStandardDerivatives.

You shouldn't have to do anything nontrivial there, as this class only has to expose the constants that were already defined in the IDL, so it's already inheriting them. I would implement the (empty) constructor and destructor inline there, to save the hassle of having to add a new .cpp file just for them.

Exposing the Extension

Now let's do the work to actually expose this extension, in WebGL getExtension and getSupportedExtensions methods. The file to edit is mozilla-central/content/canvas/src/WebGLContext.cpp

The functions exposed to scripts are WebGLContext::GetExtension and WebGLContext::GetSupportedExtensions. They call another internal function that you'll have to edit as well: WebGLContext::IsExtensionSupported. That's where you'll have to decide whether this extension is supported or not by the client. If you have to query the GL_EXTENSIONS string of the underlying OpenGL context, call gl->IsExtensionSupported(enum value). Indeed, we are checking for a list of extensions when we create a OpenGL context, so that subsequent checking for them is really fast. If the OpenGL extensions you need to check for haven't yet been added to the list that we check for, you'll have to edit these files in a straightforward way (search for IsExtensionSupported):

• mozilla-central/gfx/gl/GLContext.h
• mozilla-central/gfx/gl/GLContext.cpp

Implementing the Extension

Finally, let's implement what this extension actually does. The spec describes that. The file you have to edit to do that, is mozilla-central/content/canvas/src/WebGLContextGL.cpp, though you might also need to make some more edits to GLContext, or elsewhere in WebGLContext.

Sometimes, what this extension does is modify the behavior of some existing methods, it should be straightforward to see what to do there. The only thing is that you'll need to use these new symbolic constants, like TEXTURE_MAX_ANISOTROPY. Rather than trying to use the constants from the extension class, add #defines for them in mozilla-central/gfx/gl/GLDefs.h. (Notice we use LOCAL_GL_ prefixes there to avoid some conflicts)

Adding Tests

Unfortunately, odds are fairly high that we don't have a test case for this particular extension! It would be very useful to write one, and even more useful to write one that can be added to the conformance test suite.

Other Remarks

• General info about getting started with Gfx hacking: Platform/GFX/Contribute

• MXR and DXR are useful code search tools.

• Test your code by running the WebGL conformance test suite. (compare to a test run without your patch)

  Or as a mozilla mochitest: (assuming that you are in your mozilla-central directory, and that obj-firefox-debug is your object directory)

  TEST_PATH=content/canvas/test/webgl/test_webgl_conformance_test_suite.html make -C obj-firefox-debug/ mochitest-plain