I'm getting a bit confused with all this.
Let's put it simple: I'm writing a WebGL application through Emscripten
(not doing any Javascript code at all, pure C++), and I want to use the
fairly new WEBGL_draw_buffers WebGL extension that is available on my
browser's version.
In theory WEBGL_draw_buffers extension, when written in pure javascript,
needs to be fetched with a call gl.getExtension('WEBGL_draw_buffers'),
which returns a function allowing us to do the same as native OpenGL ES
glDrawBuffers function.
But when I include the GL headers found in the Emscripten folder, I do get
the glDrawBuffers function, and I can call it in my C++ code, no link error
either upon Emscripten compilation. I tried it and it *seems* to work,
since in my fragment shader I can write to gl_FragData[i] as opposed to the
usual gl_FragColor. It does require me to add #extension
GL_EXT_draw_buffers : require in my fragment shader at the top to make sure
the extension is supported, but it all *seems* to work?
Can anyone explain me what is happening under the hood because I'm a bit
confused. Perhaps we don't need any change to Emscripten at all as it is?
On Tuesday, August 26, 2014 7:21:24 PM UTC-4, msc wrote:
>
> On 2014/08/26 15:47, Alon Zakai wrote:
>
> Oh, so in normal non-WebGL GL, there isn't a method to enable an
> extension?
>
> Correct.
>
> They are all enabled by default?
>
> Yes.
>
> Then yes, it sounds like we need to add an option to enable an
> extension, emscripten_enable_webgl_ext(const char*) or such. I'm surprised
> we didn't run into this before. I think emscripten might already be
> enabling some extensions by default, that sounds familiar.
>
> Yes I think this option is needed.
>
> In native OpenGL {,ES} the tokens are all defined in the .h files so can
> be used in code without any additional checks. Function pointers are also
> defined. To use an extension that does not add any functions, the
> application simply uses it, preferably checking for its existence first.
> For extensions that add functions, the app queries the function pointer
> values setting those into the existence function pointer variables.
>
> In WebGL a call to getExtension, which is defined as "object?
> getExtension(DOMString name", returns an object which contains any
> constants or functions defined by the extension. This has to be reconciled
> with the native OpenGL way of doing things.
>
> Regards
>
> -Mark
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