[Public WebGL] WEBGL_debug_shader_precision extension proposal

Tibor Ouden, den [email protected]
Fri Nov 14 05:53:03 PST 2014

To easily change the precision to inspect the effect on the computation is
useful debug functionality.

But a next request could be to have control over the exact count of bits in
the significand.
And then I would like to specify the amount of guard bits used.
And then for sqrt() I would like to be able to specify a certain look up
table algorithm because the gpu
in device xyz uses that.
Change the treatment of subnormal numbers ?
Emulate the 'exact' numerical behaviour of gpu xyz ?
... etc.

I think this is part of a larger set of debug features related to numerical
precision issues.
I think extensions should be related to the functionality of WebGL (running
code from a web page on the gpu)

Everything related to enhancing the implementation process of a polished
WebGL application is 'tool' functionality
and should be done in a library if possible in my opinion. It allows for
more flexibility and prevents the WebGL
spec from increasing.
If you allow this functionality to be an extension where do you draw the
line ?

I have nothing against a browser vendor implementing a full blown shader
debugger in their browser.
But that is not part of the WebGL spec.

While attempting to implement a physics engine on the gpu (
using WebGL, I encountered some interesting numerical issues.

Due to the experiences with that project I started working on a shader
debugger in javascript which allows
the user to inspect every single variable by rewriting the shader on the
fly and warn against common mistakes (at least for me :-) ).
Specifying reduced precision is an interesting feature, as is selecting the
'exact' numerical
profile of target device xyz.
Also warning against constructs like vec3(float, vec3) (silent drop of last
component of 2nd argument)
Not setting gl_PointSize when rendering points

This done in in-between-projects-time so will take a while.



2014-11-14 11:10 GMT+01:00 Olli Etuaho <[email protected]>:

>   I agree that having a tool for detecting other kinds of undefined
> shader behavior would be useful. There's multiple kinds:
> -Math function limitations (asin, acos, atan, pow, log, log2, sqrt,
> inversesqrt, clamp, smoothstep all have these)
> -Accessing textures inside non-uniform control flow
> -Reads from uninitialized shader variables and missing return values
> -Not writing to gl_Position
> -Some details of integer computations
>  But even with all of these possibilities of undefined behavior, most of
> the errors I've seen are still definitely related to precision. To put some
> data behind my claims that they're extremely widespread: they're in some
> three.js examples, some Blend4Web demos, all except the latest version of
> Babylon.js, turbulenz engine, some other proprietary WebGL content, and as
> a guesstimate in half of recent shader demos in glslsandbox and shadertoy.
> Of course you won't ever see the precision issues unless testing on a
> variety of mobile hardware.
>  If someone is willing to put in the work to implement a more versatile
> shader debugging library, that would be useful, but I think the precision
> emulation can still stand on its own. Having it as an extension in browsers
> doesn't prevent building more things on top of it.
>  -Olli
>  ------------------------------
> *From:* Gregg Tavares <[email protected]>
> *Sent:* Friday, November 14, 2014 3:31 AM
> *To:* Olli Etuaho
> *Cc:* Jeff Gilbert; Gregg Tavares; Mark Callow; Florian Bösch; Kenneth
> Russell; public webgl
> *Subject:* Re: [Public WebGL] WEBGL_debug_shader_precision extension
> proposal
>  As an example of something that I'd want added to this and an argument
> for making it a library,
>  I'd like to see something that re-wrote the shaders to find all the
> undefined behavior. For example I just tried to use this shader on iOS
>       http://threejs.org/examples/webgl_shader.html
>  It turns out it's calling pow(x, y) with x < 0 which is undefined
> according the spec and therefore doesn't work on all GPUs.
>  That seems like something a shader debugging re-writing library could
> easily do, maybe by rewriting pow to some kind of expression that returns a
> different color by mod(gl_FragCoord, 2) or something such that the results
> hopefully stick out. Personally I've found these errors far more common
> than precision errors but that might just be my experience.
>  It seems like other re-writes for debugging would be useful too. You
> could probably implement shader debugger. But if you make it an extension
> no one else can't augment it.
>  Also not every browser uses ANGLE AFAIK.
>  -gregg
> On Wed, Nov 12, 2014 at 3:57 AM, Olli Etuaho <[email protected]> wrote:
>> I do see the upsides of having this as a library, but as I stated before,
>> the best way to implement said library would be to run ANGLE's shader
>> compiler through emscripten. This is possible to do whether the extension
>> is accepted or not, but from a purely technical perspective, it's much more
>> work and overhead. As counterpoints to Gregg's message:
>> -The specification is fairly small, so making it exact is not very hard.
>> -The specification can still spend a while as a proposal/draft, so it can
>> be more freely edited and the issues can be worked out.
>> -I'll be doing the implementation work in ANGLE. ANGLE maintainers
>> already expressed that they'd likely be willing to accept the patches.
>> After that, it's fairly trivial to expose the extension. I already have a
>> working prototype for Chromium. So I hope it will require only a minimal
>> amount of work from anyone else.
>> -I can't foresee any pressing need to extend and update the extension.
>> The extension should be compatible with both ESSL 1.00 and ESSL 3.00
>> already in its current form. The need to do large updates to it would arise
>> only if WebGL switched to a drastically different shading language.
>> -This is an extension for testing, so not having support in every browser
>> is more of a slight inconvenience rather than something that would greatly
>> hinder its usefulness.
>> -If it was a library, a spec like this would still be beneficial, so that
>> what it does would be clear to the user.
>> I also can't stress enough how widespread precision-related shader bugs
>> are. I've seen them frequently in content developed by professional and
>> hobbyist developers alike, every once in a while even in content that was
>> specifically written with mobile devices in mind. If you're still not
>> convinced, I'll have to look at other alternatives besides the extension,
>> but something needs to be done, and I think tooling like this is a big part
>> of the answer.
>> -Olli
>> ________________________________________
>> From: Jeff Gilbert <[email protected]>
>> Sent: Wednesday, November 12, 2014 4:26 AM
>> To: Gregg Tavares
>> Cc: Mark Callow; Florian Bösch; Olli Etuaho; Kenneth Russell; public webgl
>>  Subject: Re: [Public WebGL] WEBGL_debug_shader_precision extension
>> proposal
>> I agree with Gregg.
>> I will add that if it's something that we feel is important enough as a
>> working group, we could canonize the library and maintain it as part of our
>> github repo.
>> -Jeff
>> ----- Original Message -----
>> From: "Gregg Tavares" <[email protected]>
>> To: "Mark Callow" <[email protected]>
>> Cc: "Florian Bösch" <[email protected]>, "Jeff Gilbert" <
>> [email protected]>, "Olli Etuaho" <[email protected]>, "Kenneth
>> Russell" <[email protected]>, "public webgl" <[email protected]>
>> Sent: Tuesday, November 11, 2014 6:16:19 PM
>> Subject: Re: [Public WebGL] WEBGL_debug_shader_precision extension
>> proposal
>> If this works just fine as a JavaScript library why add it as an
>> extension?
>> As an extension what it does has to be specifically specified.
>> As an extension it can't be upgraded without making and proposing a new
>> extension.
>> As an extension it passes all work to the browser vendors who each need to
>> implement it
>> As a library it can be updated and extended whenever
>> As a library it only needs one implementation and everyone can use it
>> As a library it can do whatever it wants, no spec needed
>> From the discussion above it doesn't seems like it needs to be an
>> extension. It doesn't seem like there is some specific OpenGL
>> functionality
>> that needs to be exposed to make it possible. It also doesn't sound like a
>> speed issue given that the resulting shaders are up to 10x slower.
>> Also as a library it should be easy to patch it the same way the WebGL
>> Inspector patches itself in or various other libraries that patch things
>> like WebGLRenderingContext.prototype.compileShader
>> On Tue, Nov 11, 2014 at 2:23 PM, Mark Callow <[email protected]> wrote:
>> >
>> >
>> > > On Nov 12, 2014, at 7:19 AM, Florian Bösch <[email protected]> wrote:
>> > >
>> > > What's wrong with it is that it does not allow you to isolate an issue
>> > with any of your shader code buried in use somewhere in your
>> application.
>> > >
>> >
>> > You have to find either the buried shader code or the buried call to
>> > compileShader for that shader. These efforts may or may not be much
>> > different, depending on the structure of your code. I would not object
>> to
>> > supporting both an API toggle and a pragma, getting the best of both
>> worlds.
>> >
>> > Regards
>> >
>> >     -Mark
>> >
>> >
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