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Re: [Public WebGL] WEBGL_debug_shader_precision extension proposal




First, I think this discussion has slipped sideways into the wrong thread so I've just replied to the list only so this doesn't continue. I think replies to the main thread on this topic would be best if possible.


Second, we're really happy to provide the background on why we've developed the proposal as is. We really value the feedback and constructive criticism provided so far and I hope this can help resolve some of the angst this also seems to be generating 8/

Our first idea was simply to use RGBA/UNSIGNED_SHORT_4_4_4_4 but the initial feedback we got (which made good sense to us) was that a 3 channel data structure like RGB/UNSIGNED_BYTE would save a number of steps in the conversion/unpacking process and would therefore be more efficient.

We created some tests and ran them across various browsers and during that process also realised that we could get better performance and channel usage by using RGB/UNSIGNED_SHORT_5_6_5 - already a unit16 array and only 3 channels instead of 4.

And this is what we then took to this wider group for more feedback and suggestions.

As far as I'm aware using LUMINANCE_ALPHA/UNSIGNED_BYTE is not as effective as the LUMINANCE_ALPHA is internally converted to an RGBA with the luminance spread across the R, G and B channels. But please correct me if this is not right?

Our primary goal here was simply to find a pragmatic approach that could easily be developed against WebGL 1.x right now - so people could start using Depth Camera Streams via WebGL in the "very near future".

Then in WebGL 2.x we could just move to using RED_INTEGER so we would then move back to using just standard WebGL with no extension. But waiting for only this option seemed like a long delay when a feasible stop-gap approach seemed available.

If there are other options that could help us meet our primary goal then we'd definitely like to hear about it.

Yet it does seem to me that the WEBGL_texture_from_depth_video extension does define a very minimal "novel behavior of a piece of software". At the moment there is no way to upload a <video> frame that includes a depth data track. This extension enables that - which makes this novel.

Florian, it sounds like you're saying that our only option is that users/devs really have to wait for WebGL 2.x to access Depth Camera Tracks - am I understanding that correctly?

roBman


On 12/11/14 6:19 PM, Florian Bösch wrote:
A specifications purpose is to describe a novel behavior of a piece of software/hardware, and how a user has to use those new capabilities. I believe that every specification for OpenGL, OpenGL ES and WebGL follows this idea, and I believe every extension does too (in any case, there is no functional difference between an extension and the core specification, since an extension modifies the specification). And so, it follows that any extension has to pass muster required to pass, as if it where to be included in the specification (because that's where it might end up in in due time).

The fixation on the 5-6-5 format primarily has one motivation: Depth data happens to be 16-bit for arbitrary reasons, and 5-6-5 happens to be a 16-bit internal format, so the first idea anyone would have, just mash them together and call it a day, no wait, draft an extension for it, too. It's an "it happens to work" cum extension.

There are in fact other 16-bit formats that depth could be packed into:

  * unsigned byte and luminance alpha
  * rgba and unsigned short 4-4-4-4
  * rgba and unsigned short 5-5-5-1

And many more coming with WebGL 2.0

Why 5-6-5 should be superior to any of the others is a mystery to me. Personally, I think luminance alpha is more convenient, because then conversion to a 0-1 scaled depth can be done much simpler:

    vec2 texel = texture2D(mydepth, texcoord).xw;
    float depth = texel.x + texel.y/255.0;


The real problem is, this isn't a technical specification about a behavior. This is a specification how the USER has to behave. It's a leaky abstraction. To my knowledge, there isn't any piece of specification or extension that resorts to a similar hack. It's a precedent.


A WebGL (or OpenGL or ES) extension modifies the specification. Let's suppose you where to encode that behavior in the core specification (that would never fly). But what would a core functionality likely do? Well let's assume that, for whatever reason, you're lacking an appropriate internal format for the kind of format you'd like to store.

Introduce a new internal format (gl.DEPTH), and a new external format (gl.UNSIGNED_SHORT_DEPTH) modify the gl.tex(Sub)Image2D call to accept these parameters, such that you've fully described a useful internal format (mipmappable, interpolatable, mixable, blendable, coveragable, renderable gl.DEPTH) and fully specified a transfer format (it's an unsigned short 16bpp/16bbc). You might introduce a new GLSL sampler type (samplerDepth) and a new texturing function (textureDepth), although that's a bit frowned upon I think, and instead the behavior would probably be that texture2D just returns the depth on all channels of the returned vec4. Afaik, this is the kind of thing that could pass muster for inclusion in the core specification. And in fact, it has. All of those things, have passed muster previously.

So I'm strictly against an extension, that encodes a "it happens to work" behavior, that is less about a technical specification, but an external format description and specifying how a user has to behave. That's not an extension, that's a hack.


On Wed, Nov 12, 2014 at 3:26 AM, Jeff Gilbert <jgilbert@mozilla.com <mailto:jgilbert@mozilla.com>> wrote:


    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" <khronos@greggman.com
    <mailto:khronos@greggman.com>>
    To: "Mark Callow" <khronos@callow.im <mailto:khronos@callow.im>>
    Cc: "Florian Bösch" <pyalot@gmail.com <mailto:pyalot@gmail.com>>,
    "Jeff Gilbert" <jgilbert@mozilla.com
    <mailto:jgilbert@mozilla.com>>, "Olli Etuaho" <oetuaho@nvidia.com
    <mailto:oetuaho@nvidia.com>>, "Kenneth Russell" <kbr@google.com
    <mailto:kbr@google.com>>, "public webgl" <public_webgl@khronos.org
    <mailto:public_webgl@khronos.org>>
    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 <khronos@callow.im <mailto:khronos@callow.im>> wrote:

    >
    >
    > > On Nov 12, 2014, at 7:19 AM, Florian Bösch <pyalot@gmail.com
    <mailto:pyalot@gmail.com>> 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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