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Re: [Public WebGL] The state of MRTs



On Tue, Mar 27, 2012 at 2:01 PM, Marco Di Benedetto <spattija@gmail.com> wrote:
> ÂOn Tue, Mar 27, 2012 at 9:52 PM, Florian BÃschÂ<pyalot@gmail.com>Âwrote:
>
>> For some weird reason OpenGL ES choose not to forward-compatible the FBO
>> featureset by allowing for N>1 rendertargets. I don't understand why that
>> was done this way, if it hadn't been, we could get a clear picture now what
>> mobile devices actually support MRT and they would not have to
>> discuss/ratify another set of extensions for ES. Any insight as to what the
>> ES-WG plan in regards to MRT would be, would be appreciated.
>
>
> MRT is NOT ONLY a matter of defining GL_COLOR_ATTACHMENTi, i > 0.
> As I wrote in a couple of mails (that seem to have been ignored...)Âin this
> thread, there are functionalities exposed glDrawBuffers() that are extremely
> handy, that allow to decouple shaders from algorithms, and that are at the
> core of MRT.
> Simply defining other enums without glDrawBuffers() doesn't "give justice"
> to MRT idea.
> Nonetheless, to strictly define MRT including glDrawBuffers() is not
> complex, even if not as easy as just increasing N.
>
> On Tue, Mar 27, 2012 at 9:52 PM, Florian BÃschÂ<pyalot@gmail.com>Âwrote:
>>>
>>> MRT is a perf improvement. A huge one.
>>
>> I do actually think that MRT support is a bigger boon to mobile devices
>> than to desktops. Desktops often do have enough juice to simply render a
>> scene multiple times. Doing the same on mobiles is disproportionally more
>> expensive because they have far less GPU power. So speeding up techniques
>> that require multiple outputs (like normal, depth, color, specularity etc.)
>> is a disproportional advantage in mobiles.
>
>
> To my memory, it's been since the first rumors about WebGL that the benefits
> of MRT have been enumerated. If we still need to bring examples of why they
> are good if not crucialÂ(@Florian: I quoted you just for reasoning (: ),ÂI
> think that something bigger is under the hood that prevent MRT to be
> included in WebGL. If it is like so, I'd like to hear a clear NO from the
> WG, so we can just stop dreaming/speculating and don't be stuck in a loop.

No individual in the WebGL WG has, or should have, the authority to
definitively say "no" to any of these proposals. We should
collectively come to agreement.

The capabilities of mobile GPUs are advancing, and I would not be
surprised if MRT support began to appear in that arena. If or when it
does, I think that that would be the right time to advance the WebGL
specification to add MRT support. In the interim, I personally think
it is not the right time to add it. Doing so would fragment the WebGL
ecosystem between desktop and mobile devices before it has had a
chance to crystallize between them, and I think there are more basic
improvements in current implementations that should be made first.

-Ken


> On Tue, Mar 27, 2012 at 9:52 PM, Florian BÃsch <pyalot@gmail.com> wrote:
>>
>> On Tue, Mar 27, 2012 at 9:38 PM, Gregg Tavares (å) <gman@google.com>
>> wrote:
>>>
>>> All devices currently support MRT. Low end devices support N render
>>> targets where N = 1.
>>
>> For some weird reason OpenGL ES choose not to forward-compatible the FBO
>> featureset by allowing for N>1 rendertargets. I don't understand why that
>> was done this way, if it hadn't been, we could get a clear picture now what
>> mobile devices actually support MRT and they would not have to
>> discuss/ratify another set of extensions for ES. Any insight as to what the
>> ES-WG plan in regards to MRT would be, would be appreciated.
>>
>>> MRT was introduced in 2003, over 9 years ago. It's been used nearly every
>>> PC game since Half Life 2. Getting more adoption by top game developers
>>> means adding more features they need to make their games.
>>>
>>> This problem won't go away ever. Waiting a year or 2 years, or 5 years
>>> won't change the fact that there will always be devices where
>>> MAX_COLOR_ATTACHMENTS is less than other devices.
>>
>> With a support rate around 80%, everybody on desktops has to do an
>> alternative renderingpath to MRTs anway. But that number isn't gonna go
>> down. 2 or 5 years from now, not only will the problem not go away, it will
>> be worse, because 2-5 years from now 98% of desktops will have MRT support.
>> If you do it now, like right now, it's the least pain you're ever gonna get,
>> the pain's not gonna get any smaller by any amount of waiting.
>>
>>> MRT is a perf improvement. A huge one.
>>
>> I do actually think that MRT support is a bigger boon to mobile devices
>> than to desktops. Desktops often do have enough juice to simply render a
>> scene multiple times. Doing the same on mobiles is disproportionally more
>> expensive because they have far less GPU power. So speeding up techniques
>> that require multiple outputs (like normal, depth, color, specularity etc.)
>> is a disproportional advantage in mobiles.
>
>
>
>
> --
> Marco Di Benedetto, Ph.D.
>
> Researcher at the Visual Computing Lab
> Istituto di Scienza e Technologie dell'Informazione "A. Faedo" (ISTI)
> Consiglio Nazionale delle Ricerche (CNR) - Pisa - Italy
> Office Phone: +39 050 315 2921
> E-Mail: marco.dibenedetto@isti.cnr.it - spattija@gmail.com
>
>

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