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Always use VAO or no?


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#1 melbow   Members   -  Reputation: 221

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Posted 17 January 2013 - 10:19 PM

I'm sure there has to be a similar thread somewhere, but I can't find it. I was wondering, is there any reason not to use a VAO? And in the latest version of OpenGL, are all other forms of rendering deprecated?

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#2 irreversible   Crossbones+   -  Reputation: 1409

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Posted 17 January 2013 - 10:55 PM

Yes, to comply with what's been standard in OpenGL since 3.0 pretty much deprecated the fixed function pipeline, it's good to be always using a VAOs. Furthermore, VAOs have been around in OpenGL forever, so there really is no other excuse to not use them than laziness. The fixed function pipeline is useful for quick-and-dirty mockups and testing, but not really for any kind of serious work.

 

I personally wrote a fixed-allocation wrapper for objects with up to a few thousand vertices that is filled up in software using emulated fixed function notation and then render it as a VAO. It follows the same notational paradigm as OpenGL, but provides a number of benefits:

 

1) it's cross-compatible with D3D once you fill in the buffers

2) it's easy for QnD testing

3) it's both forward-compatible and backwards-compatible

4) it's transparent to the application

5) it's object-oriented so it provides functionality encapsulation

6) it's far easier to write debug code for than horrible GL fixed function calls

7) it can do tessellation on the fly (in particular stuff like quad->triangle conversions)

8) state changes and streams are handled internally (eg no texture coord stream is written or enabled if the first vertex doesn't have a texture coordinate - you don't need to worry about any of this when using it)

9) it's easily extensible to support TNB or any other streams you may need using good old fixed function style notation

 

An example of its use looks like:

 

 

drv->Begin(GD_QUADS);
   drv->TexCoord(0, 0);
   drv->Binormal(bn0);
   drv->Vertex(v0);
   ...
drv->End();

 

 

The only drawback is that you have to write it.

 

For actual time-critical rendering you should always use VBO-s that you keep on the chip at all times. Moreover, once you have your data in a VAO, you can potentially use stuff like transform feedback to cut the CPU out of the loop altogether.

Think of it this way: fixed function is a toy. When you want to play, you get your toys. When you want to get work done, you bring out the tools.


Edited by irreversible, 17 January 2013 - 11:07 PM.


#3 melbow   Members   -  Reputation: 221

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Posted 18 January 2013 - 12:17 AM

Thanks Irreversible. I know fixed-function is dead (as it should be), but wasn't sure if there was some scenerio where it might be better to call glVertexAttribPointer and not use a vertex array object.

I'm writing a wrapper for GL ES (for educational as well as practical applications) and was considering forcing the usage of VAOs. I guess I will go ahead with my plan then. Thanks again.

#4 Aks9   Members   -  Reputation: 918

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Posted 18 January 2013 - 02:44 AM

I'm sure there has to be a similar thread somewhere, but I can't find it. I was wondering, is there any reason not to use a VAO? And in the latest version of OpenGL, are all other forms of rendering deprecated?

 

In core profile VAO is mandatory. But it can be created, selected and forgotten. It is the way I'm using VAO. smile.png

VAO is not always the best way to do things. Try to make several VAOs and select them and compare to changing just one attribute in a single VAO. If you have to change many attributes at once, than just selecting a VAO is better. For a single attribute, or few of them... hm, I wouldn't bet. Also, mixing VAO and NVIDIA bindless is not a good idea.



#5 melbow   Members   -  Reputation: 221

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Posted 18 January 2013 - 04:03 AM

<blockquote class='ipsBlockquote'data-author="Aks9" data-cid="5022808" data-time="1358498689"><p>
<blockquote class='ipsBlockquote'data-author="melbow" data-cid="5022767" data-time="1358482777"><p>I'm sure there has to be a similar thread somewhere, but I can't find it. I was wondering, is there any reason not to use a VAO? And in the latest version of OpenGL, are all other forms of rendering deprecated?</p></blockquote>
<br />
In core profile VAO is mandatory. But it can be created, selected and forgotten. It is the way I'm using VAO. <span rel='lightbox'><img src='http://public.gamedev.net//public/style_emoticons/default/smile.png' alt='Posted Image' class='bbc_img' /></span><br />
VAO is not always the best way to do things. Try to make several VAOs and select them and compare to changing just one attribute in a single VAO. If you have to change many attributes at once, than just selecting a VAO is better. For a single attribute, or few of them... hm, I wouldn't bet. Also, mixing VAO and NVIDIA bindless is not a good idea.</p></blockquote>

Thank you! That is exactly what I needed to know.

#6 melbow   Members   -  Reputation: 221

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Posted 18 January 2013 - 04:06 AM

OFF TOPIC: And gamedev.net desperately needs to update the mobile site. It has some very annoying design flaws and bugs...

#7 tanzanite7   Members   -  Reputation: 1378

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Posted 18 January 2013 - 08:44 AM

OT: The quick reply is broken from where-ever one uses it - not specific to modile site. It has been that way for ages (years?) so, do not hold your breath.

#8 swiftcoder   Senior Moderators   -  Reputation: 10430

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Posted 18 January 2013 - 09:51 AM

It has been that way for ages (years?)

Months, if that. It seems to be a bug in the most recent version of IP.Board.


Tristam MacDonald - Software Engineer @Amazon - [swiftcoding]





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