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Time consuming OGL func calls...


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#1 smitty1276   Members   -  Reputation: 560

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Posted 14 July 2001 - 10:28 PM

I know certain OpenGL functions are supposed to be very time consuming (for the computer, not the programmer), but I''m a little fuzzy on which ones exactly they are... I''ve heard that glPushAttrib() etc. are bad. What about glPush/PopMatrix()? I also have heard that "state changes" are time consuming. What does this include? Is enabling/disabling culling and alpha-blending and that sort of thing something to be avoided as much as possible? In a nutshell, I''m wanting to know which functions to avoid or use conservatively to get the best performance. Thanks in advance for your responses.

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#2 Obelix   Members   -  Reputation: 122

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Posted 15 July 2001 - 01:45 AM

This is the kind of information that you have to search at the hardware manufacturers like nvidia, ati.
I think that you are right about the state changes but pop/push matrices is OK.
If you want better performance should your application keep track of the states. Setting an already enabled state is expensive. After that should you try to sort so you can use the same state as long as possible.
Particulary changing the texture seems to kill the performance.

#3 zedzeek   Members   -  Reputation: 528

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Posted 15 July 2001 - 11:00 AM

there is no list that i know of. it would change from card to card and even colour depth to colour depth (think stencil with 16bit + 32bit colour)
but yes push/pop attrib are slow, also changing textures etc check the faq perhaps theres more info there

http://members.xoom.com/myBollux

#4 koom   Members   -  Reputation: 122

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Posted 15 July 2001 - 11:15 AM

on 3dcard, main slow things are bitmap operations, ie glBitmap etc, since they use computer memory, state changes and retrieving data from card. on software are things different since everything is in main computer memory:-) for example, on 3dcard font outlines are faster then font bitmaps, on software vice versa.
look for performance faq on sgi opengl site.


regards
Jindrich Kolman




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