From: Benedikt Kessler (bjk++at++munich.sgi.com)
Date: 08/14/2002 08:18:29
Hi!
w.price++at++electronic-alchemy.co.uk wrote:
>
> csklu_pf++at++yahoo.com wrote:
> > Performers,
> >
> > Again maybe another OpenGL question: I'm writing
> > values to the depth buffer directly using:
> >
> > glDrawPixels( width,height, GL_DEPTH_COMPONENT,
> > GL_FLOAT, depths).
> >
> > This is much slower than the corresponding write using
> > GL_RGBA and GL_UNSIGNED_INT (with depths converted to
> > rgba values). Performer reports that the depth buffer
> > size is 24 (on a GeForce3, RedHat 7.3). Could this
> > slowness be because sizeof(GL_FLOAT) !=
> > depthbuffersize/8? Would it make any difference if I
> > use a depth buffer depth of 16 and GL_UNSIGNED_SHORTs?
> > Any other suggestions/explanations?
>
> GL_UNSIGNED_SHORT data will be faster than GL_UNSIGNED_INT since you have half as much data being sent (sizeof(int) is usually 4, sizeof(unsigned short) usually 2).
>
> Another possible speedup which I have found across using glDrawPixels() on Linux is to NOT send the entire chunk of data in one go. i.e. instead of using
>
> glDrawPixels(1024, 768, ....)
>
> use
>
> glRasterPos2i(0, 0);
> glDrawPixels(128, 128, ...., buffer);
> glRasterPos2i(128, 0);
> glDrawPixels(128, 128, ...., buffer + offset);
> .....
> .....
> (most likely this will be in a loop)
>
> I don't know the details of why, but this was twice as fast for me to draw pixels to the screen. Best tile sizes were 128x128 and 64x64, I think.
>
> You would have thought that the one call would have been faster since there's less overhead than multiple calls, but experiments have proved otherwise.
Probably you were using an InfiniteReality* system.
One these systems you might do some experiments with the two environment
variables GLKONA_DMATHRESHOLD_IMAGE and GLKONA_DMATHRESHOLD_SCANLINE.
These threshold values (in bytes) control weather a PIO (eats up CPU
time) or DMA (CPY might be available for other things) will happen.
You might try to:
setenv GLKONA_DMATHRESHOLD_IMAGE 301500
Restart your application and check if the performance changes.
Bye! Benedikt
>
> Wayne
> --
> ___________________________________________________________________________
> Wayne Price W.Price++at++electronic-alchemy.co.uk Electronic Alchemy Ltd
> Mobile: +44 (0) 7770 376383 Home: +44 (0) 1483 531235
>
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-- +---------------------------------+----------------------------------+ |Benedikt J. Kessler | Silicon Graphics GmbH | |Professional Services | Am Hochacker 3 - Technopark | |SGI | 85630 Grasbrunn-Neukeferloh, FRG | | --- __o ,__o | | | ------_ \<,_ _-\_<, | Phone: (+49) 89 46108-366 or -0 | |----- (*)/ (*) (*)/'(*) | Fax: (+49) 89 46107-366 | +---------------------------------+----------------------------------+ |E-Mail: bjk++at++sgi.com Web (private): http://reality.exsgi.org/bjk | | Web: http://www.sgi.de | +--------------------------------------------------------------------+
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