Re: glTexSubImage2D long download time with 1x1 texel level on IR
Angus Dorbie (dorbie++at++sgi.com)
Tue, 09 Mar 1999 15:34:35 -0800
Mark Lewis wrote:
>
> Dear Performers,
>
> Although not strictly a Performer issue, I would like to hear from
> anyone who can shed light on a problem I have with glTexSubImage2D (and
> glTexSubImage2DEXT). This function can be used to download specific
> sub-areas of specific LODs of a textures in TRAM. It works fine for me
> for all but the highest LOD (the 1x1 texel level), when it takes an
> unusually long time to complete. The sort of times I am measuring are
> shown below:
>
> 3 components: 512 x 512 : 3.53295 ms
> 3 components: 256 x 256 : 1.24431 ms
> 3 components: 128 x 128 : 0.658368 ms
> 3 components: 64 x 64 : 0.410448 ms
> 3 components: 32 x 32 : 0.277744 ms
> 3 components: 16 x 16 : 0.267592 ms
> 3 components: 8 x 8 : 0.30048 ms
> 3 components: 4 x 4 : 0.354984 ms
> 3 components: 2 x 2 : 0.87924 ms
> 3 components: 1 x 1 : 3.0324 ms
>
> You can see that the 1x1 level takes 3.0324 milliseconds, compared to
> only 0.87924 ms for the 2x2 level.
>
> It is also worth noting that the same program run on an O2 behaves much
> better, in that the time to download the 1x1 level is about the same as
> the 2x2 level.
>
> Is there a way of getting more sensible performance out of the IR in
> this respect (perhaps a patch) ?
>
The 2x2 image looks like it takes excessive time also.
I'm making inquiries about this. A work around would be to never load
the 1x1 MIP level on iR and set the texture maximum lod to the 2x2 image
using the OpenGL extensions on that machine. I very much doubt you'd
notice any aliasing from this. You could also slightly blur the 2x2
image if aliasing was a problem. If you're feeling brave then don't use
the 2x2 either.
Cheers,Angus.
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on Tue Mar 09 1999 - 15:34:40 PST