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Dark Photon
06-20-2012, 06:18 AM
(See poll above)

On which vendors?

aqnuep
06-20-2012, 11:49 AM
I don't think you can see any major performance benefit from using immutable textures as long as the driver has to support mutable textures. I believe immutable textures could provide quite visible benefits if mutable textures were to be removed, however, that's not going to happen due to backward compatibility.

Dark Photon
06-20-2012, 05:41 PM
That's what I was suspecting but thought I'd check with everyone. Suspect there's probably a "validated" bit (or bits) on the texture that the driver tracks for every texture and checks to avoid needless revalidation every time. It'd just be true for TexStorage built textures.

aqnuep
06-20-2012, 07:37 PM
Don't get me wrong, there way be situations that can already benefit from using ARB_texture_storage, in the worst case, just by having cleaner code. Not to mention that you can create an "empty" mipmapped texture with a single GL call instead of log(size) calls. Also, it forces you to write proper, efficient code as with immutable textures you cannot do such craziness like these:

// pulling out the storage from under a framebuffer attachment
// (it would work, just it's likely to hit your performance hard, not to mention that your framebuffer can easily become incomplete "behind your back")
glBindFramebuffer(GL_FRAMEBUFFER, myFBO);
glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, myTexture, 0);
...
glBindTexture(GL_TEXTURE_2D, myTexture);
glTexImage2D(GL_TEXTURE_2D, ...);

// specifying mipmap levels with mismatching internal format or sizes that don't respect mipmap size rules
// (not sure if this is even allowed by the core or compatibility spec, maybe the texture ends up incomplete)
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 512, 512, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glTexImage2D(GL_TEXTURE_2D, 1, GL_RG32F, 384, 128, 0, GL_RG, GL_FLOAT, NULL);

Note: never ever do something like these if you don't want to make problems for yourself.

Btw, if you use texture storage, you can't get into such situations.

Zenja
06-23-2012, 03:42 AM
No noticeable performance gain (<1%) on Linux 3.2.8, 4.2 compatible profile, AMD HD 6350