Hardware AA

[Ken2012]

Unashamedly n00b query:-


Why don't we have hardware AA? That is, AA that is hard-coded into the GPU that runs automatically all the time.

Other than the fact that it might make already expensive video cards even more expensive, what's the reason(s) that it hasn't been done so far?

[Xmas]

We do have hardware AA. But it can be disabled because it's not required in some cases.

[Zengar]

Hardware AA was there all the time, where have you been? It is only in their last cards that ATI uses shader units to perform AA.

[no-X]

ATi uses ALUs to perform resolve pass. Not to perform whole MSAA.

[Davros]

I think thats what ken is getting confused with shader units doing aa

[Ken2012]

Why is it then that enabling AA is still such a massive performance hit (subjectively, and depending on the app)?

Sorry for the stupid questions, I'm out of touch with this stuff :(

[silent_guy]

Why is it then that enabling AA is still such a massive performance hit (subjectively, and depending on the app)?
Because it requires quite a bit of additional memory traffic.

E.g. say you have a previously completely covered pixel, with uniform color. Now the edge of a new polygon goes through this pixel, such that some samples receive a new color and others don't.

You now need to read the old value and write some samples with the old value and some with the new value.

Also, when the full frame is completed, you need to resolve the different samples values into 1 pixel value.