Back to the point, this kind of inter-pixel communication is only possible with "high quality" derivatives. With "low quality" derivatives, only 3 out of every 4 pixels can communicate, with 1 out of 4 pixels being isolated and unable to talk to the others in it's "2x2 quad".
Wow. Ok. Yes, this is was I was after.
Acknowledgement that there actually _are_ different levels of derivative "quality" on modern hardware.
I help author a high-end tool which runs on the most modern of cards. We're very quality concious ( rather than speed ).
So I'm going to set this hint to HighQuality regardless in our app from now on.
I know derivatives are used to select mip-map levels during texturing.
Do you know what happens if a polygon occupies only 1 pixel on screen? ( or even 2x1 )
Any idea what the derivatives will give in that scenario? And what mip-map level will be chosen?