the grids

Listening to: Nothing, i've lost my iPod charger cable

So to wrap up my previous post about the mysterious grid lines..

It turns out that in multiplying my derivatives used in the texture map lookup in order to blur the map, the usually unnoticeable discontinuities in derivatives around the shading grid boundaries had been magnified (by the multiplication).

I was doing this:-

duds * 25 = ...
dvds * 25 = 0.03 // for arguments sake, it's there or thereabouts
dudt * 25 = ... // similar order of result for the others
dvdt * 25 = ...

but sometimes the derivatives were very different around shading grid boundaries, so the tiny aliasing that is normally unnoticed, was being amplified and appeared as a grid.

The solution was to add a number to the derivatives to get roughly the 0.03 I wanted (so not 25, something much smaller instead). That way the aliasing would stay small, but I would still get the blurring I wanted.

So that's that.
Hurray!