shader development

[joevenzon]

There's probably already room for using another sampler for normal maps.

The ambient cubemap gives you different colors and intensities in different directions, which you don't get with an approximation based only on the y coordinate of the normal. If you look at the sides of the tents in your screenshots you can see the difference. I'll grant that it's a very subtle effect, though. If you want to avoid using the texture but match the effect exactly, an approximation with low order spherical harmonics would look almost identical, although it would be more costly than the approximations you proposed.

I'm not really against using something else or scrapping the effect, though. I think the final product just has to look good. By the way, if you haven't found it yet, VDrift has a key you can bind to instantly reload the shaders, which is verrry handy while working on shader development.