Quote from: Matt on February 01, 2008, 12:53:29 PM
Hi Jim,
I agree with your observations about the hard edge on the terminator of hard surfaces and how they can be improved by using an area light, but I think the Earth images show something different. In those Earth terminator images most of that softness comes from the scattering effects of the atmosphere. The angular diameter of the sun as viewed from the Earth is only about 0.5 degrees, so on a hard surface there is only about 0.5 degrees of additional softness that wouldn't be there with a point light source.
Enabling soft shadows in TG2 may help to slightly soften the terminator on hard surfaces, but it doesn't simulate all the softening effects you'd get from a true area light source. Not yet anyway.
Matt
Good point Matt. However, we don't see a harsh terminator on planets without atmosphere. I personally put it down to the Sun being much larger than any of the planets, which means light is hitting the 'spheres' from a wider source and wrapping around slightly. I've used the technique before and you get two desirable things; a soft division between the day and night side, and a more even spread of light on the day side which smooths out the distribution of light. The latter means you don't get a very harsh polarisation of light where the planet's surface is pointing more directly towards the Sun, which we see in actual planetary photos (it was one of those things that always bugged me about singularity light sources).
That 0.5 degrees makes a big difference because the surface area being illuminated is actually quite large.
I did a very quick test in TG2 last night and you are definitely correct that the Atmosphere is diffusing the light and softening the terminator. However, if you have clouds that are very (and unrealistically) high they show a harsh terminator, possibly because of the scattering which might be similar to specular shading?
I think my point is, more importantly, that when applying a specular surface, be wary of making the specularity too rough and tweak it. Far better to use proper soft reflections with just a little bit of specularity thrown in. There are CGI lighting techniques that can throw out ambient occlusion and final gathering completely by using soft reflections only.