Quote from: JimB on February 02, 2008, 07:45:03 AM
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.... That 0.5 degrees makes a big difference because the surface area being illuminated is actually quite large.
The absolute size of the sun isn't important, only the angle subtended by its visible disc at the distance from the sun to the planet. As Nikita says that wraps around about 0.4% to 0.5% of the diameter of the planet (or cylinder, whatever). In some scenes you might notice the difference. On the moon, however, near the terminator the roughness of the terrain casts shadows over much larger distances than that, so you would mostly notice the difference on smooth plains or if you look at the edges of individual shadows.
QuoteI'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).
Yes, that wrap around is an important effect that should be simulated. TG2 doesn't do this yet. You will get better results with a proper area light.
The second benefit you mention above about reducing the extreme variation in brightness is something you should only see if you use a much larger angle. The real reason the moon, for example, doesn't behave like a CG sphere (has a much flatter appearance at full moon etc.), is mostly down to its reflective properties, and 0.5 degrees of area light can't correct for that (or if it does then there is a bug in the area light model). If a large area light solves that problem then I wouldn't argue that you shouldn't use it
but I'd worry about it then creating too much softness elsewhere.
Quote
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 wouldn't rely on TG2 to test these theories
You are right that high clouds in TG2 show a hard terminator, and that's a combination of various limitations in TG2's models, including the lack of decent area light support that you're looking for.
Quote
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.
Definitely.
Matt
