This looks quite nice I think. The wet, slick, shiny skidmarks are particularly great (though indeed there needs to be one under the front tires too). I think the water in the ditch is maybe too transparent, either it should be a bit murky, or maybe just darken the surface below the waterline. But otherwise it's a beautiful scene.
I agree though that a 2nd sun is not a good solution and I'll explain why. Adding a 2nd light source *can never make shadows darker*. Think about it: you are adding *more* light. It only *appears* darker because the rest of the scene is lighter and there is a lighter 2nd shadow. So *by comparison* it's darker, but it's not actually a darker shadow than in your original image! It's like this classic illusion:
http://en.wikipedia.org/wiki/Checker_shadow_illusionWhat I would suggest is making sure that GISD is enabled, then keep at the defaults (bounce and occlusion both at 1), but increase Radius (in pixels) to 50 or 100 and uncheck Falloff. That should darken the underside of the vehicle a fair amount (it will also darken other shadow areas of course). This is a quick way that does not impact render time results significantly.
However what you're really seeing with the underside of the vehicle being too light is the result of low GI accuracy, so for best results you'll want to increase GI Cache Detail and Sample Quality. What are they set at now? If they're not 4 or higher, try values of 4/4 (four for both) or 6/6. The higher you can go, the better that area under the vehicle will darken up. Still, GISD is going to be a much faster way to approximate it, even though it's less accurate without higher cache detail and sample quality. Some combination of the two may be best, e.g. GI at 4/4 with GISD at pixel radius 100 and no falloff.
- Oshyan
