These are some reference videos based on outdoor lighting. Starting with what a white box or sphere would look like only lit by the sky. Then adding a sun directly above. Lastly adding and angle of the sun at 30 degrees. My interest was in how the light would bounce within the shadows and to find out why the sides of the cubes have the tones that they do base on what light is hitting them.
These first two have no ground plane. I wanted to see what universal lighting on an object looked like with no interruptions to the light rays except the single object. As expected the object is lit uniformly. The biggest note is that the object is darker that its light source.
For these I have added a ground plane. The light to the objects is now blocked from the bottom by the ground plane so the object is affected by only half the dome. Much like an object out in the open in real life. This simulates what would happen on an overcast day when the light is diffused and the sunlight is not able to make direct contact. The darkest shadow is where the sphere or box meets the ground plane. The fall off of the light is very soft and gradual from top to bottom and the cast shadow is soft, both because of the extreme scattering of light rays.
I have added a light source directly above to mimic the sun. The box has no cast shadow. But the core shadow on the sphere and the top edge of the sides of the box have the same value. The change is just sharper on the box. In other words the box also has a core shadow. The light bouncing off of the ground back into the shadow is similar on both objects with one exception. The cast shadow on the sphere bounces back as well making the bottom of the sphere move back into the darkness of its occlusion with the ground.
The sun is now angled to 30 degrees. I had always wondered why the sides of boxes had differing values. This gave me that answer. The direct light is mainly hitting the top and front of the box. The back is the darkest because the cast shadow is what is bouncing back into the shadow of that back plane of the cube. But the two sides are a step lighter because they have the light of the fully lit ground plane bouncing back into them. Another note on the back plane and the cast shadow. The darkest point is where the box meets the ground. The cast shadow lightens as it moves away from the box because the sky light is able to bounce freely into it unobstructed.
The sphere reacts very similarly except all of the bounced light blends with the curve of the sphere. The darkness of the occluded area also blends smoothly back into the reflected light areas.
Since the sphere wouldn’t change with the horizontal angle of the sun I only included the box. I shifted the horizontal angle of the light source 30 degrees as well. The main change on this is the sides of the box. the top and front are still being hit the hardest but now one of the sides is included in that direct light. But because the angle is not as direct it is about a half step darker thank the top and front planes. The other side is darker than the previous example because now it has a sliver of cast shadow bouncing up and into it. The back most plane of the box screen left on the video and at the bottom, because of that horizontal shift in the light source there is now an angle of light from the ground bouncing into it that wasn’t there before.