Advanced use of skylights

The skylight technique works because it projects diffuse shadows on the ground and around the objects. Another more subtle reason is that because the skylight illuminates only the +Y part of the scene (from the ground floor and up), there are 2 times less rays that reaches the sides of the objects than the top. The number of rays that illuminate a surface gradually diminishes to one half as the surfaces goes from horizontal to vertical. This produces a nice subtle modeling of the objects with light.

OK. Now that we have proven that we can make our own "à la Arnold" images, what's next?

Adding skylight color to the scene

I have already mentioned the possibility of changing the light color for the skylight. For instance, one might want to set the skylight to blue and add a yellow sun light in the scene to simulate a sunny day. While it is very possible to do that, a better solution is to add an hemisphere model that fits just under the lights of the skylight and add a transparent material to that hemisphere (50% transparency is a good start) with the ambiance value set to 100%. When the light passes through the hemisphere, it is tinted from the color of the transparent patches under it and cast tinted light.

When the material is of uniform color, the scene is tinted with that color. We still gets that variation in brightness but the image looks monochromatic.

Continuing on the same idea, a gradient material can also be applied. With a gradient that sets the zenith color to a deep blue and gradually changes to a light gazing blue at the equator, it is possible to simulate the light that would come from a clear sky.

This simple technique adds considerable depth and solidity to the scene as can be observed in the image above. Normally, when a skylight is used, the top of the objects are brighter than the sides due to the fact that there are 2 times more rays coming from above than from the sides. By using such a gradient material, a variation in saturation is added over the brightness variation which helps sit the objects in their environment.

Experimenting and animating with materials on a dome

Working with a transparent dome can be annoying because the background color will filter through. The technique is improved be placing a second copy of the dome right under the first one and scaled slightly smaller like 99%. Now the material as well as the dome model attributes are set to be completely opaque. The transparency will be adjusted in the choreography. In the choreography, the inner dome is set to 0% transparency and receives or cast no shadows but cast reflections. The outer dome is set to 50% transparency and cast shadows but receives no shadows and cast no reflections.

The material on the dome technique allows for easy experimentation when trying to find the correct mood in a scene. Simply tweak the colors of the material until it is perfect.

One of the easiest adjustment is performed by varying the transparency of the outer dome.

Decreasing the transparency below 50% have little effect but increasing the transparency have the effect of shifting the filtered color toward white (assuming that the skylight light color is white, of course).

The light intensity will need to be adjusted accordingly though. For instance, increasing the transparency from 50% to 75% generally means the the intensity of the light will have to be reduced to 50/75 = 66% of its original intensity. Actually, it is not so simple because changing the transparency of the dome also change the quality of the light but this equation is a good start.

From this example, it is not difficult to figure that the material could be animated through time during an animation sequence. What about a sky that goes from blue to a more reddish tint while our hero gets really mad? Or a sky that gets darker and darker as a storm approaches?

Using images, photos and even movies on a dome

But wait! There's more! 

It is also possible to map a nice sunset or any other photograph onto the hemisphere. But why stop there? Think about the possibilities of mapping a video sequence of a rising sunset or a thunderstorm formation.

Use of additional lights

It is generally not a good approach to use only one skylight in a scene. In cinema, natural light is always complemented with a whole array of projectors, reflectors and the like. That is because natural light alone is rarely suitable for helping separate foreground objects from the background and does not help either in directing the eye to specific locations of the image.

In 3DCG, the use of a sun light or main light complemented with fill lights, back lights and all variation of hair lights should not be neglected even with skylights.

For outdoor scene, skylights may be quite prominent with only one sun light to add shadows. For indoor scenes, skylights may be used to replace the ambiance setting. In this case, the skylight need not be so prominent.

Conclusion

The use of a skylight in a scene allows much more creative light placement than the traditional 3 point light. For instance, since the skylight alone already gives the scene a lot of depth and pops out the geometry of the objects, it becomes possible to put only one main light that will light the scene from behind in addition to a dark skylight.

The skylight library can be downloaded here.