Part 3 - Key-to-Fill Ratio
Key- to-Fill ratio is a simple equation that describes the relationship between the Key light and the Fill light. A key light that is twice the brightness of its fill light has a Key-to-Fill ratio of 2:1. This ratio would be considered a Low Key-to-Fill ratio. A scene with a ratio of 18:1 would have a High Key-to-fill ratio.
Low Key-to-Fill ratios are most often used to depict a snow or overcast day or bright interiors. Shadows would be washed out by the reflected light. Typically Low Key-to-Fill ratios create more of a happy lighting scheme. When you are buying your Super Slushy in the Quicky-Mart you are in a Low key-to-fill ratio environment. We will be discussing ways in the Time of Day and Mood section on how to create more gloomy lighting using lower ratios.
High Key-too Fill ratios can be used for bright daytime light with dark shadows, night scenes or to build suspense and drama through severe lighting. This technique of using high ratios is best seen in Film Noir.
The well executed use of these techniques can go greatly to enhance mood and suspense throughout a scene or sequence. Remember that continuity from scene to scene is important, as well as the use of temporal contrast to enhance a change of location or even the story conflict.
Part 4 - Quality of Light and Colour Balance
In this example you can see the difference between Hard light and Soft light
Without getting to deep into color theory I would like to discuss Quality of Light and Color Balance.
There are only two real Qualities of Light, Soft and Hard. Thus creating two types of shadows, soft and hard. We can also call it sunny or cloudy. The hardest light would be generated by the sun on a very clear day, and the softest light would be created by the sun on a very overcast day. It is the millions of particles floating in the air that bounce and catch the light. These particles then refract, reflect and scatter the light from the sun creating all the wonderful variants of light we see everyday. The more the light bounces around and gets diffused, the softer shadows become. This is also the cause of atmospheric perspective. Atmospheric Perspective is the viewable phenomenon of things getting bluer or less saturated and fuzzier in the extreme distance. The diffusion of light in the air is what we can sense, even if we are unaware of it happening. The ability to control this gives the artist explicit control over the feeling of a scene. We will keep these two qualities of mind when we talk about Time of Day and Mood.
Colored lights can be very powerful, but can also get out of hand very easily. When using colored lights, try to stay away from saturated and dark colors. That said there is always a time and a place for vibrant lights, just keep in mind that is not very natural.
The color of a light, on film, greatly effected by the balance of that film. Because in 3D graphics there is no film actually being exposed we need to simulate it. On a side note: It is a strange phenomenon that people perceive an image that has all the flaws, distortions and artifacts of film to be more "real" that a well light and clean render. So where does that leave us? Well, we try to approximate what the camera lens is seeing, in order to accommodate the tastes of our audience. Of course, a chosen style should prevail.
For simplicity sake we will focus on two different types of balanced film. "Indoor" or Tungsten-Balanced and "Outdoor" or Daylight. There are other kinds of film and gels and filters and post effects that all can contribute to the overall color of a scene. A good thing to keep in mind, is that the dominate light source in the scene should dictate the balance of the film. The balance that is referred to is on the Kelvin Scale. It is a scale that measures color temperature. The range of the Kelvin scale is from 0K - 10000K. The higher the temperature the bluer the color and the lower the temperature the redder the color. An Outdoor film would be balanced for 5500K and an Indoor film would be balanced for 3200K.
So what does the light balance of film mean to a CG artist? It means that in a "balanced" light set up, the dominant light source would be close to white, all others would range from warmer to cooler. For example: In a daylight balanced scene, the sun would be a very pale yellow and the light from the sky would be a light blue. You knew this already? Well how a bout an indoor scene lit with a torch near a doorway to the outside? With an Indoor balanced scene the torch would be a pale orange while the sunlight from the doorway would be bluish.
A final word about Quality and Color. These, like all the other principles that are being discussed in this guide, are only part of the equation and are in no way absolute. Let story and character drive your lighting set ups. Not the other way around.
We will talk further about overall lighting pallets in the Time of Day and Mood section. There we will go more in depth about tinting a scene to convey emotion.