Adjusting light falloff distance

So we left the previous part of this tutorial with a scene that looked like the render at right: Obviously, there are too much light.

This is kind of normal because we now have 5 lights with 100% intensity each where we previously put only one. So let's reduce the light intensity to 1/5 which means the light intensity should be set to 20%. Together, the 5 lights will give 5 times 20% = 100% intensity. That is what we want back from the previous lighting setting which looked good, at least in term of overall lighting intensity.

Adjust the intensity on the light model. Not on the light shortcut in choreography. This way, you only need to change one model instead of 5 instances. Thus having less probabilities of missing one of the 5 instances when doing lighting changes. In addition, it is much quicker to make adjustment to one light and immediately see the resuolt in a progressive render rather than having to wait until the 5 instances were changed.

Now, render. You should get something like the image on the right:

Notice how everything look flat. The walls are almost uniformly red with no gradients in the shadings. The table is also uniformly brown. Apart from the obvious perspective and a few reflection effects, there is no depth to this picture.

This is the place to discuss the light falloff property and how it works. The light falloff distance defines the volume around the light inside which, light intensity is constant. In other words, anything within the falloff distance will receive the full light intensity as specified in the intensity property. What we just did is define a light where the light intensity will be 20% everywhere inside the falloff sphere.

Everything look flat in this render because the light falloff distance is large enough to enclose all the objects in the room including the room itself as can be seen in the illustration to the right. Every object in this scene is illuminated with exactly the same intensity no matter its distance from the light.

This is not what natural lights do in reality. In reality, light intensity decreases as the distance from light increases. The amount of light increase or decrease is governed by the Inverse Square Law.

If you increase distance between an object and a light by 2 times, lets say from 5 feet to 10 feet, then the light intensity that the object will receive is 2 ² = 4 times darker. If distance changes to 1/3 the previous distance from the light, then the light intensity that now shines on the object is 3² = 9 times brighter. Decreasing the distance from the light have the same effect as increasing the light intensity.

Lights in A:M also follow the inverse square law. Except that there is this important difference. The inverse square law in only applied starting from the falloff distance and going further away. Within the falloff distance, light intensity is constant.

So in the case of our 500cm falloff light and 20% intensity, an object at 500cm from the light will receive 20% light intensity. Double this distance, at 1000cm, the object will receive only 1/2² = 1/4 of 20% = 5% intensity. Triple the distance, at 1500cm the object will receive 1/3² = 1/9 of 20% = 2.22% intensity. But because the falloff distance is 500cm, an object 100cm from the light will also receive 20% intensity.

Using falloff distance, in A:M, allows us to control light intensity for far off objects without having to specify a huge intensity value for the light. And you don't have to figure the correct intensity value by trial and error too.

The procedure is very simple. You know that at the falloff distance you have the specified light intensity and from that distance and away, light attenuation behaves in a normal way. So if you want to light a scene, you just need to set the falloff distance of each light to a distance where it just touches the first object that it needs to light in a realistic way. Once this is done, you simply adjust the light to its proper intensity. 100% intensity is a good starting value.

So the next step is to do just that.

On the light model,

• Set the light intensity to 100%
• Set the light falloff distance to 100cm so that it just gets near the turkey on the table.

If you render, you should get something like the render on the right:

Now, we get nice shading gradients on the walls and on the table. The credence behind the table and the chairs are also darker than the table and the floor is darker. And while the table top is a little bit brigher than in the previous render, observe that the turkey is much brighter than it was. All this, together, already gives a better sense of depth in that scene.

Recall that all rendered images in this tutorial have been Gamma corrected with a gamma of 2.2. If your renders don't look the same as mine, then this is most probably due to missing to gamma correct your renders.