We have now covered some of the project settings which need to be considered before beginning work on lighting your scenes in Unity. Hopefully at this point you should have your project configured appropriately for your target platform (generally Baked GI and Gamma Color Space for mobile, Precomputed Realtime GI and Linear Color Space for Standalone PC or recent games consoles).
Let’s move on to looking at the tools available for achieving the lighting you want in your game.
‘Directional Lights’ are very useful for creating effects such as sunlight in your scenes. Behaving in many ways like the sun, Directional Lights can be thought of as distant light sources which exist infinitely far away.
Light rays emitted from Directional Lights are parallel to one another and do not diverge like those from other light types. As a result, shadows cast by Directional Lights look the same, regardless of their position relative to the source. This is useful to us, especially when lighting outdoor scenes.
As Directional Lights do not have a source position, they can be placed anywhere in your scene without changing the effect of the light. Rotating the light however does greatly affect the visual result.
With other light types where there is an obvious source position, such as Spotlights, character shadows will change as the character moves closer to the light source. This can be a problem when trying to generate character shadows in interior levels. Directional lights are advantageous in these situations as shadows remain consistent regardless of proximity to the light source.
Directional Lights do not diminish over distance. As they affect all surfaces in your scene (unless culled), they pose a performance cost when using the Deferred Rendering path. Remember that when using this rendering technique, the performance cost of a light is relative to the number of pixels it illuminates. However, despite the cost, performance will at least be consistent and as such, easier to balance.
By default, every new Unity scene contains a Directional Light. In Unity 5, this is linked to the procedural sky system defined in the Environment Lighting section of the Lighting Panel (Lighting>Scene>Skybox). You can change this behaviour by deleting the default Directional Light and creating a new light or simply by specifying a different GameObject from the ‘Sun’ parameter (Lighting>Scene>Sun).
Rotating the default Directional Light (or ‘Sun’) causes the ‘Skybox’ to update. With the light angled to the side, parallel to the ground, sunset effects can be achieved. Additionally, pointing the light upwards causes the sky to turn black, as if it’s nighttime. With the light angled from above, the sky will resemble daylight.
If the Skybox is selected as the ambient source, Ambient Lighting will change in relation to these colors.
A Point Light can be thought of as a point in 3D space from which light is emitted in all directions. These are useful for creating effects like light bulbs, weapon glow or explosions where you expect light to radiate out from an object.
The intensity of of Point Lights in Unity diminishes quadratically from full intensity at the centre of the light, to zero at the limit of the light’s reach defined by the ‘Range’ property of the component in the Inspector. Light intensity is inversely proportional to the square of the distance from the source. This is known as ‘inverse square law’ and is similar to how light behaves in the real world.
Point Lights emit light in all directions from their position in the world. The spherical gizmo represents the ‘Range’ of the Light. Direct Light will ‘Falloff’ to zero when it reaches this limit, however bounced - or indirect light can continue much further.
Enabling shadows for Point Lights can be expensive and so must be used sparingly. Point Lights require that shadows have to be rendered six times for the six world directions and on slower hardware this can be an unacceptable performance cost.
When adding Point Lights to a scene it’s worth noting that currently they do not support indirect bounce light shadowing.This means that the light created by Point Lights will continue through objects and bounce on the other side unless attenuated by range. This can lead to light ‘leaks’ through walls and floors and therefore lights must be carefully placed to avoid such problems. This however is not a problem when using Baked GI.
Spotlights project a cone of light in their forward (+Z) direction. The width of this cone is defined by the light’s ‘Spot Angle’ parameter. Light will ‘falloff’ from the source position towards the extent of the light’s range, where it will eventually diminish to zero. Light also diminishes at the edges of the Spotlight’s cone. Widening the Spot Angle increases the width of the cone and with it, increases the size of this fade, known as the ‘penumbra’.
Spotlights have many useful applications for scene lighting. They can be used to great effect as street lights, wall downlights or used dynamically, for creating effects like a flashlight. As their area of influence can be precisely controlled, Spotlights are extremely useful for creating focus on a character or for creating dramatic stage lighting effects
Light will diminish as distance increases from the source. Notice how light also fades towards the edges of the cone. We call this the Light’s penumbra and this increases as the angle of the cone increases.
Like Point Lights, Spotlights do not presently support indirect shadowing when using Precomputed Realtime GI. This means that light produced by Spotlights will travel through geometry and will bounce on the other side. Placement therefore needs to be carefully considered.
Area Lights can be thought of as similar to a photographer’s softbox. In Unity they are defined as rectangles from which light is emitted in all directions, from one side only - the object’s +Z direction. Presently only available in Baked GI, these Area Lights illuminate uniformly across their surface area. There is no manual control for the range of an Area Light, however intensity will diminish at inverse square of the distance as it travels away from the source.
Light is emitted across the surface of an Area Light producing a diffuse light with soft shadowing.
Area Lights are useful in situations where you wish to create soft lighting effects. As light is emitted in all directions across the surface of the light, the rays produced travel in many directions - creating a diffuse lighting effect across a subject. A common use for this might be a ceiling striplight or a backlit panel.
In order to achieve this, we must fire a number of rays from each lightmap texel in the world, back towards the light in order to determine whether the light can be seen. This means that Area Lights can be quite computationally expensive and can increase bake times. However, used well, they can add a great depth of realism to your scene lighting and this extra precomputation may be justified. Note that as they are baked only, gameplay performance is not affected.