Simulating Real Lighting using a Parametric Light Setup

While there are many different ways to create realistic lighting setups using raytraced solutions, these techniques are very slow to render and for real-world animation productions such render overheads are prohibitive. This tutorial overviews how comparable lighting setups can be created using the simplest of lights in maya.

In analysing real world lighting you should ask yourself the following questions;

For Key Lights
1) What are the Key Lights? (the sun, atmospheric reflection/scattering of the sun, artificial lights etc...)
2) What is the nature of the key lights (directional, point, spot)
3) What is the position and orientation of the key lights
4) What are the qualities of the key lights (direct unfiltered, sun or open lamp, indirect filtered light: through a diffuser, clouds, lampshade etc)
5) What is the colour/temperature of the key (remembering that colour and temperature are interrelated).
6) What is the key Intensity
7) Are the shadows soft or hard

For secondary Lights and Fills: The same points should be noted as above. The secondary lights are not so crucial to the mood and context of the scene: but these details are where the realism creep in.

For Bounced Light:
1) Where is the centre of energy/origin of light that will be bounced?
2) What is the colour of the surface that light is being bounced off of?
3) What is the nature of the bounced light (normally fairly diffused scattered light that casts very soft if any shadows)

The following is a step by step analysis and synthesis of several different lighting scenarios

Scenario 1: An Incandescent Lamp

n this scene we have a roof, two walls and a floor with an omnidirecitonal light on the roof to simulate a typical lighting setup of a room having an incandescent light globe on toward the roof. Rendering this scene we can now see that the distribution of light is not at all typical of a light globe.  The light cast on to the roof is minimal and the light on the floor and roof is much too great given their distance from the light. An artificial light source usually follows and inverse square rule.

Intensity of illumination of the light source decreases exponentially with its distance from the light source.

The roof should be very bright (distance = 1)

     ie 100% illumination

At the bottom (distance = 9) the light should be considerably duller

ie 1% illumination

We can adjust this by making the decay rate of the point light Quadratic and adjusting its intensity accordingly (~35). Also lets make the light a more yellowish to make it look more like a real light Dealing with Radiosity. In real lighting situations we will have some of the light that hits each surface be bounced off and add to the overall room illumination.Note: the bounced light is the unabsorbed light from the light from the key light hitting the surface, so it should be the same colour as that of the surface (after all light absorption is what gives things colours in the first place).

Note also that bounced light won’t create specular hightlights so it is best to make the spots emit diffuse only.

The most simple solution to create this light bouncing would be to create a light source having the correct bounce colour and place it under the strongest point of illumination.  The images below show the positioning and penumbra setup for the blue bouncelight.  The colour is the same blue as the floor and the intensity it 0.2

(a) Fill from Floor Bounce	(b) Blue Bounce combined with Key light	(c) Extra two bounce lights from opposing walls

Scenario 2: Light from a window. Using Area Lights for light bouncing

This scene is the same as the previous scene only that the source of light is different.  We have an external light source (probably through a window) casting light into our scene.  As there are no other lights in our scene all the other geometry appears black.

The rest of the scene will be illuminated by bounced light.  The problem is that the concentration of light on the floor isn’t the same as for the previous example.  So a spot light, whilst it would do some of the job, it wouldn’t capture the nature of light scattering from the square hotspot.  Even if we include the exponential dropoff  the effect still isn’t convincing.

(a) Light from outside	(b) bounced light from a spot light source.	(c) bounced light from a spot using exponential decay

Alternatively we can use an area light.  Area lights create a softer more scattered light than spot lights.  Create an area light and position/scale it to the hotspot.  Have it facing the roof.  You should get the following rendering.

The light is a bit intense so adjust the intensity of the area light to your taste.

Now we just need to account for the bouncing of the bounced light and our scene is about finished.  This can be taken care with using three extra spot lights positioned as we did for our first example.

	Roof Secondary Bounce Intensity = 0.3 Colour - White (like the roof) Wide penumbra
	Back Wall Secondary Bounce Intensity = 0.3 Colour - White (like the roof) Wide penumbra
	Side Wall Secondary Bounce Intensity = 0.3 Colour - White (like the roof) Wide penumbra

And that gives you a fairly good approximation!

Rendering note: this solution takes 1 second to render…  To achieve the same illumination results using a photon tracing solution may take up 30 times longer!

Scenario 3: Textured Floors.

What would we do if there were say a rug on the floor of our room in the previous example?  Here we are faced with a more complex problem of light bouncing.

Unfortunately we can’t get away with the simple solution of a straight colour for an area light.  We need to bake the light from the floor into an area light. To do this we first need to bake the lighting (and shadows if there were any) into our floor plane.  Select all the surfaces except the ground plane and turn their primary visibility off Windows > General Editors > Attribute Spreadsheet Render Tab > Primary Visibility – Turn to off (enter 0 for all the surfaces) Now change the render globals to render a 256x256 image and then render from the top viewport.  Save this image as RugAreaLight.iff Turn the primary visibility back on for the surfaces. Now we are going to recreate a similar area light as before only have it cover the entire floor space. In the attribute editor, click on the colour swatch and choose a file texture.  Load the RugAreaLight.iff file and render. The result isn’t that fabulous.  We need to introduce a little trick.  Select the area light and change its scale in Z to –2 and re-render.  Now its too bright, so change the decay rate to quadratic and the primary bounce is looking nice.	256x256 Image rendered from top viewport

(a) No scaling	(b) Scale Z = -2	(c) Quadratic Decay rate.

We can now repeat the process we saw in the prior two examples to introduce the secondary fills to the scene (noting that the secondary fills will again be of the same colour as the walls they hit).