Doc:Tutorials/Lighting/Pseudo Area Light

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[edit] Pseudo-Area Light

The concept of Light coming from a point is an approximation. No real world light source is dimensionless. All light is shed by surfaces, not by points. This has a couple of interesting implications, mainly on shadows:

1. Sharp shadows do not exist: shadows have blurry edges.
2. Shadow edge blurriness depends on the relative positions and sizes of the light, the shadow casting object and the object receiving the shadow.

The first of these issues is approximated with the Soft setting of the Spot light, but the second is not. To have a clearer understanding of this point imagine a tall thin pole in the middle of a flat plain illuminated by the Sun. The Sun is not a point, it has a dimension and, for us Earthlings, it is half of a degree wide. If you look at the shadow you will notice that it is very sharp at the base of the pole and that it grows blurrier as you go toward the shadow of the tip. If the pole is tall and thin enough its shadow will vanish.

To better grasp this concept have a look at (Area light and its shadow). The Sun sheds light while the middle object completely obstructs the Sun's rays only in the dark blue region. For a point in the light blue region the Sun is partially visible, hence each of these areas is partially lit.

The light blue region is a partial shadow region where illumination drops smoothly from full light to full shadow. It is also evident, from (Area light and its shadow) that this transition region is smaller next to the shadow casting object and grows larger farther away from it. Furthermore, if the shadow casting object is smaller than the light casting object -- and if the light casting object is the Sun this is often the case -- there is a distance beyond which only partial shadow remains (Area light and its shadow 2).

If we place a single Spot light at a fixed distance from an initial plane object and look at the shadow cast at a second plane object, as this second plane object gets further away we notice that the shadow gets larger but not softer (Spot light and its shadow).

[edit] Building a fake Area Light

Of course it isn't really necessary to create a fake Area light because there already is an Area light built in. But if it can't configure exactly type of Area light you are looking for you can always enhance a fake one!

To fake an Area light we can use several Spots, as if we were sampling the area casting light with a discrete number of point lights. This can either be achieved by placing several Spot lights by hand, or by using the more efficient DupliVert feature.

1. First add a Mesh Grid using the Toolbox, (SPACE), Add>>Mesh>>Grid menus that is 4x4 in size. Place the grid exactly where the Spot light is and be sure the grid's normals are pointing down. You can inspect the normals from Editing context (F9) in the Mesh Tools 1 panel by enabling Draw Normals button. If they are pointed upwards then click the Flip Normals button in the Mesh Tools panel.
2. Parent the Spot light to the Grid by selecting the light and then the grid and press (CTRL P). Remember the last object selected will be the parent.
3. Select just the Grid and in the Object Context, (F7), press the DupliVert and Rot buttons in the Anim Settings Panel (F7). This will duplicate the children of the parent-child relationship which means the light (or child) will be duplicated at each of the parent's vertices. You should now see 16 Spot lights arranged in a grid pattern all pointing downwards.

Rot is not strictly necessary but will help you in positioning the Area Light later on. You will have a set of Spot lights as shown in (Spot light and its dupliverts).

Grid setup

Spot light and its dupliverts

We need to decrease the Energy of the child Spot light (the original spot light). If for a single Spot light you used a certain Energy, you must now subdivide or distribute that Energy among all the duplicates; currently you have 16 times as much energy being injected into the scene. There is so much energy now that the lights almost washout the shadows.

Because we have 16 Spot lights, each should be allotted 1/16 of Energy (that is Energy=0.0625). The same two renderings of above, with this new hacked area light will yield the results in (Fake Area light with multiple Spots).

The result is far from that expected, because the Spot light sampling of the Area light is too coarse. On the other hand, a finer sampling would yield a higher number of duplicated Spot lights and cause unacceptably long rendering times.

A much better result can be attained by softening the Spot lights by the following configuration:

• SpotBl=0.45,
• Sample=12,
• Soft=24 and
• Bias=1.5

The results is shown in (Fake area light with multiple soft Spot lights).

We now apply this Pseudo area light to Cornelius as shown in (Cornelius under Area Light). The results shows what happens to Cornelius once the Key Light is substituted with 65 duplicated Spot lights with an Energy=0.0154 in a circular pattern. Note how the shadow softly transitions from sharp shadows next to the feet to progressively softer shadows as it gets further away from him. This is the correct physical behavior.