Light domes & environment maps

In the previous section we talked about indirect lighting and how it can create a more realistic image at the cost of rendering speed. In those examples we looked at how the light from singular light sources can bounce around a scene. We also looked at how to fake lighting from the environment using environment maps and / or fresnel shaders.

But there is a way to combine the two into a single approach that offers the better aspects of both of those operatios. Of course this will, again, be at the cost of rendering speed.

What we're talking about is the use of light probes, light domes, and how HDR imagery can be used in conjunction with both to create a more realistic scene. The key factor is how most indirect lighting algorithms allow us to project light FROM a self-illuminated surface. This means the texture applied to an object that creates the look of "incadescence" can actually CREATE light within a scene.

The phrase "light probe" actually can refer to both a digital and real-world object.

A "light dome" refers to a literal dome shape within a 3d space

Because of the way that environment maps are applied to light domes in individual programs there is not much to be said about them in the general, software non-specific section. We'll discuss them more in the software specific pages. Basically you can think of them as applying a texture to a sphere shape you can't actually see in the scene.

Dome lights are a fairly constant idea between programs. This involves creating an actual spherical mesh and applying an emissive / self-illuminating texture map to it.

Creating an in-scene light probe

In some cases you can save time on rendering by creating static background images and sampling the surrounding scene with an in-engine probe you make yourself. This is especially useful for real-time video games where you don't want to continuously re-render background imagery that isn't going to be seen from a different perspective other than "far away". "far away" is the technical term.

This approach is also useful to create environment maps for objects within the scene if you don't have bouncing light turned on.

The basic operation should be identical to the process used in the "baked lighting" section of the previous page where we used baking to "save" shadows on a surface to a texture.

Real world physical light probes

Generally you won't have to worry about the reflection of the photographer being in the light probe itself. Unless there is a perfectly reflective object within the scene (such as the examples here) then you won't actually SEE the reflection of the photographer.

Once you have the projection that's right for you you'll need to assemble the various exposures into a single HDR file. If you are unaware of how to go about this process follow the link below to read about HDRI merging in the graphic design section.

Graphic Design : Image Preparation

Graphic Design : Image Preparation in Photoshop

Rendering with HDR Images

Under construction.

Taking pictures with a light probe

  • It doesn't hurt to take multiple exposures of your background plate as well. If you've set up for steady shots of a chrome ball, then why not go ahead and make it so you can adjust the background image as well?
  • Your probe photos should be from the same ANGLE as the clean plate.
  • Your probe photos should be from the same DISTANCe as the clean plate.

If you don't have a fancy motion controlled rig as large VFX houses would then you can simulate an animated light probe setup by *fading* between light probe images taken at multiple positions. This is how some video game engines work. Once a game level is built a level artist places digital light probes within a scene to create cube maps that sample light in the scene. Characters moving within the scene adopt the qualities of nearby light probes based on how close they are to those probes. This allows for fast environment lighting without constant environment sampling.

Processing the photos

Adobe photoshop unfortunately lacks the ability to make maps from images of reflective light probes / chrome balls.

Since the subject of HDRI crosses over fairly heavily with Photography and Graphic Design the instructions on compiling individual images into a single HDRI image that can be used within your 3D software can be found in a different section.

The link below will take you to the pertinent section.

Graphic Design : Image Preparation in Photoshop

Manual UV editing

While the process may differ for various circumstances the general appraoch to creating an in scene dome is simple.

  1. Make a sphere.
  2. Rotate it so that one pole (where points converge) faces what will be the front.
  3. Cut off the circle of faces (the triangleS) on the other side (the back).
  4. Use a planar (or "flat") projection to get a UV map of the sphere from the front.
  5. From the center point select the circular rows of the sphere from front to back and scale them down towards the center (or start from the back and scale them up.

Once none of the circular rows are overlapping you can fit it within the image of the reflective sphere. You may need to adjust it to obtain the appropriate distortion towards the edges of the sphere (the uv quads will become *very* thin towards the edge).

Implementing light probes within a 3D scene

Under construction.