Creating Reflections

Reflection & Environment Maps

Introduction - Creating a Reflection

In PRMan, reflections are generally simulated through the use of maps. Maps can often be used to simulate reflections more quickly and efficiently than the alternative of raytracing, especially for large, complicated scenes.

Reflections are simulated in PRMan by generating "mirrored" image maps from reflection cameras. When properly set up, the resulting reflections, while not physically accurate, are usually quite convincing to the human eye. MTOR makes the process of setting up reflections fairly trivial, and once the process is understood it is straightforward.

PRMan uses two different types of maps to generate reflections: reflection maps & environment maps. The appropriate type of map is determined by what kind of object it will be attached to. For flat surfaces, a reflection map is used. For objects with volume, like spheres and cubes, an environment map is used.

In this lesson we'll set up reflections using both a reflection map and an environment map.

1 - Open the file

For this tutorial, follow along with the Maya scene:

mtor/scenes/mirror-reflections/

mirror.ma

In this scene, we'll set up reflections using Slim.

First, we'll add a reflection to the mirror, using a reflection map. 

Second, we'll add an environment map to the metal rock on the table.

   
Open the scene, mirror.ma 

2 - Render the File

For the sake of comparison, render the scene to see what it looks like without reflections.

The scene has no reflections.

 Creating a Reflection Map

Reflection Maps create reflections on flat surfaces, like a floor or a wall..

3 - Importing a Reflection Map Shader

First we'll create a reflection map.

To add a reflections, we'll import a shader by choosing, from a Slim palette: 

File-> Import Appearance 

The file picker will open. Go to the "RAT Shaders" directory (On NT, this directory is located under the "Go to:" menu. On SGI, this directory is listed under the folder presets at the top of the file picker.). 

When the "RAT Shaders" directory is open, import "ratReflection.slo".

Your palette should look similar to the one on the right.

 

4 - Enable the Reflection Map

Open ratReflection in the appearance editor, and expand the "make reflection map" collection. 

a) Change "Frequency" from "Never" to "EveryFrame." This will compute a reflection map for each frame.

b) For "Objects in Map" enter "mirror1". Later, we'll create a Maya set named "mirror1" to define what objects will appear in the reflection. 

c) For "Reflection" we need to make sure the shader refers to the reflection map. To do this, click on the blue and purple Texture button and select:

Refer to-> Reflection 

Next, attach the ratReflection shader to the mirror surface.

 

5 - Defining the Objects for the Reflection

For a reflection to work correctly, it is crucial to define which objects will appear in the reflection. We can define those objects using a Maya set. 

Important: The trick is that we only want the objects in front of the reflecting surface to appear in the reflection. You will get incorrect reflections if objects behind the mirror (or the mirror itself) are set to appear in the reflection. 

To define the correct objects for this case, select all of the objects in the scene . . . except for the walls and the mirror surface, as is shown in the image on the right. 

Now create a new set named "mirror1":

Edit-> Sets-> Create Set [Option Box]

6 - Rendering the Reflection Map

Now try rendering the scene.
RenderMan-> Render

The rendered image should look like something on the right.

 Creating an Environment Map

Environment Maps create reflections on objects with volume, like a sphere.

7  - Import an Environment Map

From the same place that we imported ratReflection, now import the shader called ratEnvironment.

Your palette should look somewhat like the image on the right.

8 - Enable the Environment Map

This step is very similar to enabling the reflection map.

First, open ratEnvironment in the appearance editor, and expand the collection, "make reflection map." 

a) Change "Frequency" from "Never" to "EveryFrame." This will compute an environment map for each frame.

b) For "Objects in Map" enter "rock1". As we did above, we'll create a Maya set to define what objects will appear in the reflection. 

c) For "Picture" we need to make sure the shader refers to the environment map. To do this, click on the blue and purple Texture button and select:

Refer to-> Environment 

d) Also, for the sake of speed, lower "Map Resolution" to 128.

Now, attach the ratEnvironment shader to the rock object on the table.
 

9 - Define Objects for the Environment Map

As described above, create a Maya set named "rock1", but in this case, the Maya set should include all the objects in the scene, except the rock itself. 
   
The environment map will use images generated from inside the rock, and if the rock is included in the reflection set, then the environment map will be the dark insides of a rock.
 

10 - Rendering the environment map

Now try and render the scene.
RenderMan-> Render

The rendered image should look like something on the right.

Reflection Notes - Tips and Tricks

Tips and tricks for working with reflections.

Note A - View the reflection map

You can view the reflection map using the same technique used to view shadow maps. (Review technique.)
Note that the files generated for reflections are put in a different project directory than shadows:
/rmantex/env
In the reflection map on the right, it is easier to see how important it is to select the correct objects to appear in reflections. If the back walls or the mirror were visible in this reflection map, it would create a strange reflection. 
 

A view of the reflection map,
frame.mirrorShape4.ref.0001.tex

Note B - View the environment map

You can view the environment map using the same technique used to view shadow maps. (Review technique.)

Again, files generated for reflections are put in a different project directory than shadows: /rmantex/env/

Here we can see that an environment map is composed of six different images, all knitted together. It can take a long time to compute six image maps, and that's why it will always speed things up to use the lowest "Map Resolution" that is acceptable.
 

Note C - Reusing Reflections

Much time can be gained be reusing reflection maps. In this case, reflection maps are saved to disk, and do not have to be recalculated for each job.

To reuse reflections, ensure that map cleanup is off. To turn map cleanup off, open the "Job Setup" tab of the RenderMan Globals, and turn map cleanup off by clicking on "map" so it is raised, like the image on the right.

Now we need to tell MTOR to reuse maps. Open the "Acceleration" tab of the RenderMan Globals, as seen in the image on the right. Change the "Lazy Compute" setting from "Off" to "Maps". Now the reflections will be reused until "Lazy Compute" is turned off.

 
The Job Setup Tab  
The Acceleration Tab