Shadows and Shadow Maps

Casting shadows with MTOR
Tutorial 2

Introduction -Creating Shadows 

    The scene from the last tutorial has shaders attached to the objects and lights in the scene, but it still requires shadows for a more convincing look. 

   PhotoRealistic RenderMan creates shadows through the use of shadow maps. Shadow maps are (usually) generated by lights, and the shadow map itself is a texture file that is stored in the Maya project and used by Prman to calculate a shadow. 

   Shadows maps offer several advantages. Once a shadow map is generated, it can be kept and reused. Also, shadow maps are versatile, allowing arbitrary control over lighting. For instance, a light can cast a shadow generated from another light.

1 - Opening the Scene

Open the file ladder2.ma, or use the completed scene from the last tutorial. 

The trick to creating shadows with MTOR, is remembering that shadows are created by textures, shadow maps, and these maps must be managed properly. There are two steps in using shadow maps successfully: 

 

a) A valid shadow map must first be generated.

b) That shadow map must be then referenced by the appropriate light(s).

2 - Generate shadows

In the palette, double-click on the light shader, "keyLight." (This light shader should be connected to the key light in the scene.)

The appearance editor will open.

To enable shadows . . .

Scroll to the parameter "Make shadow map," and double-click on the colored rectangles. That will toggle open a menu.

To enable shadows, change the Frequency setting from "Never" to "Every Frame." This will cause the light to generate a shadow map for each frame that is rendered. 

(Read more about computed maps in the  Shader Concepts section: Using Computed Maps. They're a very useful thing to know about.)

This generates a shadow map.

3 - Refer to the shadow map

Now here's the trick . . .

The light shader will now generate a shadow map from whatever light it is attached to. But we must now tell this light shader to use the shadow map it has created. 

Under the shadowname parameter the name of the shadow map must be entered. To do this, click on the colored texture button (the yellow, blue, purple, and green button in the image on the right). A pull-down menu will appear. Select: 
Refer to-> Shadow

Text should now appear in the field identical to the text on the right [shdmap $OBJNAME].

(In the section Using the Slim Interface, you can find out more about referencing shadow maps: The Reference Texture Menu.)
 

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.
. This parameter is crucial.
It gets this entire section here.

4 - Render and adjust the shadow

When the new image renders, the ladder has a shadow. 

In the keyLight appearance editor, there are a number of basic shadow controls to adjust: shadow color, blur, etc. Clicking the info "i" buttons will reveal some helpful information.

Also note that the shading rate has an enormous effect on shadow quality. Shading rate is controlled from the "Quality" tab of the RenderMan Globals. 

RenderMan-> RenderMan Globals

A shading rate of "1" will create good looking shadows, a lower quality setting of "5" will create shadows that seem "chunky."

Conclusion

   And that's how shadows are created. In this example a MTOR Spot Light was used to create the shadow. In practice, there are several ways to create shadows, but remember that in every case a shadow map must first be enabled and then the shader must be set to refer to it.
    The next couple steps will show a couple of techniques that will make working with maps a little easier.
 

Shadow Notes - Tips and Tricks

Tips and tricks for working with shadow maps.

Note A  - Viewing Shadow Maps #1

Next we'll take a look at the shadow map, but first MTOR must be told to keep the shadow map on disk. By default, shadow maps are "cleaned up" after every job, to keep the maps from cluttering up the directory. In this case, however, it is desirable to keep the shadow maps. To do this, open the RenderMan Globals.

RenderMan-> RenderMan Globals

Open the "Spool/Job Setup" tab of the RenderMan globals. Notice the "Cleanup" parameter on the panel on the right. Since we want to keep the maps, click on the "map" button so it is raised, like it is on the right. Now shadow maps will remain. 

Note A - Viewing Shadow Maps #2

Render again, and this time the shadow map will not be cleaned up after the render job is over.

After the shadow map is generated, go to a command prompt. The shadow maps are placed in the current Maya project directory, inside a directory created by MTOR called rmantex/shd/

This is the area where shadow maps are stored. There should be a file there called:

ladder.keyShape.shd.0001.tex

Now type at the prompt:

sho ladder.keyShape.shd.0001.tex

An image similar to the one on the right will be displayed in it. There is much valuable information to be gleaned from map.

The cone angle is used to define the area of the shadow map, and this is important because shadows of objects can only be as detailed as the pixels it is created from in the shadow map.

So a couple basic things to remember when generating shadow maps:

1) Tighten the cone angle of a light to frame objects that cast shadows.

2) Set the map resolution to the required resolution.

In the shadow map on the right, note that most of the objects in the scene which won't be casting shadows, like the floor, have had their "Cast Shadows" attributes purposely turned off, which makes the shadow map more efficient.

Note B - Reusing Shadow Maps

Once a map is saved, it can be reused. This means shaders can be adjusted and the scene can be rerendered, but a shadow map doesn't need to be generated every time. This can speed up workflow quite a bit.

The reused shadow will be correct and true as long as the light isn't moved, the coneangle isn't changed, and all the shadowing objects remain in the same place.

To reuse shadow maps, ensure that map cleanup is off, like has been done above. 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 shadow map will be reused until "Lazy Compute" is turned off.

In the image on the right, this technique is emphasized by deleting the ladder and reusing a previous shadow map. The effect is a shadow of a ladder cast where there is no longer any ladder. Now to update the shadow maps correctly we'd just turn "Lazy Compute" off. 

So reusing maps can speed things up, but don't leave them on by accident. 

Note C - Monitoring shadow generation

Now take a quick look in Alfred and see what happens when a shadow map is created. First, start a render. The Alfred render queue should open.  Next click on the black triangle by to the render in the queue, that render's job window will open.

Looking in the job window,  the boxes represent different tasks of the render. The empty boxes represent finished tasks, the green boxes are in progress, and the solid boxes are yet to be worked on.

Job window #1 is a single frame job with no maps being generated. The green box is the final image being rendered.

In job window #2, the same scene is rendered, but now an additional shadow map is being created. This map is actually displayed now in job window #2. The green box is showing the progress of the shadow map. If you're successfully generating a shadow map, your job should look like window #2. 

The Alfred job window gives a quick and clear overview about what is going on in a render job. It can be very helpful when working with larger, complicated scenes.

The Alfred Render Queue 
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Job Window #1
Without shadow map
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Job Window #2
With a shadow map

Note D - Building shadows into Slim Shader Networks

It's easy to build shadows into Slim shading networks. Once it is built into the network, you can control them with the methods outlined above. To see how this is done read through the Shadow recipe under Shader Design section.

Note E - Sharing Maps Between Lights

Further Reading

You may find yourself using computed maps quite a bit. They can be used to create shadows, reflections, reference images, depth maps, and more. In practice the use computed maps is straight forward. You can read more about computed maps under the Shader Concepts section. Application Note #15 has more information about shadows.

Next:

Learn how to create soft shadows, which simulate a diffuse light source.
Now save this file and use it in the next tutorial.