Shadow Ray Manipulation

Suppose your scene is set inside a cave, and you want ambient light infused throughout your cave. An ideal solution might be to place an environment light outside of the cave and “ignore” your cave for the purposes of shadowing. There are, in fact, a number of reasons you might wish to ignore occluding geometry: to bring more light in, to avoid self-shadowing, to prevent non-hero objects from casting distracting shadows, and so on.

MoonRay provides a number of different controls that allow us to ignore occluding objects:

What are shadow rays? Permalink to What are shadow rays?

At each shading point in the scene, we must determine whether (and how much) the point is in shadow. The way we measure this is by shooting a shadow ray toward the point we’ve sampled on the light. If our shadow ray intersects with some geometry in the scene before it reaches the light, it’s considered to be in shadow, and the radiance we might have added from the light is discarded. We shoot as many shadow rays as there are light samples.

Shadow Ray Illustration
In this example, we have a shading point p, a RectLight, and a pink sphere geometry in between. In order to determine how much p is in shadow, we shoot (in this case, four) shadow rays toward the RectLight and find that two intersect with the pink geometry. Therefore, our point is partially in shadow.

Each of our shadow ray settings allow us to ignore shadow-casting objects by shortening the shadow rays, ensuring that they do not find intersections with geometry within a certain distance. While the mechanism is similar for each attribute, they differ in the following ways:

  • They either move the start or end of the shadow ray
  • They measure distance from either the shading point or the light intersection
  • They are defined either on a per-geometry basis or a per-light basis

Shadow Ray Epsilon Permalink to Shadow Ray Epsilon

The shadow_ray_epsilon attribute effectively establishes a radius around the shading point within which no objects can cast shadows. It does this by moving the origin of the shadow ray the specified distance away from the shading point, preventing it from finding the nearby intersections.
Moves the start of the shadow ray Measures distance from the shading point Defined on the geometry
Shadow Ray Epsilon Illustration
In this example, we have a shading point p, a RectLight, and a pink sphere geometry in between. We have a shadow_ray_epsilon set to 5 units. We therefore move the origin of the shadow ray 5 units from the shading point, which means we don't find the intersection with the pink geometry. Note that rays originating from Geom2 do not get the same treatment, since shadow_ray_epsilon is set only on Geom1.
In this scene, you see a ground plane and a few thin cubes arranged in a stair-stepping pattern above it. In the upper right, we have a green light casting blue shadows, and in the upper middle, we have a blue light casting green shadows. A shadow_ray_epsilon is set on the ground plane geometry. Note that, as shadow_ray_epsilon increases, and the origins of the shadow rays start to miss the stair-stepping geometry, all shadows on the ground plane start to disappear. The shadows on the stair-stepping geometry are unaffected because this setting applies only to the ground plane geometry.

Min Shadow Distance Permalink to Min Shadow Distance

The min_shadow_distance attribute is identical to shadow_ray_epsilon, except that it applies to a specific light instead of a geometry.
Moves the start of the shadow ray Measures distance from the shading point Defined on the light
Min Shadow Distance Illustration
You can see that the mechanism for min_shadow_distance is identical to shadow_ray_epsilon. However, it's applied to the light instead of the geometry, so we see it affect shadow rays from all geometry in the scene.
In this scene, you see a ground plane and a few thin cubes arranged in a stair-stepping pattern above it. In the upper right, we have a green light casting blue shadows, and in the upper middle, we have a blue light casting green shadows. A min_shadow_distance is set on the green light on the right. Note that, as min_shadow_distance increases, all of the blue shadows cast by the green light start to disappear. The green shadows cast by the blue light are unaffected, as this setting applies only to the shadow rays cast toward the green light.

Shadow Ray Epsilon vs. Min Shadow Distance Permalink to Shadow Ray Epsilon vs. Min Shadow Distance

shadow ray epsilon Moves the start of the shadow ray Measures distance from the shading point Defined on the geometry
min shadow distance Moves the start of the shadow ray Measures distance from the shading point Defined on the light
In this scene, you see a ground plane and a few thin cubes arranged in a stair-stepping pattern above it. In the upper right, we have a green light casting blue shadows, and in the upper middle, we have a blue light casting green shadows. A min_shadow_distance is set on the green light on the right, and a shadow_ray_epsilon is set on the ground plane. As both increase at equal rates, you'll notice that shadow_ray_epsilon clears more and more shadows on the ground plane, while min_shadow_distance clears more and more of the blue shadows cast by the green light, regardless of geometry.

Max Shadow Distance Permalink to Max Shadow Distance

The max_shadow_distance attribute establishes a radius around the shading point outside of which objects don’t cast shadows. It does this by moving the end of any shadow ray the specified distance away from the shading point.
Moves the end of the shadow ray Measures distance from the shading point Defined on the light
Max Shadow Distance Illustration
In this image, our RectLight has a max_shadow_distance of 5 units. That means we only want to include any occluder intersections a maximum of 5 units from the shading point in all directions. We've therefore established a sphere of influence outside of which all intersections with occluding geometry are ignored.
In this scene, you see a ground plane and a few thin cubes arranged in a stair-stepping pattern above it. In the upper right, we have a green light casting blue shadows, and in the upper middle, we have a blue light casting green shadows. A max_shadow_distance is set on the green light on the right. Note that, as max_shadow_distance increases, and the ends of the shadow rays start to pass the stair-stepping geometry, the shadows cast by the green light start to appear. Again, note that the green shadows are unaffected, as max_shadow_distance is set on a per-light basis.

Min vs Max Shadow Distance Permalink to Min vs Max Shadow Distance

min_shadow_distance Moves the start of the shadow ray Measures distance from the shading point Defined on the light
max shadow distance Moves the end of the shadow ray Measures distance from the shading point Defined on the light

Clear Radius Permalink to Clear Radius

The clear_radius attribute establishes a radius around the light within which objects cannot cast shadows. It does this by moving the end of the shadow ray the specified distance away from the light.
Moves the end of the shadow ray Measures distance from the light intersection Defined on the light
Clear Radius Illustration
In this example, our clear radius is set to 5 units. This means that any intersections with occluders within 5 units of the light intersection point will be ignored. We do this by moving the end of the shadow ray 5 units from the light intersection point.
In this scene, you see a ground plane and a few thin cubes arranged in a stair-stepping pattern above it. In the upper right, we have a green light casting blue shadows, and in the upper middle, we have a blue light casting green shadows. A clear_radius is set on the green light on the right. Note that, as clear_radius increases, and the ends of the shadow rays start to move closer and closer to their origins, the shadows start to clear.

Combining All Attributes Permalink to Combining All Attributes

How do all of these attributes work together? Well, there are a few different scenarios:

  1. Both attributes move the origin of the shadow ray (shadow_ray_epsilon, min_shadow_distance). We take the maximum distance between the two.
Moving the Origin of the Shadow Ray Illustration
On the left, we start with applying a shadow_ray_epsilon of 2 to the blue geometry. However, when we apply the min_shadow_distance of 4, we take the max distance, and end up with a shadow ray origin 4 units from the shading point.
  1. Both attributes move the end of the shadow ray (max_shadow_distance, clear_radius). We take the minimum distance between the two.
Moving the End of the Shadow Ray Illustration
On the left, we start with applying a clear_radius of 2 to the light. Let's assume our shadow ray is 5 units long. That means the end of the shadow ray is now 3 units from the shading point. When we apply a max_shadow_distance of 2 units (on the right), we take the minimum, and end up moving the end of the shadow ray 2 units from the shading point.
  1. One attribute moves the origin of the shadow ray, and one moves the end. We move both the start and end of the ray accordingly. If either the start or the end of the ray overlaps the other, we end up with a shadow ray with zero length (since we can’t have negative length), so we ignore all occluders along the ray.
Moving the Start and End of the Ray Illustration
On the left, we start by applying a clear_radius of 2. On the right, we then add a min_shadow_distance of 1. We simply move the start and end of the shadow ray the appropriate distances.
Moving the Start and End of the Ray Illustration 2
On the left, we once again apply a clear_radius of 2. On the right, we then apply a min_shadow_distance of 4. Let's again assume the ray is of length 5. The start and end of the ray now overlap each other, leaving us with a shadow ray of length zero.

Quick Reference Permalink to Quick Reference

Shadow Ray Epsilon Min Shadow Distance Max Shadow Distance Clear Radius
Moves the start of the shadow ray Moves the start of the shadow ray Moves the end of the shadow ray Moves the end of the shadow ray
Measures distance from the shading point Measures distance from the shading point Measures distance from the shading point Measures distance from the light intersection
Defined on the geometry Defined on the light Defined on the light Defined on the light