Light object

Light Objects cast light on other objects in a scene.

All Parameters Local variables

Light Objects are those objects which cast light onto other objects in a scene. With the light parameters you can control the color, shadows, atmosphere and render quality of objects lit by the light. Lights can also be viewed through and used as cameras (Viewport > Camera menu).

Deep shadows

Deep shadows were added in Houdini 8. Their advantage include:

  • Shadows of transparent objects, including volumetric objects.

  • Motion-blurred shadows.

  • Better shadow anti-aliasing.

Deep shadow maps store the opacity of any translucent objects the light ray passes through as well as the depth of the final opaque surface.

See the discussion for the Shadow Style parameter on the Shadow tab for additional information on deep shadows.

Note

Deep shadows use significantly more disk space than plain depth maps.

Parameters

Transform

Keep Position When Parenting

When the object is re-parented, maintain its current world position by changing the object’s transform parameters.

Pre-transform menu

This menu contains options for manipulating the pre-transform values. The pre-transform is an internal transform that is applied prior to the regular transform parameters. This allows you to change the frame of reference for the translate, rotate, scale parameter values below without changing the overall transform.

Clean Transform

This reverts the translate, rotate, scale parameters to their default values while maintaining the same overall transform.

Clean Translates

This sets the translate parameter to (0, 0, 0) while maintaining the same overall transform.

Clean Rotates

This sets the rotate parameter to (0, 0, 0) while maintaining the same overall transform.

Clean Scales

This sets the scale parameter to (1, 1, 1) while maintaining the same overall transform.

Extract Pre-transform

This removes the pre-transform by setting the translate, rotate, and scale parameters in order to maintain the same overall transform. Note that if there were shears in the pre-transform, it can not be completely removed.

Reset Pre-transform

This completely removes the pre-transform without changing any parameters. This will change the overall transform of the object if there are any non-default values in the translate, rotate, and scale parameters.

Transform Order

The left menu chooses the order in which transforms are applied (for example, scale, then rotate, then translate). This can change the position and orientation of the object, in the same way that going a block and turning east takes you to a different place than turning east and then going a block.

The right menu chooses the order in which to rotate around the X, Y, and Z axes. Certain orders can make character joint transforms easier to use, depending on the character.

Translate

Translation along XYZ axes.

Rotation

Degrees rotation about XYZ axes.

Viewing Options

Display

Whether or not this object is displayed. Turn on the checkbox to have Houdini use this parameter, then set the value to 0 to hide the object, or 1 to show the object. If the checkbox is off, Houdini ignores the value.

Icon Scale

Scales the viewport geometry. This parameter is only for display purposes.

Cache Object Transform

Caches object transforms once Houdini calculates them. This is especially useful for objects whose world space position is expensive to calculate (such as Sticky objects), and objects at the end of long parenting chains (such as Bones). This option is turned on by default for Sticky and Bone objects.

See OBJ cache preferences for how to control the size of the object transform cache.

Viewport Selecting Enabled

Object is capable of being picked in viewport.

Select Script

Script to run when the object is picked in the viewport. See select scripts .

Lookat Options

Look At

Object to point to. Constrains an object so its -Z axis always points at another object’s origin.

Look At Up Vector

When specifying a look at, it is possible to specify an up vector for the look at. This controls the roll of this object when looking at the look at object.

Don’t Use Up Vector

Use this option if the look at object does not pass through the Y axis of this object.

Use Up Vector

This precisely defines the direction of the Y axis of this object while looking. The up vector specified should not be parallel to the look at direction. The value used is specified by the Orient Up Vector parameter below.

Use Quaternions

Quaternions are a mathematical representation of a 3D rotation. This method finds the most efficient means of moving from one point to another on a sphere.

Path Options

Path Object

A reference to the curve node or object you want this object to follow.

Roll

The orientation of the object around the path.

Position

The position of the object along the path. 0.0 means at the beginning of the path, and 1.0 means the end. The integer portion of the position is used as the primitive number inside the geometry while the fractional part indicates the u value on the primitive.

If the primitive number does not exist, then it is wrapped back to the beginning. So for numbers greater than 1.0 in a path object that only have a single primitive, it wraps around to the beginning again. You can easily animate something moving around a circular track simply by continuously increasing the position value.

Keyframe this value to animate the object moving along the path.

Note

The Follow Path tool on the shelf automatically sets keyframes on this parameter that you may want to edit or replace.

The Parameterization option controls how position values between 0 and 1 correspond to knots on the path curve.

Parameterization

Controls how the Position parameter is translated into a point on the path curve. Use Arc-length (the default) to interpolate the position values evenly along the physical length of the curve.

Use Uniform to distribute the position values between the knots of the curve. This lets you slow down the object at certain points by bunching up knots in the path curve. However, it is much more convenient to simply edit the animation curve to control the speed of the object along the path.

Orient Along Path

Orient the object according to the path’s curvature.

Orient Up Vector

The direction vector of the object’s Y axis to orient with.

Auto-Bank factor

Controls automatic banking of the object as it turns corners. Set this to 0 to turn automatic banking off.

Light

Light Type

The type of light. See the Lights and Cameras shelf help for light descriptions of light types.

Light Color

The color of the light source.

Light Intensity

A scale on the color. If the intensity is 0, the light disabled. The light will only be sent to the renderer if the object is included in the “Force Lights” parameter of the output driver.

Light Enabled

Turning this toggle off is equivalent to setting the light intensity to 0.

Enable Light In Viewport

Turning this off removes the light from consideration in the viewport. This is useful for lights which only make sense at render time.

Diffuse contribution

The light will contribute to diffuse illumination when illuminating a surface.

Specular contribution

The light will provide specular highlights on the surface when illuminating a surface.

Categories

The categories defining the set membership of this object.

Attenuation

Controls how light decreases in intensity the farther you get from the light source.

No Attenuation

The light is the same intensity, regardless of how far away a surface is from the light.

Half Distance Attenuation

You specify a distance at which the light attains half the intensity that it is at the origin of the light. This is modeled mathematically as

Cl = A / ( A + D )

where A is the attenuation and D is the distance from the light source. When D is 0, we end up with A/A which is 1. When D is equal to A (the half-distance) the intensity is A/(A+A), which is half the intensity.

The light is never fully extinguished no matter the distance from the source.

Physically Correct Attenuation

In reality, light falls off based on the the inverse square law. This attenuation mode implements this type of fall off. Mathematically, the intensity is modeled by

Cl = 1 / (D * D)

Quadratic Attenuation

This is a general form of attenuation which covers many different types of light fall-off. Three coefficients are specified to create a quadratic equation.

C = Constant coefficient L = Linear coefficient Q = Quadratic coefficient

The mathematics of this model are represented by

Cl = 1 / ( C + L*D + Q*D*D)

To get a physically correct light source

C = 0 L = 0 Q = 1 Cl = 1 / ( 0 + 0*D + 1*D*D) = 1 / (D*D)

But, by adjusting the L, C, and Q coefficients, many different types of non-physical fall-off may be created.

Half-distance fall-off may be created using

C = 1 L = 1/A Q = 0 Cl = 1 / ( 1 + 1/A*D + 0*D*D ) = 1 / (1 + D/A)

When D == A, the light has half the intensity as it would at the origin of the light (when D == 0).

No Attenuation can be created using

C = 1 L = 0 Q = 0 Cl = 1 / ( 1 + 0*D + 0*D*D ) = 1 / 1 = 1

Attenuation Distance

A in the attenuation formula (see above).

Constant Coefficient

C in the attenuation formula (see above).

Linear Coefficient

L in the attenuation formula (see above).

Quadratic Coefficient

Q in the attenuation formula (see above).

Spot Light Options

The area within the center ring receives full illumination. The light falls off between the inner ring (defined by the Cone angle) and the outer ring (defined by the Cone delta). The dashed line represents where illumination is half as bright as within the cone angle. The Cone rolloff parameter moves this line between the center and outer rings.

Cone Angle

The size of the spotlight’s beam. Enter the number of degrees wide the spotlight’s cone should be. You can only change this parameter if you are using a cone light.

Cone Delta

Number of degrees of penumbra for spotlights.

Cone Rolloff

Shape exponent of penumbra (larger = sharper).

Projection Map

Specifies the image that gets projected. The viewing angle of the light will determine the area of projection.

Use cone angle for field of view

When rendering shadow maps or using a projection map, the cone angle will be used to define the field of view. However, to be able to get better shadows on a certain section of geometry, it is useful to be able to de-couple the field of view control. Toggling this parameter off will enable the Projection Angle or Orthographic Width parameters.

Projection Angle

The viewing angle used for depth map generation when perspective view is used.

Orthographic Width

Controls the field of view for spot or distance light sources when decoupled from the cone angle when orthographic view is used.

Clamp Projected Map to Frustum

When you have a projection map from the light source, you can either have the map streak its color outside the viewing frustum or you can have the map clamped to the viewing frustum.

Area Light Options

Area Shape

Line

A line light which is from (-0.5, 0, 0) to (0.5, 0, 0) in the space of the light.

Grid

A rectangular grid from (-0.5, -0.5, 0) to (0.5, 0.5, 0) in the space of the light.

Disk

A disk shaped light. The disk is a unit circle in the XY plane in the space of the light.

Sphere

A sphere shaped light. The sphere is a unit sphere in the space of the light.

Environment

A hemisphere (or sphere) environment light. The position of the light does not matter. You can rotate the light object to orient the light source. With no rotation, hemisphere lights are oriented to cover the region above the XZ plane.

An environment light may use a texture map to provide HDRI illumination from an environment map. With no rotation, the environment map is oriented so that the top face aligns with the positive Y axis.

See how to create an environment/reflection map.

Area Samples

The number of lighting samples used to evaluate the light source. Depending on the area of the light source, more samples might be needed to provide smoother illumination.

Area Size

A uniform scale on the area light.

Use full sphere for environment

When using an environment light, illumination is typically computed over the hemisphere. This option turns on full sphere illumination.

Use Area Map

This option is only currently used for environment lights.

Area Map

This option is only currently used for environment lights. The area map will be tinted by the color of the light source.

Shadows

Shadow Type

None

No shadows will be cast by this light.

Ray-Traced Shadows

Ray-tracing will be used to compute shadows from this light source.

Depth Map Shadows

Shadows will be computed using a depth map. When transparent shadows are turned on, deep shadow maps will be used. Otherwise, simple z-depth shadow maps will be used.

Shadow Mask

A mask of objects which will be considered occluders for the light source. When ray-traced shadows are used, only these objects will be intersected by shadow rays. When depth map shadows are used, only these objects will appear in the depth map.

Note

It is possible to use categories by adding the shadowcategories rendering parameter.

Shadow Bias

The bias when computing shadows from this light source. This is a small distance. If occluders are within this distance, they will not be considered shadowers. This is to prevent self-shadowing artifacts due to numerical precision.

Shadow Intensity

A scale on how much light is blocked by occluders. By decreasing the shadow intensity, the occluders will not block all the light, allowing some illumination through.

Shadow Quality

Adjust the quality of lookups when evaluating standard shadow maps or blurred shadows

Shadow Softness

A softness control on the shadows.

Ray-traced blurred shadows may be expensive to compute and may require a higher shadow quality to produce smooth shadows. It may be better to use area lights which give more accurate and slightly more efficient soft shadows.

A blur on the shadow map when using depth mapped shadows.

Shadow Blur

Controls the fraction of the shadow map image to blur when rendering.

This parameter can be used to blur the map disregarding the surface derivatives, which are taken into account but the Shadow Softness parameter.

Transparent Shadows

When this option is turned on, the surface shader of occluders will be evaluated to determine the opacity. Turning this option on will cause deep shadow maps to be used when shadowing using Depth Map Shadows.

Depth Map Options

Auto-generate Shadow Map

Generate a shadow map from this light source during the next render.

Shadow Map

The path to the shadow map.

Resolution

The resolution of the shadow map. It is a good idea to keep the resolution the same in X and Y.

Pixel Samples

The number of pixel samples to be used when rendering shadow maps. This is more important when deep shadow maps (transparent shadows) are being used.

Near Clipping

Position of near clipping plane.

Far Clipping

Position of far clipping plane.

Depth Map Motion Blur

When rendering deep shadow maps, it is possible to capture motion blur in the shadow map. If motion blurred shadows are desired, it’s important to turn on Transparent Shadows.

Local variables

IPT

This is typically -1. However, if the object is performing point instancing, then this variable will be set to the point number of the template geometry. For the IPT variable to be active, the Point Instancing parameter must be turned on in this object.

Note

This variable is deprecated. Use the instancepoint expression function instead.