Lights

Lights are standard UsdLux prims. Crust attributes only decide whether the camera sees a light’s surface.

Supported lights

Crust Render reads these UsdLux light types with their standard inputs:intensity, inputs:exposure, inputs:color, inputs:normalize, inputs:enableColorTemperature and inputs:colorTemperature attributes:

inputs:color and inputs:shaping:focusTint are in the working colour space, as UsdLux specifies, unless a colour space is authored for them: colorSpace metadata on the attribute, or a UsdColorSpaceAPI colorSpace:name on the light or an ancestor (see Constant colours). A colour temperature's tint is computed straight into the working space, at unit luminance there. An image file is decoded from the colour space authored for it the same way; with none, an 8-bit image is sRGB and a float image (EXR, .hdr) is taken as already in the working space.

Area lights also read ShapingAPI: the cone (inputs:shaping:cone:angle, inputs:shaping:cone:softness), focus (inputs:shaping:focus, inputs:shaping:focusTint) and IES profiles (inputs:shaping:ies:file, inputs:shaping:ies:angleScale, inputs:shaping:ies:normalize).

Light linking and shadow linking use the standard collection:lightLink and collection:shadowLink collections. See samples/light_linking.usda.

Not supported: mesh lights (MeshLightAPI), portal lights, light filters, and shaping on distant and dome lights. inputs:diffuse and inputs:specular are ignored with a warning.

Camera visibility

By default the camera doesn't see the surface of an area light, which is the usual convention in Arnold, RenderMan and Karma. A light can sit inside the frame without showing up, while it still lights the scene. Shadow and indirect rays still see the surface: it casts shadows, and appears in reflections.

Lights at infinity (dome and distant lights) are the opposite: the camera does see them by default, as the sky behind the scene.

crust:light:cameraVisible

bool, default false for area lights, true for dome and distant lights.

Whether camera rays see this light.

# A Cornell-box style light, visible as a bright patch in the image.
def RectLight "Ceiling"
{
    float inputs:width = 1
    float inputs:height = 1
    float inputs:intensity = 10
    bool crust:light:cameraVisible = 1
}

# A dome that lights the scene but leaves the background black.
def DomeLight "Sky"
{
    asset inputs:texture:file = @sky.exr@
    bool crust:light:cameraVisible = 0
}

If crust:light:cameraVisible isn't authored, RenderMan's primvars:ri:attributes:visibility:camera is read instead (an int, non-zero meaning visible). Published assets such as the Moana Island carry it on their lights.

To hide every dome and distant light from the camera at once, use crust:domeLightCameraVisibility on the RenderSettings prim.

crust:rayMask on a light

int, default: bits 1 and 2 (shadow and indirect), plus bit 0 (camera) when the light is camera-visible.

crust:rayMask works on the surface of an area light too. When it is authored, it replaces the visibility above completely, and crust:light:cameraVisible is ignored:

def SphereLight "Masked"
{
    float inputs:radius = 0.3
    int crust:rayMask = 7      # camera, shadow and indirect: fully visible
}

crust:rayMask has no effect on dome and distant lights: they have no surface to mask.

samples/light_visibility.usda shows the three cases side by side: the default, a light with crust:light:cameraVisible = 1, and a light with crust:rayMask = 7.

Light groups

crust:light:lpeTag

token, unset by default.

The light's group for light path expression AOVs: the label its L events carry, so C.*<L.'key'> holds the light of every light tagged key. Any light type takes it, dome and distant lights included. An empty tag is no tag.

def RectLight "Key"
{
    token crust:light:lpeTag = "key"
}

Other renderers' light-group attributes (Karma's, RenderMan's, Arnold's) are not read yet.

Emissive materials

An emissive material (for example a crust:openpbr shader with inputs:emissionLuminance above 0) lights the scene through the path tracer. It is not sampled like a UsdLux light, so use a UsdLux light for a scene's main light sources.

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