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The sky can be displayed in Houdini’s viewport which changes interactively as any of the parameters change. The correct preview requires a camera in the scene (default camera parameters will be used if no camera has been added).

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The sky is drawn in a sphere which can be defined on the main interfaceas a viewport background hook.

  • Show In Viewport: enables / disables displaying sky preview in the viewport.
  • RadiusResolution: radius of the virtual sky sphere.Segments: number of segments of the virtual sky spherex and y segments to draw in the viewport, specifies the detailed look of the preview.

There are four environment lighting options:

  1. None: no environment lighting will be used. All the lighting in the scene will be provided by emitter materials (described later).
  2. Sky Dome: creates a consistent color or gradient dome for the scene.
  3. Physical Sky: a sophisticated atmosphere model that reproduces the skylight conditions after physical parameters. The atmosphere parameters allow users to customize the look of the sky and the resulting light in the scene.
  4. Imaged Based Lighting (IBL): allows users to light the scene by applying an HDR image to a virtual sphere that encompasses the scene.


Sky Dome

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The colored Sky Dome can be specified via the following parameters:

  • Intensity (cd/m2): specifies the intensity of the Sky Dome in Candela per square meter which means how much light is emitted.
  • Zenith: specifies the color of the dome at the zenith (the highest point above our head).
  • Horizon: specifies the color of the dome at the horizon.
  • Mid Point (degrees): specifies the transition between the Zenith and Horizon colors. This value indicates the angle at which the contribution of the colors is equal, where 0° and 90° correspond to Horizon and Zenith respectively. For a consistent color dome, the same color for both Zenith and Horizon has to be choosen.
  • Enable Sun: enables / disables rendering the Sky Dome together with the sun from the current Physical Sky settings.


Physical Sky

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Maxwell Render provides a realistic simulation of the sky via the Physical Sky lighting option. Pre-sets of the sky settings can be loaded from and saved to a file (.sky) to share with other users.

  • Load Sky Preset: specifies the path of the preset file to load.
  • Save Sky Preset: saves the sky settings to the preset file.

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The Location tab owns location and time settings.

  • City: list of cities to quickly choose a location. The list is a text file which can be found in the Maxwell installation folder (cities.txt). The content can be freely edited to add or delete locations.
  • Latitude / Longitude: Earth positions to calculate the sky / sun light direction.
  • GMT Offset: specifies the Greenwich Mean Time offset of the location.
  • Day of Month: specifies the day field of the date.
  • Month: specifies the month field of the date.
  • Hour / Minute / Second: specifies the time.
  • Now: sets the time and date to the computer’s current time and date.
  • Ground Rotation (°): allows users to rotate the north direction. This is useful when the sunlight should be repositioned without changing the location or date/ time settings which would change the sky illumination.
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The second tab holds settings for the Sun.

  • Type: defines the type of the sunlight.

    • Disabled: turns off the Sun's direct contribution.
    • Physical: the color of the sunlight is derived from the Sun
    : enables / disables direct sunlight
    • position in the sky by the Physical Sky model.
    • Custom: the sunlight color can be freely defined by the user.
  • Power: specifies the amount of light emitted from the sun. Higher values than the default 1 will make the sun emit more light, lower values will emit less. A value of 2 means the sun in the scene emits twice as much light as the Earth’s sun.
  • Radius Factor: a multiplier of the sun's radius. Useful when you need blurrier shadows, or to create larger/smaller visible sun disks in the image.
  • Temperature (°K): specifies the temperature of the sun’s spectral radiation. The default value of 5777°K is the most common value measured outside the Earth’s atmosphere. Lowering this value will give the sky and scene illumination a yellow tint, higher values will give the illumination a blue tint. Although it is possible to change this value for different looks, it is better to leave it at default and instead change the other atmosphere parameters for accurate and predictable results.
  • Color: user defined sunlight color.
  • Sun Direction: sets the method of defining direction of sunlight in the scene.
    • Longitude / Latitude: specifies geographical location.
    • Zenith / Azimuth: specifies sun direction with zenith and azimuth angles.
    • Direction: specifies a vector for the sun direction.
    • Track Object: specifies an object from the scene (Geometry node) which is used to set the position or direction of the sunlight, for instance a directional light.

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Image Based Lighting (IBL) allows users to apply high dynamic range data to a virtual sphere to light the scene, providing accurate environment lighting. The scene will be lit according to the properties of the HDR image and reflections from this environment will also be shown on the models. HDR, MXI or EXR image formats are accepted. The images should be in the Latitude/ Longitude format (the width should be twice the height), images in “strobes” or “cross” formats will not work correctly.

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The following options define the behavior of the image based lighting:

  • Use for Disabled: adds extra control over the environment. If any of the IBL channels are disabled it’s possible to use the Sky Dome, Physical Sky or None to replace the channel. For example, if the IBL Illumination channel is disabled and “Use for Disabled” is set to Physical Sky, then the illumination for the scene will be provided by the Physical Sky settings.
  • Intensity: controls the lighting intensity which affects all channels.
  • Interpolation: useful for low resolution map to avoid pixelisation. The pixels in the map will be interpolated to try and “blend” the pixels together to avoid the pixelated look.
  • Screen Mapping: controls if the map is applied spherically (disabled) or in a planar screen fashion (enabled).

The lighting of the scene can be controlled through four channels:

  1. Background: addition of an MXI, HDR or EXR map as a background environment, supplying a background image in the scene. Screen mapping can be used to map the given image to screen coordinates. This channel does not provide any scene illumination or reflections.
  2. Reflection: addition of an MXI, HDR or EXR map for reflections on scene objects.
  3. Refraction: addition of an MXI, HDR or EXR map for refractions on scene objects.
  4. Illumination: addition of an MXI, HDR or EXR map for scene illumination.

Each channel can be edited separately with the following properties:

  • Type: sets the channel type which can be one of the following:
    • Disabled: disables the channel for rendering.
    • HDR Image: the channel is rendered by the given map.
    • Active Sky: the channel is rendered by the sky settings set in the Use for Disabled parameter.
  • Same as background: the same map and settings will be used as for the Background channel.
  • Map: specifies the MXI, HDR or EXR map.
  • Intensity: specifies the contribution of the map to scene illumination, reflections or refractions.
  • Scale: specifies scale for the map.
  • Offset: specifies rotation of the spherical environment in the X and Y axis. 0 – 100 range represents rotation from 0° to 360°.

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