This page provides information on the V-Ray Sun/Sky package that can be added to a Modo directional light.

 

Page Content

 

Overview


The V-Ray Sun and V-Ray Sky are special features which are provided by the V-Ray renderer. Developed to work together, the V-Ray Sun and V-Ray Sky reproduce the real-life Sun and Sky environment of the Earth. Both are coded so that they change their appearance depending on the direction of the V-Ray Sun.

The V-Ray Sun and Sky are based largely on the SIGGRAPH'99 paper "A Practical Analytic Model for Daylight" by A. J. Preetham, Peter Shirley, Brian Smits. For a more complete list of references, please see the Links and references section.

 


© Alex Hogrefe

 

V-Ray Sun/Sky Parameters


To create a V-Ray Sun you need to add the V-Ray Sun/Sky package to a Modo directional light. See the Lights page for information on how to do this.


 

 

Intensity Multiplier – The multiplier for the V-Ray Sun's intensity. Used to reduce the sun's brightness, because by default it can be very bright. See the Notes section below for more information.

Turbidity – Determines the amount of dust in the air and affects the color of the sun and sky. Smaller values produce a clear, blue sky and sun as seen in rural areas, while larger values make them yellow and orange as seen in a big city's atmosphere.

Ozone – Affects the color of the sun light. Available in the range between 0.0 and 1.0. Smaller values make the sunlight more yellow, while larger values make it blue. For more information, see Example: Ozone Value below.

Size Multiplier – Controls the visible size of the sun. This affects the appearance of the sun disc as seen by the camera and in reflections, as well as the blurriness of the sun's shadows. For more information, see Example: Size Multiplier.

Water Vapour – Controls the amount of water vapor in the atmosphere which affects the sky color .

Sky Model – Specifies the procedural model that will be used to generate the V-Ray Sky texture. In order to have the Sky texture rendered, you need to select Physically-based Daylight as the Environment Type in the Modo Environment material.

Hosek et al. – The V-Ray Sky procedural texture will be generated based on the Hosek et al. method.
Preetham et al.
– The V-Ray Sky procedural texture will be generated based on the Preetham et al. method.
CIE Clear
– The V-Ray Sky procedural texture will be generated based on the CIE method for clear sky.
CIE Overcast
– The V-Ray Sky procedural texture will be generated based on the CIE method for cloudy sky.

Horiz Illum – Specifies the intensity (in lx) of the illumination on horizontal surfaces coming from a CIE Clear or CIE Overcast sky model.

Filter Color – Shifts the hue of the V-Ray Sun and Sky system towards the color specified in the field.

Ground Albedo – Sets the ground color of the V-Ray Sun and Sky system.

Blend Angle – Specifies the angle in degrees up from the horizon where the sky starts to blend with the ground albedo. Values close to 0.0 produce a sharper horizon line, while large values produce a softer horizon line.

Horizon Offset – Specifies an offset for the horizon under the sky-dome. Allows the user to manually lower the horizon line.

Invisible – When enabled, the sun becomes invisible to both the camera and reflections. This is useful to prevent bright speckles on glossy surfaces where a ray with low probability hits the extremely bright sun disk.

Shadow Subdivs – Controls the number of samples for the area shadow of the sun. More subdivs produce area shadows with better quality but render slower. Use Local Subdivs must be enabled in the DMC Sampler to specify Shadow subdivs at this level.

 


 

Example: Turbidity

 

Common settings in this example are Intensity multiplier: 1, Size multiplier: 5.0

 

 


Turbidity: 2.0

 


Turbidity:
5.0

 


Turbidity:
10.0

 

 

 

 


 

Example: Ozone value

 

Common settings in this example are Turbidity: 3.0, Intensity multiplier: 1, Size multiplier: 1.0

 

 


Ozone:
0.0

 


Ozone:
1.0

 

 

 



 

Example: Blend Angle

 

The Blend Angle specifies the angle (in degrees) up from the horizon where the sky starts to blend with the ground albedo. So values close to 0.0 produce a sharper horizon line, while large values produce a softer horizon line.

In the examples below some geometry has been hidden and Aerial Perspective has been turned off for the sake of visual clarity in the examples.

Common settings in this example are Turbidity: 3.0, Intensity multiplier: 1, Size multiplier: 10.0

 

 


Blend Angle 0.0

 


Blend Angle
5.7392

 

 

 

 


 

Example: Horizon Offset

 

The Horizon Offset the offset for the horizon under the sky-dome to be specified. So the horizon line can be manually lowered or raised .

In the examples below some geometry has been hidden and Aerial Perspective has been turned off for the sake of visual clarity in the examples.

Common settings in this example are Turbidity: 3.0, Intensity multiplier: 1, Size multiplier: 1.0

 

 


Horizon Offset:
0.0

 


Horizon Offset: 10.0

 


Horizon Offset: 15.0

 

 

 

 


 

Example: Ground Albedo


Common settings in this example are Turbidity: 3.0, Shadow subdivision: 8, Intensity multiplier: 1

 

 


Ground Albedo:
mid grey

 


Ground Albedo: green

 

 

 

 



Example: Size multiplier

(Size & Area shadows effect)

 

Common settings in this example are Turbidity: 3.0, Shadow subdivision: 8, Intensity multiplier: 1

 

 


Size Multiplier: 15.0

 


Size Multiplier: 25.0

 


Size Multiplier: 50.0

 

 

 

 


 

Notes


By default, the V-Ray Sun and V-Ray Sky are very bright. In the real world, the average solar irradiance is about 1000 W/m^2 (see the references below). Since the image output in V-Ray is in W/m^2/sr, you will typically find that the average RGB values produced by the sun and the sky are about 200.0-300.0 units. This is quite correct from a physical point of view, but is not enough for a nice image. You can either use Color Mapping to bring these values to a smaller range (which is the preferred way) or you can use the Intensity Multiplier to make the sun and sky less bright. Using the V-Ray Physical Camera with suitable values also produces a correct result without changing the Sun and Sky parameters.

Enabling Modo's built-in Physical Sun settings in addition to V-Ray Sun/Sky allows for precise control over time of day.

 

Links & References


Here is a list of links and references about the V-Ray Sun and Sky implementation, as well as general information about the illumination of the Sun.

[1] A.J. Preetham, P. Shirley, and B. Smits, A Practical Analytic Model for Daylight, SIGGRAPH 1999, Computer Graphics Proceedings
An online version can be found at http://www.cs.utah.edu/~shirley/papers/sunsky/
This paper includes source code examples and is the base for the V-Ray Sun and V-Ray Sky plugins.

[2] R. H. B. Exell, The intensity of solar radiation, 2000
This document is available at http://www.jgsee.kmutt.ac.th/exell/IntensitySolarRad.pdf
This document contains information about the average intensity of the solar radiation, as well as some specific measurements.

[3] R. Cahalan, Sun & Earth Radiation
This page can be found at http://climate.gsfc.nasa.gov/static/cahalan/Radiation/ (Please note that this link is no longer valid).
These pages contain a list of accurate solar irradiances across a large portion of the electromagnetic spectrum.

[4] D. Robinson-Boonstra, Venus Transit: Activity 3, Sun & Earth Day 2004
This document can be found online at http://sunearth.gsfc.nasa.gov/sunearthday/2004/2004images/VT_Activity3.pdf (Please note that this link is no longer valid).
Among other things, this document gives the distance from the Sun to the Earth and the size of the Sun derived from astronomic observations.

[5] Hosek L, et al, An Analytic Model for Full Spectral Sky-Dome Radiance
This document can be found online at http://cgg.mff.cuni.cz/projects/SkylightModelling/HosekWilkie_SkylightModel_SIGGRAPH2012_Preprint_lowres.pdf
Describes the Hosek sky model used by the VRaySun and VRaySky