Daylight system > Modify panel > Daylight Parameters rollout > Sunlight/Skylight drop-down lists > mr Sun/mr Sky
The mental ray Sun & Sky solution is designed to enable physically plausible daylight simulations and accurate renderings of daylight scenarios. In Autodesk VIZ, this is achieved through the use of two special photometric lights and an environment shader that all work together:
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The mr Sun photometric light is responsible for the sunlight; the direct light from the sun.
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The mr Sky photometric light is responsible for the skylight; it simulates the real-world phenomenon of indirect light created by the scattering of sunlight in the atmosphere.
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The mr Physical Sky environment shader is responsible for the visible representation of the sun disk and the sky, both to the camera and in reflections and refraction, as well as for the virtual ground plane (gray in the following illustration).
While you can use these separately, their intended functionality is unleashed only when used together. mr Sun and mr Sky are meant primarily for use within the Autodesk VIZ Daylight system. When used in combination, this solution is called Sun & Sky.
Note: Sun & Sky is a true high-dynamic range photometric lighting system that requires the Logarithmic Exposure Control to be enabled. In some of the mental ray rendering presets, such as those starting with “mental.ray.daylight,” this is done automatically. Also, when you add a Daylight System from the Create menu, you have the option to activate Logarithmic Exposure Control automatically.
To use a mental ray fast SSS material together with the high-dynamic range Sun & Sky solution, be sure to turn on Scatter Indirect Illumination on the material's Advanced Options rollout so that the material can scatter the skylight, which is considered indirect.
Also, turn off Screen (Soft) Compositing Of Layers, also on the Advanced Options rollout; otherwise the output of the SSS shaders is clamped to a low dynamic range and will appear to render black.
To use mental ray Sun & Sky:
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Make sure mental ray is the active renderer.
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From the Create menu, choose Lights > Daylight System.
You can also find Daylight System on the Create menu under Systems.
Note: The Daylight System is also available from Create panel > Systems, but if you go that route you're not prompted to turn on Logarithmic Exposure control (see next step), so when using Sun & Sky it's best to make a habit of choosing Daylight System from the Create menu.
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You're prompted to activate Logarithmic Exposure Control with the Exterior Daylight Flag, if both are not already active. Click Yes to do so automatically.
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Create a Daylight system in the Perspective (or a Camera) viewport: Drag and release to create the compass rose, move the mouse to position the light, and then click to place the light and finish creating the system.
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Go to the Modify panel, and on the Daylight Parameters rollout, set Sunlight to mr Sun and Skylight to mr Sky.
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When you choose mr Sky, you're prompted to add a mr Physical Sky environment map. Click Yes to do so.
The parameters rollouts for mr Sun and mr Sky now appear on the Modify panel.
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Open the Environment dialog (press 8) and make sure Common Parameters > Environment Map is set to mr Physical Sky, Exposure Control is set to Logarithmic Exposure Control, and Logarithmic Exposure Control Parameters has Exterior Daylight on.
Tip: Should you wish to edit the mr Physical Sky parameters, an easy way to access them is to open the Material Editor (press M) and then drag the Environment Map button from the Environment panel > Common Parameters rollout to a material slot (sample sphere) in the Material Editor. When prompted for the copy method, choose Instance.
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Open the Render Scene dialog (F10) to the Indirect Illumination panel and turn on Enable Final Gather. Choose the Draft preset as a starting point.
Important: Don't forget to turn on Enable Final Gather when using Sun & Sky, since the skylight is a form of indirect light, which can be rendered only with the help of Final Gather. If this is not done, Sun shadows will be unnaturally dark or black rather than bluish, as they should be.
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Render with Sun & Sky. To specify different times, dates, and locations in the Daylight system for different effects, use the Motion panel > Control Parameters settings or turn on Manual Override to place the sun by hand.
Tip: Adjust the Brightness and Contrast of the Logarithmic Exposure Control for a pleasing image.
Certain parameters do the same things in mr Sun, mr Sky, and mr Physical Sky. For physical correctness, it is necessary to keep these parameters in sync with each other in all three elements. For example, a sun with a different Haze value than the sky cannot be guaranteed to be physically plausible.
For this reason the concept of parameter inheritance is included. Both the mr Sun light and the mr Physical Sky shader have check boxes labeled “Inherit from mr Sky” that are on by default. When these are on, the common parameters are all guided by the mr Sky light, which is the central point of control.
With parameter inheritance, you can concentrate on tweaking the parameters in one spot (mr Sky) and, as long as Inherit From mr Sky is on for both mr Sun and mr Physical Sky, you're sure to obtain consistent results.
The most important common parameters are those that drive the entire shading and colorization model.
Note: mr Sun and mr Physical Sky can inherit parameters from mr Sky even if the latter is off.
Haze—The amount of water vapor or other particulate matter in the air. Possible values range from 0.0 (a completely clear day) to 15.0 (extremely overcast, or a sandstorm in the Sahara). The Haze value influences the intensity and color of the sky and horizon, the intensity and color of sunlight, the softness of the sun's shadows, the softness of the glow around the sun, and the strength of the aerial perspective.
Horizon Height—The vertical position of the horizon.
The default value, 0.0, places the horizon at a standard height. However, because the horizon is infinitely far away, this can cause trouble joining up with any finite geometry that is supposed to represent the ground. It can also cause issues rendering locations that are supposed to be at a high altitude, like mountain tops or the top of New York skyscrapers where the horizon really is visibly “below” the viewer.
This parameter allows tuning the position of the horizon. Note that this horizon doesn’t actually exist at a specific height in 3D space; it is a shading effect for rays that go below a certain angle. This parameter tweaks that angle. The total range available range is somewhat extreme, reaching from -10.0 (the horizon is “straight down”) to 10.0 (the horizon is at the zenith). In practice, only much smaller values are actually useful. For example, to push the horizon down just below the edge of a finite visible ground plane, use –0.2.
Note: The Horizon height affects not only the visual representation of the horizon in the mr Sky light, but also the color of the mr Sun itself. In other words, the point where the sun “sets” will change for a Horizon height settings other than 0.0.
Horizon Blur—The “blurriness” with which the horizon is rendered.
At 0.0 the horizon is completely sharp. Generally only values lower than 0.5 are useful, but the full range is up to 10.0 for a horizon that consists of blur only, with no actual horizon at all.
Ground Color—The color of the virtual ground plane. Note that this is a diffuse reflectance value (that is, albedo). The ground appears as a Lambertian reflector with this diffuse color, lit by the sun and sky only, and does not receive any shadows.
Note: In the above images, bounce-light from the ground tints the walls of the house. Also, the virtual ground plane does not receive shadows.
Tip: Some sky models neglect the influence of bounce light from the ground, assuming only the sky is illuminating the scene. To compare the output of mr Sky with, for example, the IES Sky light, set Ground Color to black.
Night Color—The minimum color of the sky: The sky will never become darker than this value. It can be useful for adding things like moon, stars, high-altitude cirrus clouds that remain lit long after sunset, etc. As the sun sets and the sky darkens, the contribution from Night Color is unaffected and remains as the base light level.
Red/Blue Tint—Gives artistic control over the redness of the light. The default value of 0.0 is the physically correct value (calculated for a 6500K whitepoint), but can be changed with this parameter, which ranges from -1.0 (extremely blue) to 1.0 (extremely red).
Saturation—Also an artistic control, where 1.0 is the physically calculated saturation level. The parameter ranges from 0.0 (black and white) to 2.0 (extremely boosted saturation).