Ai Sky Shader

This scene uses a background Sky shader with a constant color which is sufficient to 'light' the scene with GI Diffuse rays

 

This shader is intended to be used as a global environment shader (or a background shader in Arnold terms), thus affecting all objects in the scene. By default it has a simple white color, and will illuminate the whole scene evenly using this color. For image based lighting, use the Skydome light node instead.

 

Format

How the environment / image will be interpreted. The type of map connected can be set to Mirrored Ball, Angular or Lat-long.

Color

The color to be used as the environment. This could be mapped to an image probe.

Intensity

A multiplier for the color.

Hardware Texturing

Texture Resolution

Texture resolution used to display sky shading in the viewport.

Opacity

The opacity settings of the sky shader, as seen in the camera viewport. Useful when scene geometry visibility is blocked by the sky node. 

Viewport

Sky Radius

The size of the sky in the Maya viewport.

Facing

The direction that the HDR image faces in the viewport. Choose between Front, Back or Both.

Note that 'Hardware Texturing' in Maya's viewport will only work when using a Maya file texture with the Ai Sky shader. Using an 'Ai Image', for example, will return black in the viewport.

 

Workflow

The following information demonstrates how to render a scene using the Ai Sky shader with a HDR map.

Note that all the examples below are rendered with the Sky shader plus one quad area light acting as key light. The Dragon model has around 7 million triangles and is rendered as is without any displacement/bump maps. The following sampling settings were used in all the examples:

  • Camera (AA) Samples: 5
  • Indirect Diffuse Samples: 3
  • Indirect Specular Samples: 3

Below is a version of the dragon with the Sky shader's color set to blue. 

Dragon model lit with sky shader set to a flat blue color.

This section is mostly obsolete, you should use the Skydome light to light your scene.

If you select the sky shader on the pass and open the render tree, you can use any image to illuminate the scene by simply hooking up a normal Image node to the "Color" slot of the Sky shader. 

Note that it's important to set the proper mapping format in the Sky shader. We'll use the following HDR image in "lat-long" format:

Dragon model lit with sky shader using a high-contrast HDR image 

Even though this image is using the exact same GI sampling setting as the previous one, you can see that it has a lot more noise. This is because the environment map has a lot of high contrast areas with bright spots (windows, ceiling lights) and dark shadows, which is difficult to sample.

Reducing Noise Using The Ray Switch Node

This section is mostly obsolete, you should be using the Skydome light instead.


Using a very high contrast HDR image for lighting a scene can produce noise which can be hard to get rid of simply by increasing the Camera or Hemi/Glossy samples. An HDR image contains a tiny sun disc whose pixel values are in the hundreds of thousands or even millions, is going to need a lot of samples for Arnold's brute-force hemispherical sampling to find. On the other hand, a HDRI of a thick overcast day with no sun at sight is very easy to sample.

One way to fix the excessive sampling noise would be simply to blur the environment map so that it's easier for the GI and glossy rays to hit bright areas. However that would cause the render to lose all the nice highlights seen in the reflections from the environment map. Instead, we will be using different environment maps for each ray type in the scene.

Typically, you will need 2 or 3 different versions of the same environment:

  • One high-res and sharp map for reflection and refraction rays.
  • One mid-res and slightly blurred map for glossy rays.
  • One low-res and very blurred map for Indirect Diffuse rays.

For this purpose you can use "Ray_Switch".

Below are the images used for the different ray types, using a gaussian blur in PhotoShop.

 

Depending on the dynamic range of the HDRI you may need to blur more or less to achieve optimal results. Also, how blurry the map used for glossy rays should be depends on the material; for very sharp materials (almost mirror like) you don't need as much blurring, but if your glossy reflections are very soft you should use a bigger blur.

Here's the resulting image from this setup:

Dragon model lit with sky shader using separate HDRIs for each ray type.

Note how with this setup the noise is now gone and the illumination is very similar to the version using a single high-res map for all ray types.

Different amounts of blur for the diffuse environment map

This section is mostly obsolete, you should be using the Skydome light instead.

The following images illustrate the effect of varying the amount of blur in the HDRI's used for Indirect Diffuse rays. To make things simpler, all the illumination comes from the HDRI - there is no explicit key light. As expected, the more you blur, the lower the noise, at the cost of reduced lighting detail (for example, softer shadows):

 

Ray Depth

The scene below is lit soley using an Ai Sky shader with the color set to white. The number of Diffuse rays has been increased to 4 in order to reduce noise from the Ai Sky around the floor.

You can clearly see the effect increasing the Diffuse ray depth has on the amount of bounced illumination in the scene.

 

Using Smart IBL

You can automate this process with Smart IBL from HDRLabs. Smart IBL offers a unified browser of different HDRI setups which have three basic ingredients:

  • a super high-res panorama as camera background
  • a high-res HDR as specular component for reflection
  • a blurred low-res HDR as diffuse environment lighting

There is an Arnold template for sIBL that you can download here. It provides a bridge between Maya, Arnold and Smart IBL.

 

A ramp texture that is set to the U direction will render incorrectly when connected to the Ai Sky. This can be corrected by scaling the Ai Sky -1 in Z and setting 'Facing' to 'back'.

 

 

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