Physical Sun and Sky in Softimage

If you have used the Physical Sun and Sky in Maya and Softimage, you may have noticed that in Softimage the setup is halfway done. Only the sky is created, no lens shader is used and the Sun is using a soft light shader instead of the physical sun shader.

If you want to have a setup similar to Maya you have to connect everything manually. I got tired of connecting all the shaders manually so I have modified the Initialize sky shader script to include all those changes.

Download the addon here

Once installed (see Softimage Help on how to install addons) you will have a new entry in the Pass > Edit menu : Initialize Physical Sun/Sky Shader

The Initialize window is similar to the default Initialize Sky Shader window. Choose your pass and you sun light then click Apply.

The plugin will create the physical Sky shader in the pass environment stack.

It will also add a photgraphic Exposure shader in the pass lens stack and set the pass lens stack mode to Add After Camera Lens Shaders.

And most importantly it will add a Physical Sun shader in the light shader, replacing the default soft light shader.

Now you have all the proper shaders to create physically accurate renders.

But some of the default values in these shaders are not correct, the plugin will take care of adjusting them correctly.

In the Physical Sun shader you will notice that most properties are controlled by an expression, exactly like in Maya.

You can edit the shadow softness and shadow samples of the Sun in this shader.


Most of the values in this shader need to match the value of the Physical Sky shader exactly. Connecting these values with expression is the easiest way to keep these values linked.

In the Physical Sky shader, the multiplier values is set to 1 instead of 0.1 and the rgb Unit Convertion is set to 0.318

As mentioned in the Sun and Sky Mental Ray documentation:
When setting the rgb Unit Convertion values to 0.318, the final rendered pixels in the image are true photmetric luminance values in candela per square meter.

These true luminance values then fit perfectly as input to the photographic tone mapper.

For the Photographic Exposure shader, the plugin sets the F-Stop Number to 16 and the c/m2 Factor to 1.
Setting the c/m2 Factor to 1 is important to stay in true photographic mode. 
These setting represent the Sunny 16 rule in photography. As discussed in the Physical Exposure Mental Ray documentation.

To adjust the brightness of the image you can simply adjust the Shutter Time and the F-Stop.

Note that the gamma value is set to 2.2. It is possible to work in linear workflow in softimage, but I decided to keep the values to the Mental Ray default.

I will discuss linear workflow using the Physical Sun and Sky in a future post ... so stay tuned.

Finally the plugin sets 2 more things: it enables the Final Gathering and set the Ambiance Color to Black. (Don't forget to set the Final Gathering for the Region Render as the plugin do not set it) 

With this setup you will have the exact same setup as Maya ... If you downloaded the Maya modification script provided in the previous post and you create a physical sun and sky setup using the Photographic exposure.

Here are renders done in Maya 2010 and Softimage 7.5. Both renders are using the default Physical Sun and Sky settings mentioned in this post and mia_materials (architectural shader).

Maya 2010

Softimage 7.5

As you can see the result is similar in both application.


I hope you like this plugin.