A render pass creates a picture layer of a scene that can be composited with any other passes to create a complete image. Passes also allow you to quickly re-render a single layer without re-rendering the entire scene. Later, you can composite the rendered passes using Softimage Illusion, the fully–integrated compositing and effects toolset, or any other editing and compositing tool. For more information about compositing in Softimage, see Compositing and Effects.
Each scene can contain as many render passes as you need. When you first create a scene in Softimage, it has a single pass named Default_pass. This is a “beauty pass” that is set to render every element of the scene—you can render a single beauty pass or you can render separate passes. You can use preset passes such as matte, shadow, toon, and highlight, or you can create your own passes.
This photograph (background pass) is the background scene over which the dinosaur will be composited.
This pass is a rendered image of the dinosaur. Compositing it over the background would make the scene rather flat and unrealistic.
The matte pass is used when the dinosaur passes in front of or behind another rendered (or non-rendered) image (such as a telephone pole, a mailbox, or a live-action background like the one in this example). The matte “cuts out” a section of another rendered image so another image can be composited over or beneath it.
The shadow pass acts much like a matte pass but is created solely for an object’s shadow that falls on other rendered images. You can manipulate this pass’s transparency or sharpness to add realism when compositing with your final scene.
The specular pass is used to edit an object’s highlights.
Here the passes are all composited together to create the final image.
Anatomy of a Render Pass
A render pass is made up of many parts. The explorer allows you to see a list of all the render passes in your scene, their contents, and their properties.
To explore a render pass
1. Open an explorer (press 8).
2. Do one of the following:
- Press p to set the scope to Passes or choose Passes from the scope menu.
- Press u to set the scope to Current Pass or choose Current Pass from the scope menu.
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Current pass. The current pass is always displayed in bold typeface.
Each pass has its own options. This lets you optimize your rendering by enabling only those options you need for each pass. For example, you could enable shadow calculations only in the shadow pass. See Pass Rendering Options.
Expanding any pass node displays its renderer options, the active camera for the pass, its partitions, and any environment, output, and/or volume shaders applied to the pass as a whole.
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Pass renderer options. Depending on which renderer you have chosen for your pass, click the Hardware Renderer or mental ray icon to edit the pass’s renderer options. See mental ray Renderer Options and Hardware Renderer Options.
You can identify whether the pass is using a local or global set of render options by the Roman or italic typeface displayed for the renderer’s node. See Setting Global or Local Render Options.
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Pass camera. Click the camera icon to define camera and lens-shader options for the pass. You can add new cameras to your scene and set them as active if needed. See Controlling the Active Camera for the Render Pass.
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Background partition. Every pass is created with two background partitions which contain the scene’s objects and lights.
Background partitions usually contain every object in your scene that isn’t modified in the pass. However, nothing is stopping you from modifying the contents of these partitions as well. See Background Partitions.
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Partition. A partition is a division of a pass, which behaves like a group. Partitions are used to organize scene elements within a pass. See Defining Partitions.
Expanding any partition node allows you to see its contents, as well as any materials, shaders, overrides, and other properties that are applied to it.
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Framebuffers. The framebuffers folder holds all the active render channels defined for the pass including its Main render channel. See Render Channels & Framebuffers.
A material is assigned to a partition. The “B” indicates that it was applied in branch mode and is propagated to every object in the partition. If any objects in the partition have local materials, they will be overridden by the partition-level material for this pass.
What is a Render Channel?
Essentially, render channels are a mechanism for outputting multiple images, each containing different information, from a single pass. When you render the pass, you can specify which channels should be output in addition to the full pass. By default a Main render channel is declared for every pass (you can think of it as the “beauty” channel rendered for each pass).
Preset render channels allow you to isolate scene attributes that are commonly rendered in separate passes. All you need is your default pass and you can quickly output the preset ambient, diffuse, specular, reflection, refraction, and irradiance render channels.
Rendering a scene in passes can be extremely useful for isolating specific elements of a scene in order to have more flexibility when creating special effects in post production. Although similar in concept, there is a big difference between passes and layers.
• Layers are used to separate elements of a scene so they can be visually organized or not rendered to optimize interaction. Layers mainly affect the 3D views and the render region.
• Passes separate scene elements for rendering so they can be individually tweaked or edited.
You can create passes based on existing layers using the Pass > Edit > New Pass > Based on Layers command from the Render toolbar.
