Speed vs. Quality Tradeoffs

Since these graphs consist of distinct nodes that communicate only by frame buffers, it is possible to switch nodes between hardware and software. This has several applications:

• Switching by shader: whenever a hardware variant of a shader is available, use it, even if it has reduced functionality. This is useful for fast previews. For example, while the user interacts with the scene by moving an object, fast preview rendering is done; when the interactions stops a high-quality image is rendered and displayed.
• Switching by layer: speed can be further accelerated by marking layers as optional, for example rendering only the base surface without the more expensive Phong highlight layer.
• Switching by functionality: specific effects can be achieved by switching functional node groups between hardware and software rendering. For example, an object may be rendered in software with accurate transparent ray-traced shadows, or it may be rendered in hardware with nontransparent shadowmapped shadows. In general, any effect that requires surface mapping can be switched in this way by rendering the map in software and the mapped object in hardware.
• Switching by object: entire objects and all their associated shaders can be switched. This is useful for scenes that are well suited for hardware rendering, except for a small number of objects that require the flexibility of software.
• Layer Persistence: recomputation of software layers can be delayed for some frames. It is often sufficient to re-use light maps for multiple frames. Not every node in the graph needs to be executed for every frame.

These modes are a central design goal because they provide a smooth continuous tradeoff between speed and quality. Traditionally, hardware rendering was considered to generate images with poor quality, good enough for fast games but not for professional video or film production. But in fact not all aspects of a scene have equally high demands on rendering complexity - some parts of a scene can easily be rendered in hardware, while others absolutely require software. mental ray 3.3 is designed to permit this arbitrary combination of hardware and software rendering.