The rendering process involves the simulation of light interactions with materials. As a result, render performance is significantly affected by lighting calculations. When preparing to render an image, consider the following.
Render time is directly proportional to the number of lights in the scene. In general, mental ray requires more time to render more lights. Consider turning off lights that are not required for the rendered image. See .
In general, an interior view takes longer to render than an exterior view. An exterior view with no natural light (that is, at nighttime) that shows many interior lights turned on takes a long time to render.
More accurate lights require more render time. The Emit from Shape setting for a light source can impact render time. For example, point lights render faster than the other shapes. Line lights are slower. Rectangle and circle lights are slowest to render. See .
Revit uses area light sources to produce more realistic images. However, area shadows are expensive to compute. If you increase the quality of soft shadows, render time increases. (In the Render Quality Settings dialog, use the Soft Shadow options. See .)
Indirect illumination simulates the interaction of light with the environment by bouncing light off surfaces, including surfaces that are not directly exposed to a light source. If you increase the precision of indirect illumination and the number of bounces, you can improve the smaller, subtle effects of lighting, and the amount of light in a scene. However, increasing the amount of indirect illumination also increases the time required to render the image. (In the Render Quality Settings dialog, use the Indirect Illumination options. See .)
When you use section boxes to limit the geometry being rendered, you can significantly reduce the amount of time required to render an image. (See .) You can also use light groups to turn off lighting fixtures, thus reducing the number of lights that will impact the rendered image. (See . Remember, however, that lights that are not within the view can still have a significant impact on the quality of the rendered image.) Section boxes exclude lights that are clipped. When planned carefully and with forethought, the combined use of section boxes and light groups can greatly reduce the amount of time required to render an image.