Max Trace Depth
A maximum raytrace limit can be set for the combined total of both reflection and refraction rays. In Figure 2.5 if the max trace value is set to less than 4 bounces and there is a requirement for evaluating 2 reflection and 2 refraction rays, then raytracing is forced to quit before completion. If the limit is set to 2, then mental ray can only evaluate 2 reflections or 2 refractions or one of each. This limit acts regardless of the reflection and refraction limits settings so that they can both be set to 10; however only a maximum of n max trace reflections may be evaluated. The fact that limits can be set independently allows you to tentatively set the reflections to a high value and refractions to a lower value and then limit the total of the two in a way that allows the renderer to make the best use of these settings, but within a reasonable range. Alternately, this also provides you with a fast option to troubleshoot or preview-render the scene with very low settings by changing only one attribute. As a rule of thumb, with raytracing, usually two raytrace evaluations will provide you with sufficient color evaluations; however, with refractions, if you do not provide enough refractions as required by the surface and the surface requires penetrating four times to evaluate the color, an unwanted value is usually returned, resulting in empty (black) spots within the image's RGB and alpha channels. We will examine these settings further in relvant sections throughout the book.
Aside from setting these global settings, within each application, shaders and surface properties further allow you to specify raytrace limits, thus disabling or enabling raytracing for a particular shader or object. This extends these settings to a per-object or -shader basis and provides a means to further optimize raytrace renders.
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