mental ray supports a number of diagnostic modes that help visualizing and optimizing the rendering process. They modify the output image to include grid lines or dot patterns that indicate coordinate spaces or sampling or photon densities. These graphs allow simple detection of insufficient or excessive sampling densities, and help to tune parameters such as numbers of photons or sampling and contrast limits.
This mode renders a grid on top of all objects in the scene, in object, camera, or world space. It's useful to get an idea of the scene scale and to enable rough estimates of distances and areas.
Photon density mode
This mode shows a false color rendering of photon density on all materials. This is useful when tuning the number of photons to trace in a scene, and to select the optimum accuracy settings for estimation of global illumination or caustics. It also works well in combination with the Grid Mode described above.
This mode shows how spatial supersamples were placed in the rendered image, by producing a grayscale image signifying sample density. This is useful when tuning the level and the contrast threshold for spatial supersampling.
This mode shows the cost of creating and traversing the BSP tree used for raytracing. Both the depth and the leaf size can be visualized. If the diagnostic image shows that mental ray has been operating near the limit in large parts of the image (indicated by red or white pixels), this helps tuning the BSP parameters in the options block.
Final gathering mode
This mode shows final gathering points, as green dots for initial raster-space final gathering points, blue dots3.4 for final gathering points from per-object finalgather map files and red dots for render-time final gathering points.
Diagnostic modes are enabled with the -diagnostic option on the command line, or the diagnostic statement in the options block in the scene description file.
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