Raytracing with POV-Ray

The Persistence of Vision Raytracer (POV-Ray) is included with Chimera. POV-Ray images can be saved:

• with Save Image
• with the copy command
• when making movies

• scene description (*.pov file)
• raytracing options (*.ini file)

See also: exporting a scene, tips on preparing images

Lights and Shadows

Chimera includes ambient light and up to three directional lights: key, fill, and back (details...). Lighting parameters can be adjusted with the Lighting tool or lighting command and are propogated to POV-Ray for raytracing. By default, only the key light produces shiny highlights and shadows. Shadows from raytracing can be made less severe by decreasing the contrast. Also, it may be useful to move the lights (change their directions) and/or adjust the key-fill ratio.

Interactive shadows can be used to preview shadow locations. If system hardware permits, these can be enabled with the Effects tool or the command set shadows. In some cases, interactive shadows may suffice as an alternative to raytracing; their appearance can be improved by increasing the shadow quality setting in the Effects tool (albeit at the cost of increased graphics memory usage).

Sometimes a considerable speedup in raytracing can be obtained with little change in output appearance by adjusting settings in the POV-Ray Options preferences. This is particularly true for movies, which are compressed to some extent during encoding. Changes to consider include:

• decreasing the quality to 4 or 5 [quality details at the POV-Ray site]
• increasing the antialias threshold to 1.5 or even higher (maximum 3.0) [antialias details at the POV-Ray site]
• if objects are not intentionally sliced, making sure that global clipping is turned off; to adjust depth cueing, use the start and end parameters in Effects rather than moving the global clipping planes, which would turn clipping on

Raytracing jobs can be monitored and canceled using the Task Panel.

Raytracing can be quite slow; see balancing time requirements and results.

Using transparency or clipping may increase rendering time significantly. If objects are not actually clipped, clipping should be turned off, as mentioned above.

The max_trace_level parameter is set to 10 to improve multi-level transparency [details at the POV-Ray site], but this also increases time and memory requirements relative to lower levels.

Only the PNG format can be saved.

If image dimensions are specified in units of length rather than pixels, they are multiplied by the resolution to give the correct number of output pixels, but the dimensions in units of length are “forgotten” (not included in the image file).

A raytraced image from POV-Ray may differ from the view in Chimera in several ways. Some differences are desirable, such as the presence of shadows. Others reflect current limitations of the POV-Ray format and/or its implementation in Chimera:

• colors may differ slightly
• shading along ribbons may be stairstepped; using the supersmooth style instead of rounded reduces this effect
• light source directions but not colors are translated from Chimera
• although object shininess and brightness levels (but not specular color) are translated from Chimera, apparent shininess may differ
• if depth cueing is used, the background of the raytraced image:
  • will not be transparent even if the transparent background option is turned on
  • will match the depth-cueing color even if the background is a different color in Chimera
• the following are omitted or otherwise not handled: silhouettes, selection highlighting, stereo, Volume Viewer transparent solid displays, and normals on dot and mesh surfaces