Path Tracing

Beginning with Terragen 4.3, "Path tracing on surfaces" is an option you can enable on the Advanced tab of the render node. Radio buttons are provided to switch between "Standard renderer" and "Path tracer". In v4.3 these are on the Advanced tab of the render node. In v4.4 and later they are on the Quality tab.

The joys of enabling "Path tracing on surfaces" include:

• Brute force GI. The GI cache is not used for surfaces (but is still used for atmosphere). Instead, GI is sampled at every pixel (every subpixel, in fact), to give you crisper contact shadows, more detailed calculation of light scattering through trees, and so on. At low AA it may be noisy, but the noise will decrease as you increase Anti-aliasing.

• Blurred/glossy reflections. Any reflective shader, water shader or default shader that has reflectivity turned up will benefit from brute-force ray-traced blurred reflections. The blurriness depends on the "specular roughness" which is controllable in all of those shaders. The "ray trace reflections" parameter is ignored, and reflections are always ray traced. As mentioned above, it maybe noisy at low AA, but noise will decrease as you increase Anti-aliasing.

• Better anti-aliasing of reflections in displaced surfaces such as water.

Things to be Aware Of

Path Tracing On Surfaces automatically uses "Defer atmo/cloud" and "Defer all shading" regardless of those checkbox settings, and the UI doesn't let you change those options when path tracing is enabled.

When "Path tracing on surfaces" is enabled, the normal GISD is not performed by the renderer. In GI Settings there is a new set of GISD options called "Exaggerate surface details" which applies to path traced renders only, and this is turned OFF by default. Use of this setting biases the result in a way that is likely to be unrealistic but may be aesthetically pleasing in some situations.

Image quality is controlled using the overall anti-aliasing settings of the renderer. We recommend using the new "Robust adaptive sampling" mode whenever you render with path tracing. This can be found in the Pixel Sampler settings ("Edit Sampling..." on the renderer's Quality tab). The old adaptive sampler has trouble dealing with some of the Monte Carlo noise created by the path tracer, especially on darker surfaces. The new robust adaptive sampler was designed to work well with the path tracer (although it is useful in other cases too).

When "Path tracing on surfaces" is enabled, the Enviro Light "Strength on surfaces" and "Colour on surfaces" settings have no effect. Bounced and occluded light is fully handled in a realistic, energy-conserving manner by the path tracer and can't currently be biased in this way. However the respective atmosphere settings are still taken into account since atmospheric lighting continues to be cache-based even when using path tracing.

Render Time Impact and Optimization

Although Path Tracing is much more accurate and in many cases more realistic-looking, it can also be a very demanding rendering method due to directly calculating all the actual light interactions in the scene, without approximations like the GI cache. As a result render times can often be much longer than with a standard render. Fortunately there are some important settings and optimizations you can use to reduce path tracing render times to a minimum while maintaining quality. Please reference the Rendering Optimization Guide for details.

Ultimately you will need to determine for yourself whether the benefits of path tracing are right for your particular scene, hardware, and goals. But it's a valuable new tool to have available, even as it may not be the ideal choice for every situation.