Speed up rendering with path guiding in Cycles for Blender 3.4

The Blender Foundation has released Blender 3.4, the latest version of this fantastic open-source 3D software. This version comes with many new features, and path guiding is the most important among them. In today’s article, let’s learn about path guiding in Cycles for Blender 3.4 and how it helps speed up rendering in Cycles renderer.

Path guiding is a new feature in Blender 3.4 that will dramatically speed up rendering scenes in Cycles containing complex lighting effects. It was integrated (75a6d3abf7) into Cycles using Intel’s Open Path Guiding Library. This feature enhances the sampling quality of individual paths, reducing noise in complex lighting scenes such as indirectly illuminated shadow areas, long indirect light bounces, or reflected light sources.

Path guiding learns an approximation of the scene’s light distribution (direct and indirect) during rendering. This information is then utilized to guide paths into critical directions that could not be effectively explored using standard directional sampling approaches that only consider local material or directly visible light sources. In simpler terms, with path guiding enabled, Cycles analyzes the light distribution in a scene and uses that information to guide the light paths that make the most important contributions to a render. By prioritizing paths interacting with surfaces in the scene, path guiding improves sampling, thus helps the render to resolve to an acceptably low level of noise faster.

Let’s see how is path guiding in Cycles really used in scenes made by artists.

Firstly, take a look at a scene from Jesus Sandoval who does a lot of interior designs.

Equal time render without and with path guiding, scene by Jesus Sandoval (Source: blender.org)

The left picture is a five minutes render without path guiding and the right one is the result after five minutes with path guiding enabled. We can see that the indirect illumination part is much clearer at the same time. One nice thing is that it does not introduce you any bias. Instead, it converts to the same result, but much faster with fewer samples.

Secondly, a scene with a lot of mirrors by Caner Aslan.

The scene has a lot of indirect illumination like having light sources reflected by a mirror. 

Equal time render without and with path guiding, scene by Caner Aslan (Source: blender.org)

There are light sources reflected by some specular materials and they actually do a lot of indirect illumination in this scene. The left picture is the render without using path guiding. After getting 1500 samples with path guiding enabled, we have the result in the right picture. Looking at the right picture, we see the path guiding actually works quite out well. With path guiding, you might still get some fireflies. However, you can put the threshold for your radiance clamping way higher because these fireflies will be there.

Thirdly, an underwater scene by Andy Goralczyk

Equal time render without and with path guiding, original scene by Andy Goralczyk (Source: blender.org)

This scenario is actually a simple caustics. If you enable path guiding, you can get the caustic on the floor and all the multiple scattering inside the volume. The two renders were generated in equal time without and with path guiding enabled in Cycles. However, path guiding helps Cycles resolve parts of scenes with complex indirect illumination – such as this water volume – more quickly.

It’s actually quite easy to use path guiding in Blender 3.4 when you have the CPU device which is currently implemented. Just enable path guiding in the checkbox like in the above picture. Here are some path guiding settings.

• • • • Path Guiding: Turns path guiding on/off
      • Surface Guiding: Turns path guiding on surfaces on/off
      • Volume Guiding: Turns path guiding inside the volumes on/off
      • Training Samples: Number of training interactions. 128-256 are usually enough. Afterward, the structure is only queried leading to faster render times.

So, what can we do with path guiding or what should we think about when we want to use path guiding?

• • • • First, let’s change your workflow: Be more experimental and try to use fewer hacks to build realistic indirect lighting effects.
      • About caustics: Path guiding in Cycles is not a caustics solver. It helps you with caustics, but with simpler reflected/refracted caustics.
      • Indirect Radiance Clamping: Change the look between path guiding on/off
      • Avoid Shadow visibility

Note: Path guiding is currently only available for CPU rendering. The current implementation allows for guiding on surfaces with diffuse BSDFs as well as inside volumes.

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Reference source: blender.org