Light Portals And Sampling: Speeding Up Blender Cycles Renders

Speeding Up Renders with Light Portals

Rendering high-quality images with ray tracing engines like Blender Cycles can take a long time due to the computational complexity of accurately simulating the behavior of light. Using optimization techniques like light portals and sample controls can drastically reduce render times while maintaining acceptable visual quality.

What Are Light Portals and Why Use Them?

Light portals are designated mesh areas in Blender that control the ray sampling rate in a scene. They guide sampling towards important bright areas and away from less important dim regions. This is a variance reduction method that reduces noise and speeds up renders.

Portals work by concentrating samples in specific emission areas like windows, lamps, and mirrors. Rays are traced from these portals at a higher sample rate to capture accurate lighting details. Other less crucial areas can use fewer samples by borrowing lighting data from the portals.

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Well-placed portals guide rays to bright spots that most influence the final render. Since brightness fluctuates less in these areas, fewer samples are needed to clean up noise compared to low-light regions. By manipulating sample counts based on luminance levels, portals offer a smarter approach to reducing render times.

Enabling Portals in Material Settings

Marking mesh surfaces as portals is easy in Blender. Simply enable the “Portal” checkbox under Ray Visibility in the Material settings. This tells Cycles to treat the material as a portal for ray guiding purposes. Any mesh can be designated as a portal, whether it emits light itself or not.

Mesh emitters like lamp objects work very well as portals due to their high luminance. Even non-emitters can produce noticeable results though, with the indirect light bounced from mirrors, glass, and glossy surfaces providing adequate sampling guidance.

Do note that only ray visibility needs to be enabled under the material properties. The surface shader itself controls the render appearance as normal. Portals simply have the additional capability of guiding samples.

Optimizing Portal Placement

The most impactful portals are placed to capture important bright features that contribute heavily to the lighting environment. Direct light sources are the highest priority targets. Position portals over the actual area emitting light from lamps, windows, open doors, glowing meshes, HDRIs, and similar objects.

Reflective and refractive materials can also guide sampling effectively in areas influenced by concentrated light bounces. Place portals on mirrors, glass, metals, glossy shaders, and water surfaces receiving focused brightness like caustics. This helps clean up noisy reflections and refractions.

For outdoor scenes, identify portal candidates that receive direct sunshine first. Then add portals in areas with noticeable secondary bounced lighting. Observe the scene during test renders to detect regions that need improved sampling.

Indoors, trace brightness features back to their light sources. Surround each emitting mesh with a portal to catch direct illumination. Also create portals on surfaces receiving focused reflections and refractions for cleaned up bounce lighting.

Controlling Samples on Portals

By default, portals inherit the render layer’s sample settings. This can be overridden by increasing or decreasing portal sub-samples under the object data properties.

Higher portal samples improve accuracy but lengthen render times too. Only raise samples if excessive noise is visible in the portal area. Usually 16-64 sub-samples are adequate for clean results.

For non-emitting portals that only receive indirect lighting, lowering samples can often suffice. Indirect areas need fewer samples to smooth out noise compared to direct illumination. Just ensure there’s no visible splotchiness.

Also try enabling the “Adaptive Sampling” option under portal object settings. This automatically regulates samples based on variance, concentrating more effort on noisy regions. This optimization can reduce grain while keeping render times low.

Using Portals with Irradiance Volumes

Combining light portals and irradiance volumes produces fast, high-quality interiors. Irradiance volumes approximate indirect lighting within boundary shapes, letting other areas skip computing bounce rays.

Place an irradiance volume over the interior space with portals on windows, lamps, and reflective surfaces. This concentrates samples on important direct and bounced lighting while approximating dimmer indirect areas. It’s a great optimization for rooms and halls.

Note that volumes work best for diffuse materials. Keep important glossy and refractive surfaces outside the volume so they render accurately. The volume will still receive lighting guidance from the portals.

Also enable Interpolated Irradiance for better gradients across volume bounds. And use the portal’s “Ambient Occlusion” option to darken indirectly lit regions near walls as in reality.

Extra Tips for Faster Renders

Some additional advice for speeding up cycles renders:

Lower sample counts in less important areas using render layers.
Use larger portal objects to minimize noise.
Set portal geometry to invisible so it does not render.
Use the GPU for faster path tracing where available.
Limit volume sizes to contained regions and enable multiple volumes if needed.
Use fewer bounces for secondary rays (Diffuse, Glossy, Transmission).

Following these light portal and sampling best practices should significantly lower your Blender Cycles render times. Render optimizations take some trial and error, but the time savings is worth the effort for complex scenes.