Octane for Houdini

The PMC kernel is a custom mutating, unbiased kernel written for GPUs. It resolves complex caustics and lighting more efficiently than the other kernels. Rendering with PMC creates physically accurate lighting and caustic effects to produce the highest quality results, but it can also take the most time to render, depending on the scene.

Figure 1: The PMC Kernel parameters 

PMC Kernel Parameters

Quality Tab

Max Samples - This sets the maximum number of samples per pixel before the rendering process stops. Higher values result in cleaner renders. There is no rule as to how many samples per pixel are required for a good render - it depends on the scene.

Diffuse Depth - The maximum number of time a ray can bounce, reflect, or refract in a high roughness or diffuse surface. Higher values mean higher render times, but more realistic results. For outdoor renders, a good setting is around 4. For lighting interiors with natural light from the sun and the sky, you need a value like 8 or higher. While high values are possible, in reality, rays will not go beyond 16 ray bounces.

Specular Depth - Controls the number of times a ray refracts before dying. Higher values mean higher render times, but more color bleeding and more details in transparent materials. Low values introduce artifacts or turn some refractions into pure black.

Scatter Depth - The maximum path depth that allows scattering.

Maximal Overlapping Volumes - Determines how much space to allocate for overlapping volumes. Ray marching is faster with lo values but it can cause artifacts where many volumes intersect.

Max Subdivision Level - The maximum subdivision level applied on the scene's geometry. A value of 0 disables subdivision.

LOD Selection Bias - Used in conjunction with Meshlets. This setting influences the LOD data selection. Decreasing this value will make the loaded LOD data cruder and increasing this value will make the loaded LOD data finer. 

GI Clamp - It clamps the contribution for each path to the specified value. By reducing this value, you can reduce the amount of fireflies caused by sparse but very strong contributing paths. Reducing this value reducess noise by removing energy.

Caustic Blur - Increasing this value results in less caustic noise.

Ray Epsilon - The distance to offset new rays so they don’t intersect with the originating geometry. This value should be left at the default setting.

Filter Size - This sets the pixel size for the render filter. This can improve aliasing artifacts in the render. If the filter is set too high, the image becomes blurry.

Alpha Shadow - This setting allows any object with transparency (specular materials, materials with opacity settings and alpha channels) to cast a shadow instead of behaving as a solid object.

Nested Dielectrics - If disabled, the surface IORs are not tracked and surface priorities are ignored.

Irradiance Mode - This setting works similar to Clay Mode, but is applied to the first bounce, disables Bump, and makes samples that are blocked by back faces transparent. 

Alpha Channel - This option removes the background and renders it as transparent (zero alpha). This can be useful if you want to composite the render over another image and don't want the background to be present.

Keep Environment - This option works in conjunction with the Alpha Channel setting. It allows the background to render with zero alpha, but is still visible in the final render. This allows even further flexibility in compositing images.

Sampling Tab

Path Termination Power - This parameter provides a system to tweak samples per second vs. convergence (how fast noise vanishes). Increasing this value causes the kernels to keep paths shorter and spend less time on dark areas, which means they stay noisy longer, but it increases the samples per second. Reducing this value causes kernels to trace longer paths on average and spend more time on dark areas. In short, high values increases the render speed, but may create more noise in dark areas.

Direct Light Rays - Specifies the number of direct light rays traced for every sample. This amount is used after camera rays, and after very smooth specular reflections or transmission.

Exploration Strength - Specifies how long the kernel investigates good paths before it tries to find a new path. Low values create a noisy image, while higher values create a splotchy image.

Direct Light Importance - Causes the kernel to prioritize ray tracing paths with indirect light. Imagine sunlight coming through a window to create a bright spot on the floor. When Direct Light Importance has a value of 1, the kernel samples this area more and reduces noise around the bright spot. If you reduce the Direct Light Importance value, the PMC kernel reduces its efforts to sample that bright area and focuses on more problematic areas that are harder to render, such as areas with more indirect lighting.

Max Rejects - Controls the render bias. Reducing this value results in more bias, but shorter render times. In rendering terminology, biased renders introduce slight blurring and other less accurate computational techniques in order to reduce render time.

Parallel Samples - Controls how many samples are calculated in parallel. Smaller values require less memory to store the sample's state, but increases render time. High values require more memory, but reduces render time. The change in performance depends on the scene and the GPU architecture.

Work Chunk Size - The number of work blocks (512K samples each) done per kernel run. Increasing this value increases the memory requirement on the system, but does not affect memory usage, and may increase render speed.

Light Tab

AI Light - Enables AI Lights. AI Light functionality learns from the scene, and the rendering becomes more efficient as more samples are rendered. When used with Adaptive Sampling, AI Light becomes even more effective, as it learns pixel and light importance in a scene and no longer samples some pixels.

AI Light Update - Available when AI Light is activated, enables dynamic light update. 

Light IDs Action - This parameter determines whether the Light ID's action enables or disables lights with matching Light Pass ID numbers.

Light IDs - This is the ID of the light pass that captures the emitter's contribution. For more information about the Light IDs and their attributes, see the Light Passes topic in this manual.

Light Linking Invert - This option inverts the light linking behavior for selected light IDs.

White Light Spectrum - Controls the appearance of colors produced by spectral emitters (daylight, environment, black body).This determines the spectrum that will produce white (before white balance) in the final image. 

• D65 - Adapts to a reasonable daylight "white" color.
• Legacy/Flat - Preserves the appearance of old projects (spectral emitters will appear more blue)

Toon Shadow Ambient - This is the ambient modifier of Toon Shadowing.