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Pixel-to-camera rays that hit some object in the scene.
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Used to resolve noise/artifacts from:
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Geometry and Curve Details
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Displacement
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Depth of Field
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Motion Blur
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Previous Rendering
Overview ¶
Sampling is the heart of modern render engines. Sampling refers to the simulated light rays Karma sends into the scene to determine the color of each pixel of the rendered image. This chapter goes into some detail about how Karma does sampling.
Tip
The terms sample, path, and ray are generally interchangeable when talking about path tracing.
Karma begins by sending samples from the camera that then hit objects in the scene. These are the primary samples. When objects are hit, Karma uses the object properties (such as surface orientation and materials) to send out secondary samples. These additional rays hit other surfaces and lights, until the final color of the pixel is determined.
Primary
Secondary (Direct)
Secondary (Indirect)
Tip
The behavior of Primary Samples can also refer to Path Traced Samples.
Primary samples ¶
Secondary samples ¶
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Rays cast from the hit position of primary samples, toward other directions in the scene.
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Direct Samples are position-to-light rays:
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Light Samples
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Indirect Samples are position-to-position rays:
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Diffuse
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Reflection
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Refraction
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Sub-Surface Scattering
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Volume
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Oversampling (sending out more rays than you need for a given quality level) and under-sampling (sending out too few rays to achieve a given quality level) are sources of slow-downs and noise when rendering. Understanding the differences between primary and secondary samples will help you to use Karma more effectively.
Estimating ray counts ¶
Understanding how primary and secondary samples are related helps estimate the maximum number of rays Karma uses when rendering. User settings, per-object overrides, and algorithms make predicting the exact number impossible. Understanding how these parameters influence one another helps debug performance issues.
The number of rays Karma uses is determined by the total number of primary samples and secondary samples fired for each pixel. The selected convergence mode also affects this estimate, however pixel oracles only affect the number of primary samples.
Automatic Convergence
Path Traced Convergence
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Oversampling (sending out more rays than you need for a given quality level) and under-sampling (sending out too few rays to achieve a given quality level) are sources of slow-downs when rendering. Understanding the differences between primary and secondary samples will help you to use Karma more effectively.
Primary samples ¶
The rays that Karma sends from the camera into the scene are called primary samples. Primary samples resolve anti-aliasing artifacts from small geometry detail (such as curves or displacement), depth of field, and motion blur. Sufficient pixel samples improves the overall sharpness and quality of the image.
Tip
Primary Samples are sometimes also called camera rays, AA samples, or pixel samples, especially by other renderers.
A single primary sample is usually too coarse for anything more than debugging. Increasing the pixel samples gives much smoother representation of the shapes in the scene. While this improves quality, even in secondary noise, it can be overkill because primary samples are a multiplier on the secondary samples. The pixel oracle can help Karma avoid over-sampling pixels.
Pixel oracle ¶
Karma supports adaptive primary samples, using pixel oracles. Pixel oracles represent the logic Karma should use when casting rays from each pixel. Currently, Karma ships with 2 pixel oracles: Variance and Uniform.
Note
Starting with Houdini 20.5, Karma XPU supports adaptive path traced samples via the pixel oracle.
By default, Karma uses the Variance oracle, which detects differences between neighboring pixels. This is to avoid sending too many primary samples by stopping when each additional sample will have very little effect on the final pixel color. Karma will send a small number of primary samples, then the Variance Oracle starts comparing pixels against the Oracle’s Variance Threshold. Once the variance between the previous samples of the current pixel and adjacent pixels fall below that threshold, Karma stops firing primary samples for that pixel.
The Uniform Oracle simply sends the same number of primary samples from every pixel in the scene. You may want to switch to the Uniform Oracle when extreme depth of field (DOF) or motion blur dominates the image. In those cases, Karma ends up fully sampling every pixel anyway, so measuring variance just adds overhead.
Pixel Oracle Render Properties
Secondary samples ¶
Once a primary ray intersects an object, Karma will cast Secondary Samples from each primary ray’s hit location. For every hit position, Karma will send direct and indirect secondary samples. Direct samples are sent towards lights. Indirect samples are sent along directions determined by the geometry and shader, at the hit position. Secondary samples can be diffuse, reflection, refraction, subsurface, or volume rays.
Volumetric objects are handled a bit different from surfaces. Instead of sampling only a single hit position, Karma marches along the ray, sampling multiple points through the volume. At each positions along the ray, direct and indirect rays are sent out into the scene. Not every position within the volume is used; instead Karma intelligently chooses a subset of positions to sample.
Min/Max Secondary Properties
Convergence modes ¶
The convergence mode is the strategy used by Karma to decide when to send secondary samples into the scene. Karma CPU supports two convergence modes: Automatic and Path Traced, while Karma XPU only supports Path Traced mode. While convergence mode specifically affects secondary rays, it can affect how many primary samples are needed, especially for Karma CPU.
The next chapters have more information on convergence modes for Karma CPU and Karma XPU specifically.
Convergence Mode Properties
Limits ¶
To avoid rays potentially traveling forever, Karma provides per-type “bounce” limits. You may need to increasing these limits to ensure Karma takes enough indirect samples to get the desired look. Refraction often needs a higher limit, to improve the quality of scenes with glass and liquid materials. Volumes may also benefit from increased limits, to capture the subtle effects of volumetric lighting.
Increasing the limits can increase render times substantially, and require higher numbers of rays to resolve noise. Be careful to only increase limits when needed to achieve a desired look.
Limits Render Properties
Light Tree ¶
Karma automatically builds a light tree to make rendering scenes with many lights more efficient. This helps Karma efficiently decide which lights should be sampled the most, and are most likely to resolve faster. Karma automatically builds a light tree, and automatically adds lights to it, when it reaches certain thresholds:
Karma CPU |
10 Lights |
Karma XPU |
2 Lights |
There are some cases where lights will not be added to the light tree, such as dome and directional lights, or lights which have light filters. Light Sampling Mode can change this behavior, and tell Karma to sample all lights in a scene. Long, thin rectangle lights are not great for the light tree. Internally it represents lighting with bounding spheres, so these lights can cause the light tree to add more noise.
Once Karma determines which light to sample at a given hit position, the Light Sampling Quality parameter determines how many rays are fired towards the light.
Light Properties
Local Sample Overrides ¶
Many of the sampling settings for each render delegate can also be set on objects of the scene. Per-light and per-object settings are usually multpliers on their global counterpart. If a property does not mention “scale” or “multiplies” in its description, then it overrides the equivalent global property.
Also it’s worth noting that if a render delegate or convergence mode does not support a given sampling style/feature, per object overrides won’t have an effect.
Note
Karma does not support per-object overrides for Primary/Path Traced Samples.
To understand sampling and noise better, follow the next chapter for your render delegate of choise.
Next Sampling: Karma CPU
Next Sampling: Karma XPU