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Path Tracing is best used for realistic results. The render times are higher than Direct Lighting, and working with small light sources and proper caustics can be difficult, but the results are photorealistic (Figure 1).
Figure 1: The Path Tracing kernel settings
Path Tracing Kernel Parameters
- Max. Samples - Sets the maximum number of samples per pixel before the rendering process stops. Higher numbers of samples per pixel results in cleaner renders. For quick animations and scenes with predominantly direct lighting, a low amount of samples (500-1000) will suffice. In scenes with lots of indirect lighting and mesh lights, OctaneRender® may require several thousand samples to get a clean render.
- Depth - Gives the maximum number of diffuse reflections if you set GI Mode to Diffuse.
- Depth - The maximum depth to trace rays (reflections/refractions) passing through specular objects. This prevents transparent objects from causing dark outlines in scenes.
- Ray Epsilon - The distance to offset new rays so they don't intersect with the originating geometry. If the scale of a scene is too large, precision artifacts like concentric circles appear. In that case, increasing this parameter makes these artifacts disappear.
- Filter Size - Sets the pixel size for filtering the render. This improves artifact aliasing in the render, and it also reduces noise. However, if you set the filter too high, the image becomes blurry.
- Alpha Shadows - Allows any object with transparency (Specular materials, materials with opacity settings and alpha channels) to cast a shadow instead of behaving like a solid object.
- Caustic Blur - Approximates caustics on rough surfaces, and increases or decreases caustic noise sharpness. A 0 value provides the sharpest caustics, and increasing this value increases the blurring effect that makes caustics appear soft.
- GI Clamp - Controls indirect light bounces, and provides a facility to balance out global illumination and noise produced by scene caustics. Reducing he GI Clamp lowers noise contributed by strong light passing through clear or specular objects.
- Irradiance Mode - Renders the first surface as a white Diffuse material. It's similar to Clay Mode, but it only applies to the first bounce. It disables the Bump channel and turns samples transparent when back faces block them.
- Alpha Channel - Removes the background and renders it transparent (zero alpha). This composites the render over another image without the background being present.
- Keep Environment - Allows the background to render with zero alpha, but is still visible in the final render. This works with the Alpha Channel setting, allowing more flexibility when compositing images.
- AI Light - Enables AI lighting.
- AI Light Update - Enables dynamic AI Light Update.
- Light IDs Action - Selects the action taken on selected Light IDs.
- Light IDs - Specifies the Light ID.
- Light Linking Invert - Inverts the light linking behavior for the selected Light IDs.
- Path Term. Power - Provides a system for tweaking samples/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 increases samples/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 also generates higher noise in dark areas.
- Coherent Ratio - When you enable Coherent Ratio for Path Tracing or Direct Lighting kernels, the render becomes noise-free faster, but the downside is flickering when rendering animations. This ratio saves time for stills and action-heavy animations.
- Static Noise - When enabled, the noise is static (doesn't change between frames). This is disabled by default. The noise is fully static as long as you use the same GPU architecture for rendering. Using different architectures produces slightly different numerical errors, which manifest as small differences in noise patterns every time rendering restarts on the frame.
- Parallel Samples - Controls how many samples are calculated in parallel. If set to a small value, OctaneRender requires less memory to store the samples state, but renders are slower. If set to a high value, then OctaneRender needs more graphics memory, but renders are faster. The change in performance depends on the scene, the GPU architecture, and the number of shader processors available on the GPU.
- Max. Tile Samples - Controls the number of samples per pixel that OctaneRender uses for rendering before it stores the result in the film buffer. Higher numbers mean results arrive less often at the film buffer, but it reduces CPU overhead during rendering, which improves performance.
- Minimize Net Traffic - When enabled, OctaneRender distributes only the same tile to the net render slaves until it reaches the max samples/pixel for that tile, and then OctaneRender distributes the next tile to the slaves. This option doesn't affect work done by local GPUs.
- Adaptive Sampling - Provides options to the kernel to disable sampling for pixels that reach a specified noise level.
- Noise Threshold - Specifies the smallest relative noise level. When a pixel's noise estimate is less than this value, then the pixel's sampling switches off. You can get good values in the range of 0.01 to 0.03. The default is 0.02.
- Min. Adaptive Samples - Specifies the minimum number of samples to calculate before adaptive sampling activates. The noise estimate of a pixel is just an estimate with a large initial error, so the higher the noise threshold, the higher you should set this parameter to avoid artifacts.
- Pixel Grouping - Specifies the number of pixels that are handled together. If all pixels of a group have reached the specified noise level, sampling stops for these pixels.
- Expected Exposure - This value should be close to the same value as the image exposure, otherwise set it to 0 to ignore these settings. The default value is 0. Adaptive sampling uses this parameter to determine the bright and dark pixels, which depends on the Exposure setting in the OctaneRender Imager. If the value is not 0, Adaptive Sampling tweaks/reduces the noise estimate of very dark areas of the image. It also increases the Min. Adaptive Samples limit for very dark areas, because very dark areas don't always find paths to light sources, resulting in over-optimistic noise estimates.
- Deep Image - Enables rendering deep pixel images used for deep image compositing.
- Maximum Depth Samples - Sets the maximum number of depth samples per pixel. The PBR Render Target uses this when you enable Deep Image rendering.
- Depth Tolerance - OctaneRender merges the depth samples whose relative depth difference falls below this tolerance value. The PBR Render Target uses this when you enable Deep Image rendering.
- Toon Shadow Ambient - Ambient modifier for Toon Shadowing.
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