Houdini 19.5 Solaris

Karma image and pixel filters

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In the Karma rendering pipeline, there are three separate types of filters.

Each filter works on AOV data at different stages of the rendering pipeline. Sample filters access raw sample data straight from ray evaluation. These filters run independent of any other samples.

Pixel filters combine multiple samples into a single pixel value.

Image filters process the entire image, after all the pixels have been resolved.

You can get a list of all filters using the command line tool by running:

karma --filters

At the current time, this doesn’t list any options to control filters.

Sample and Image filters are “global” and run on all AOVs simultaneously. For example, a sample filter might take multiple AOVs for different color components and sum them into a different AOV. An image filter might take several AOVs and use them to run a denoiser on other AOVs. There can be multiple Sample and Image filters for each render.

On the other hand, Pixel Filters are defined on each AOV individually. Thus, a color AOV might have a Pixel Filter that averages samples together, while an AOV for surface normals might take a single sample. Each AOV only has a single Pixel Filter defined.

Sample and Image filters are specified using a JSON string. Each filter is specified by a JSON list of 2 elements. The first element specifies the name of the filter, the second argument is a dictionary of arguments to control the filter’s behaviour.

Image Filters

ocio

Key

Type

Description

planes

string

Required The list of AOVs to be processed by OCIO.

output

string

Required Output color space name.

input

string

Source color space name. The default is "scene_linear".

looks

string

A comma separated list of color grading names.

Example: [ "ocio", {"planes":"C,diff*,spec*", "output":"sRGB"} ]

denoise

Key

Type

Description

engine

string

The denoiser to apply (for example “oidn” or “optix”).

aovs

list of strings

List of AOV names to de-noise. Default is ["Cf"].

albedo

boolean

Whether to pass the albedo plane to the denoiser.

use_N_input

boolean

Whether to pass the N, Ng, or normal plane to the denoiser.

use_gl_output

boolean

Whether to output directly to OpenGL.

Example: [ "denoise", {"engine":"optix", "use_n_input":true, "use_albedo_input":true ]

Sample Filters

colorlimit

Key

Type

Description

planes

string

Comma separated list of AOV names/patterns to color limit.

colorlimit

float

The maximum channel value allowed.

Example: [ "colorlimit", {"planes":"C*,color*,diff*", "colorlimit":10.0} ]

copyalpha

Key

Type

Description

planes

string

Comma separated list of AOV names/patterns to use as input.

This is added automatically by the renderer to copy the Af render global to the fourth component of any AOVs defined by LPE.

ocio

This sample filter is primarily an example since it’s much more efficient to do color correction in the image filter (after samples have been combined by pixel filters).

Key

Type

Description

planes

string

Required Comma separated list of AOV names/patterns to process with OCIO.

output

string

Required Output color space name.

input

string

Source color space name. The default is "data".

looks

string

Comma-separated list of color grading names.

Example: [ "ocio", {"planes":"C,diff*,spec*", "output":"sRGB"} ]

Tip

You can chain multiple image filters (or sample filters):

[
    [ "denoise", {"engine":"oidn", "aovs":["Cf","diffuse","specular"]} ],
    [ "ocio", { "planes":"C,diff*", "output", "sRGB" }]
]

Pixel Filters

Radial Pixel Filter

Names: bartlett, blackman, box, catrom, gauss, hanning, mitchell, sinc.

These pixel filters are radial filters which combine samples using the given filter shapes.

Key

Type

Description

width

float

Filter width.

Example: [ "gauss", {"width":2.5} ]

Unit Box Filter (ubox)

Uniform box filter. This simply averages all the samples which fall within a pixel. This is different than the box filter which blends in samples from adjacent pixels (based on the filter width).

Min Max filters (min, max)

Take only the sample with the minimum or maximum value.

omin, omax

Currently, these are synonyms for the min, max filters.

ocover, edge, idcover

Currently, these are not implemented

Pixel Oracles

Pixel Oracles are related to filters in karma. Oracles are used to tell the renderer where additional samples need to be sent. Unlike filters, Oracles take a simple string which is parsed into arguments.

Currently, there are two oracles:

uniform

Uniformly distribute rays to each pixel. Each pixel will always get the same number of ray-samples.

variance

Distribute rays based on variance in the rendered image.

Key

Type

Description

minrays

integer

The minimum number of samples.

variance

float

The pixel variance.

plane

string

The image plane to measure.

ociomode

integer

How to apply OCIO transform when measuring variance. Set to 0 to disable (gamma 2.0), 1 to choose color space based on OCIO Display and View, or 2 to specify OCIO color space directly.

ociodisplay

string

OCIO display (ignored unless ociomode is 1).

ocioview

string

OCIO view (ignored unless ociomode is 1).

ociocolorspace

string

OCIO color space (ignored unless ociomode is 2).

Example: [ "variance", {"minrays":2, "plane":"C", "variance":0.01} ]

Solaris

USD

Geometry

  • Importing SOP geometry into USD

    Details of how Houdini converts SOP geometry to USD, and how you can control the process.

  • Component Builder

    The Component Builder tool puts down a network snippet for creating a USD model from SOPs, with support for materials, variants, payloads, and layering.

Layout

  • Edit node

    Interactively transforms prims in the viewer. Can use physics collisions to position props realistically.

  • Layout node

    Provides tools for populating a scene with instanced USD assets. You can place individual components, paint/scatter components in different ways using customizable brushes, and edit existing instances.

  • Custom Layout Brushes

    How to create layout brush digital assets you can use to customize the behavior of the Layout LOP.

Shading

  • Shader Framework

    Describes the Solaris shading framework, including shader node translation to USD primitives.

  • Using MaterialX in Solaris

    Houdini has VOP node equivalents of the MaterialX shader nodes. You can build a shader network using these nodes, or import an existing MaterialX shader, and use them with Karma (Houdini’s USD renderer).

  • UDIM

    You can encode different tiles of a texture space into different texture files, each eith its own resolution. You can then specify a texture filename such as kaiju.exr, and Houdini will replace the token with the specific tile address at load time.

Karma rendering

Tutorials