Houdini 19.0 Rendering

Cryptomatte

How to generate image maps associating each pixel in the image with the object name and/or material in the original source scene.

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Overview

Cryptomatte is a set of open-source tools for generating and working with images that associate individual pixels with object IDs. More generally, a “cryptomatte” is the type of image generated by the tools, or equivalent features in other software such as Houdini. This is a deep EXR image which stores object names/materials per-pixel, with special filtering to properly support anti-aliasing and transparency.

You can use the image to mask compositing effects to only the pixels associated with certain 3D source objects, or certain materials, without having to re-render the scene.

The following example shows how you can use a cryptomatte image to isolate a specific material in a Houdini compositor network.

See Cryptomatte’s official site for specifications and plugins for popular compositing applications.

Tips and notes

  • Cryptomatte uses the primary plane’s pixel filter. Custom pixel filter plugins can be used as well, but a filter that reads data from additional image planes (for example, minmax filter reads data from Z plane) are not allowed.

  • You cannot generate cryptomatte in micropolygon rendering mode.

  • Keep in mind that opacity and transparency are two separate things: Partially opaque materials let camera rays through, while transparent materials obstruct camera rays and cast secondary (refraction) rays. The pixel coverage is computed based on opacity, so you won’t be able to generate matte for objects behind refractive materials.

How to

To...Do this

Render a USD scene to a cryptomatte image using Karma

See Cryptomatte in Karma.

Render a traditional Houdini object-level scene to a cryptomatte using Mantra

The Mantra ROP lets you generate a cryptomatte image as an AOV plane in the main rendered image, or in an extra generated file.

  1. Set up the Mantra ROP you want to use to render the scene.

  2. Click the Images tab and then the Cryptomatte sub-tab.

  3. Click the Plus button to add a layer.

  4. Use the Property field to choose how to generate cryptomatte IDs.

    Use the drop-down menu after the text field to generate IDs by object name or material name.

    or

    You can enter the name of a user-created string property you have added to objects or materials using a Properties node. See render properties for detailed information on how to add user properties to objects or materials.

  5. By default, the cryptmatte layer is generated as an image plane in the main rendered image. To render it to a separate file, turn on Different file and set it to the output path of the .exr file to generate.

Generate masks in the compositor from a cryptomatte

See the help for the Cryptomatte COP.

Instancing and unique IDs

In order to matte a particular instance among other instances, you need to generate Cryptomatte using a user string property whose value is unique to each instance.

Here’s an example method using Material Stylesheets:

  1. Set up instancing.

  2. Make sure you're using fast point instancing.

  3. Using Attribute Expression (or any method you prefer), give each point inside Instance node a unique integer.

  4. Add a user string property to the Instance node.

  5. Add a root level stylesheet, target Point Instances, override script Render Property using Inline Script, and set up appropriate bindings:

    Example inline script:

    cvex myFn(int id=0; export string myuserprop="") 
    { 
        myuserprop = sprintf("foo%d", id);
    }
    

In this example, the name of the user property is myuserprop. The name of the point integer attribute is mypointid. The unique property values that are generated are foo0, foo1, foo2 etc (the # comes from mypointid attribute).

Note that, in this example, Inline Script was only necessary to convert integer attribute to string property. If you're using a string attribute then you could simply use Attribute Binding and skip the script and data binding altogether.

See Material Stylesheets for more information on how to use stylesheets.

Rendering

Mantra user guide

Basics

Lighting

Next steps

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