How to Create Object and Material Masks in Blender with Cryptomatte

What will you learn in this guide?

Cryptomatte is a masking standard used to identify specific objects or materials in a render. In Blender, the Cryptomatte node lets you bring these selections into the compositor, where you can create a grayscale mask of the selected elements or an image containing only those elements, with the rest transparent.

This guide first covers choosing a Cryptomatte source, then selecting elements in the node and using the appropriate output. There are two distinct workflows: using passes generated by Blender during rendering, or using Cryptomatte data from a pre-rendered multilayer OpenEXR file. Depending on the intended use, the Image input may not be necessary; it can be left unconnected if you only need a grayscale mask.

Requirements

First, decide which source to use. To use render passes from Blender, enable the appropriate Cryptomatte type in the scene’s View Layer and render the scene. Choose Object to select objects, Material to select materials, or Asset to select groups of objects that share the same parent. The Asset option is not the same as Blender’s asset feature.

Alternatively, use the Cryptomatte image in a multilayer OpenEXR file as the Image source. For this method, set the Cryptomatte node’s source to Image. The manual notes that Cryptomatte sidecar files are not supported.

Both methods use the Cryptomatte node in the compositor. You can use Render Layers and Viewer nodes to see the render in the background while making selections. The Cryptomatte node’s Image input is only required if you intend to use its Image output. If you only want the grayscale Matte output, you can leave the Image input unconnected.

Create a mask step by step

1. Choose the source and masking type

If you’re using render passes, choose the Object, Material, or Asset Cryptomatte pass according to what you want to isolate in the scene. This setting is in the Passes section of the View Layer properties. Then render the scene; the Cryptomatte node uses information from these passes when set to the Render source.

If you’re working with a prepared file, use the Cryptomatte image in the multilayer OpenEXR and select the Image source in the node. The source determines which image or pass data the node uses. Choosing the correct type at the outset helps prevent issues such as the expected object or material not appearing in the selection list.

2. Set up the compositor nodes

Add a Cryptomatte node to the compositor. To view the render while making selections, connect the Image output of the Render Layers node to the Image input of a Viewer node. If the image isn’t visible in the compositor background, check whether Backdrop is enabled.

With the Render source, the relevant Scene and View Layer passes are used. With the Image option, the node uses Cryptomatte data from a multilayer OpenEXR. Depending on the source type, the node may expose different fields for choosing images and scenes.

3. Select the elements to include in the mask

With the render visible in the compositor, use the selection control in the Cryptomatte node and click the object or material you want to include in the mask. Repeat to add multiple elements to the same mask.

The node’s Matte ID field stores the names of selected objects or materials as a comma-separated list. You can fill in this field manually, but using the adjacent plus and minus controls to make selections is more convenient. After selecting multiple elements, review the field to check which ones are included in the mask.

4. Use the output that suits your needs

The Matte output provides a grayscale mask of the selected elements. You can use it for operations in the compositor that require mask data. The node’s Image output, on the other hand, applies the mask to the color image connected to the Image input: the selected elements remain visible while the other areas become transparent.

So, if you only need a mask, you don’t have to connect an image to the Image input. If you want a color image containing the selected elements, connect the render image to this input. The Pick output can also help with object or material selection by displaying Cryptomatte data in color.

Common mistakes

If the mask is empty or doesn’t contain the expected element, first check the source you’re using. For the Render source, make sure the correct Cryptomatte type is enabled as a pass and that the scene has been rendered. For the Image source, verify that the file contains Cryptomatte data in a multilayer OpenEXR.

If the image isn’t visible in the compositor background, check the connection between Render Layers and Viewer, as well as the Backdrop option. If you get only a grayscale image when you expected a color-masked render, remember that Matte and Image outputs serve different purposes. For a color output, connect the render image to the Cryptomatte node’s Image input.

Keep the limitations in mind, too: according to the Blender manual, the Cryptomatte node cannot be used inside a node group, and Volume Objects are not supported. Sidecar files are also unsupported, so don’t assume a workflow based on them will work.

Its place in architecture, interiors, and archviz workflows

In architectural visualization, separating a piece of furniture, a surface finish, or another scene element from the rest of the image can make final edits more targeted. For example, using a mask for a specific material or object in an interior render in the compositor can reduce the need to apply the same change to the entire image. Cryptomatte’s support for multiple objects in a single pixel is also useful for separating overlapping image areas.

Before incorporating the workflow into a real project, test that the render engine produces the required passes and that the output file has the appropriate structure. The Blender manual specifically notes that Cryptomatte works with transparency, motion blur, and depth of field in Cycles. So rather than assuming you’ll get the same result with other engines or output types, it’s sensible to test with a small scene. The sources do not specify particular hardware, licensing costs, or a Blender version.

Next steps

Start with a small scene, create an Object Cryptomatte pass for a single object, and inspect the Matte output. Then try selecting a finish with the Material type, and finally connect a render to the Image input to create a masked color image. These tests can help you determine which source and output you need for a production scene.

The Blender manual describes Cryptomatte as a method with a similar purpose to the Object Index and Material Index passes. Its advantages include easier setup, compatibility with different compositor applications, and support for multiple objects in a single pixel. Even so, a successful result depends on generating the right pass in advance and matching the source to the node settings.

Source and license

This guide is adapted into Turkish from the official Blender Cryptomatte Node and Passes manuals. Both sources are licensed under CC BY-SA 4.0.