Create Object Masks After Rendering with Blender Cryptomatte

What you’ll learn in this guide

When you need to edit just one piece of furniture, a light fixture, or a group of materials in an interior render, manually masking every object can take time. Blender’s Cryptomatte node lets you select objects or materials in a render from the compositor and create masks for them. This makes it possible to work on a selected area instead of the entire image.

This guide covers adding Cryptomatte data to a render, setting up the node in the compositor, selecting objects from the image, and using the mask. It also explains the difference between the Matte and Image outputs and outlines workflow limitations. The key point: Cryptomatte data must be generated during rendering for selections to work; it cannot be added afterward to a regular image that does not contain this data.

Requirements and version notes

You’ll need a scene set up in Blender, a render containing Cryptomatte data, and Blender’s Compositor. The Blender 4.0 manual marks the Cryptomatte node as “CPU Compositor Only,” so account for this CPU compositor limitation when using the node in that version. The current manual does not state the limitation in the same way. Check the documentation for your Blender version to confirm hardware and processing requirements.

Before rendering, enable the Cryptomatte Object pass in the relevant View Layer’s Passes section. Blender’s documentation refers to Cryptomatte layers for objects, materials, and asset types; the example below focuses on selecting objects. The pass and layer you use must match the type of selection you want to make.

Cryptomatte data can come from a Blender render; according to the current manual, Cryptomatte data in a multilayer OpenEXR image can also be used as a source. The manual pages cover different Blender versions: the current page describes selecting a source and layer in the node properties, while the Blender 2.92 documentation describes an older workflow in which passes are connected to the node inputs. If the interface steps do not match, follow the documentation for your version.

Step-by-step: creating a Cryptomatte mask

1. Enable the Cryptomatte Object pass. In the View Layer’s Passes settings, enable Cryptomatte Object and render the scene. Cryptomatte selection data is generated during rendering; if the pass is not enabled, the current render may not contain the required data.

2. Add a Cryptomatte node in the Compositor. The node handles object or material selection and mask outputs. If you want a color image with a mask applied, connect the render image from the Render Layers node to the Cryptomatte node’s Image input. You can leave this input unconnected if you only need a grayscale mask.

3. Make the image visible for picking. Connect the Render Layers node’s image output or the Cryptomatte node’s Pick output to a Viewer node. If the image is not visible in the compositor backdrop, check whether Backdrop is enabled in the header. The Pick output displays Cryptomatte data in color and can be used when selecting objects.

4. Choose the source and Cryptomatte layer. If you’re using data from a render, choose Render as the source; if you’re using a multilayer OpenEXR, choose Image. For an Image source, specify the image to use, then select the appropriate layer in the Cryptomatte Layer field. When Render is selected as the source, a scene option is also available.

5. Select the objects to include in the mask. Use the add button on the node and click an object in the image to include it in the mask. Repeat to select multiple objects; use the remove button to exclude a selection. You can also enter object or material names manually in the Matte ID field, separated by commas; the manual notes that using the selection buttons is easier.

6. Choose an output for your needs. The Matte output provides a grayscale mask for the selected objects or materials. The Image output masks the color image connected to the Image input, leaving only the selected areas visible and making the rest transparent. Use Matte when you only need a mask, or Image when you need a masked render.

7. Check the result and edges. Inspect the mask in the Viewer for unexpected areas; confirm that the correct Cryptomatte layer is selected and that the appropriate image is visible for picking. Blender’s documentation states that Cryptomatte can account for effects such as antialiasing, motion blur, and transparency. Check mask edges against the render in scenes where these features matter.

Common mistakes

Rendering without enabling the pass: Cryptomatte selection data is generated during rendering. If the pass is not enabled, the node cannot create object data afterward that is missing from the image.

Monitoring the wrong output in the Viewer: You can use the Pick output to sample objects. Also check the Backdrop setting if the compositor backdrop is not visible.

Confusing Matte and Image outputs: Matte provides a grayscale mask; it is not a color render on its own. To get a masked color image, connect the render to the Image input and use the Image output.

Ignoring version differences: Older Blender manuals describe connecting passes to the node separately, while the current manual places source and layer selection in the node properties. The Blender 4.0 manual also notes a CPU compositor limitation for the node. Check instructions and hardware requirements for the version you’re using.

Relying on unsupported data: Blender’s manual states that Cryptomatte sidecar files are not supported, the node cannot be used inside a node group, and volume objects are not supported. Keep these limitations in mind when planning the compositor stage and delivery format.

Impact on architecture and archviz workflows

In architectural visualization, Cryptomatte can be used to mask furniture, light fixtures, or specific surface groups separately in a render. For example, instead of manually outlining a chair or selected materials, you can select them from the render data and use the Matte output in the compositor. This approach helps manage edits to specific areas without changing the rest of the render.

For the workflow to work, the pass must be enabled during rendering and the required data must be present in the output file. If the render is going to be brought into another compositor, the target application must support the Cryptomatte standard. The CPU compositor note in the Blender 4.0 manual means teams using that version should check their processing setup in advance. Given version differences and limitations, creating a small test render before a project can help validate the workflow.

Next steps

Start by selecting a single object and inspecting the Matte and Image outputs separately. Then try adding and removing multiple objects, and check mask edges in scenes with transparent surfaces or motion blur. Confirm which node workflow and compositor limitations apply to your Blender version. If you plan to use a different compositor, test that it can read the Cryptomatte data with a small file.

Sources and license

This guide has been adapted into Turkish based on Blender’s current Cryptomatte node documentation and relevant manual pages for Blender versions 2.92, 4.0, and 2.80. The source documentation pages are licensed under CC BY-SA 4.0.