Blender Cryptomatte Manifest and Matte Isolation

This article provides a comprehensive guide to understanding the Cryptomatte manifest and using it to isolate specific scene elements during post-processing in Blender. You will learn what the metadata manifest does, how Cryptomatte handles complex visual data like motion blur and transparency, and the exact step-by-step workflow required to isolate objects or materials within the Blender Compositor.

What is the Cryptomatte Manifest?

Cryptomatte is an open-source standard originally developed by Psyop for efficiently creating identification mattes. When you render an image with Cryptomatte enabled, the renderer generates several 32-bit float channels containing hashed identification numbers and fractional coverage values for every pixel.

The Cryptomatte manifest is the metadata dictionary embedded directly into the render file (typically an OpenEXR MultiLayer file) or held in memory during the render session. This manifest maps human-readable names—such as object names, material names, or asset hierarchies—to the numerical 32-bit hash values assigned to those surfaces.

Without the manifest, artists would have to guess or manually calculate numeric hash IDs to select elements. With the manifest, the compositing system reads the embedded name table, allowing users to select elements by name or click them with a color picker to extract precise alpha masks that correctly preserve motion blur, depth of field, and anti-aliased edges.

Enabling Cryptomatte Passes in Blender

Before you can isolate elements, you must instruct the Cycles renderer to generate the necessary Cryptomatte layers:

  1. Open the View Layer Properties tab in the Properties editor.
  2. Scroll down to the Passes section and expand the Cryptomatte panel.
  3. Check the levels of isolation you require:
    • Object: Generates mattes based on individual scene objects.
    • Material: Generates mattes based on assigned materials.
    • Asset: Generates mattes based on parent-child object hierarchies or linked groups.
  4. Set the Levels value (typically 6 by default), which dictates how many overlapping objects per pixel the system can resolve.

How to Isolate Scene Elements in the Compositor

Once your scene is rendered with the Cryptomatte passes active, use Blender’s built-in Compositor to isolate your elements:

1. Set Up the Compositor Nodes

  • Switch to the Compositing workspace and ensure Use Nodes is checked in the top header.
  • Add a Cryptomatte node by pressing Shift + A and navigating to Matte > Cryptomatte.
  • Connect the Image output from your Render Layers node to the Image input of the Cryptomatte node.
  • Ensure the Crypto Object, Crypto Material, or Crypto Asset layer is selected in the Cryptomatte node dropdown depending on what you want to isolate.

2. Pick Elements to Isolate

  • Connect the Cryptomatte node’s Image output to a Viewer node (Shift + A > Output > Viewer) to view the render in the backdrop.
  • On the Cryptomatte node, click the Add eyedropper icon (the + icon next to the Matte ID list).
  • Click directly on the target object in the backdrop or Image Editor window. The object's name from the manifest will instantly appear in the Matte ID text field.
  • To remove an object, click the Remove eyedropper icon (the - icon) and click the target object, or delete its entry manually from the Matte ID field.

3. Utilize the Isolated Matte

  • Matte Output: Connect the Matte socket of the Cryptomatte node to the Factor input of a Color Correct, Mix Color, or Defocus node to apply adjustments exclusively to the selected objects.
  • Image Output: The Image socket outputs the original render with an alpha channel pre-applied, masking out everything except the selected elements, which is ideal for isolating elements for layering or exporting as standalone cutouts.