Cryptomatte and EXR workflow in V-Ray for SketchUp: A step-by-step guide

In V-Ray for SketchUp, render elements and Cryptomatte help give you greater control when editing architectural visualization output during compositing. Chaos’s SketchUp documentation describes element types, Cryptomatte criteria, EXR output options, and the VRIMG converter. This guide brings together the documented options for preparing the necessary channels during rendering and then bringing them into a compositing application. The steps are based on the V-Ray for SketchUp documentation; the ability to add multiple Cryptomatte elements is available in V-Ray 7 and later.

1. Which render elements should you choose for compositing?

Render elements keep different components of the final image in separate channels. The V-Ray for SketchUp documentation lists options such as Diffuse, Lighting, Reflection, Refraction, Shadows, Z-Depth, Ambient Occlusion, Cryptomatte, and Denoiser. The elements you need depend on which parts you plan to edit separately after rendering.

For example, Reflection contains surface reflections, Z-Depth contains surface depth information, and Lighting contains direct surface illumination. Total Light combines direct and indirect illumination. Some elements, such as Global Illumination and Caustics, produce content only when the relevant V-Ray option is enabled. Before rendering, check not just that the channels appear in the list, but also that they are appropriate for the scene settings.

Cryptomatte can create selection masks based on objects or materials. The documented ID Type options determine which naming convention the mask uses:

• Object Name: Uses object names; if no scene name is specified, object IDs are used.

• Material Name: Creates a mask based on the materials in the scene.

• Object Name with Hierarchy: Evaluates object names using the full scene hierarchy path.

• Layer Names: Uses the names of the layers to which objects are assigned.

Add Cryptomatte by right-clicking V-Ray Asset Editor › Render Elements, or by choosing V-Ray Asset Editor › Create Asset › Render Elements. According to Chaos’s documentation, this element works with Progressive and Bucket Image Sampler. Cryptomatte does not require separate preparation such as the object IDs needed by Multimatte elements; in typical use, it generates a few additional channels.

2. Set up multichannel EXR output

1. Go to V-Ray Asset Editor › Settings › Render Output. Under Save Image, specify a valid file path; the settings become available after you enter a file path.

2. Select EXR as the file format. Enable Multichannel to store render elements as channels in the same image file.

3. Check the Single File Output setting. When enabled, normally only the main channel is saved; however, according to the documentation, this setting is ignored when VRIMG or multichannel EXR is selected.

4. Review the Alpha, bit depth, compression, and Data Window options as needed. Alpha settings determine how the channel is saved in supported file formats. EXR offers 16-bit or 32-bit options.

5. After rendering, open the EXR file in the compositing application you plan to use and verify that the elements are present. Chaos’s documentation cites Nuke as an example for color correction with Cryptomatte, but does not provide application-specific steps.

The table summarizes documented settings; it does not imply that there is one ideal value for every project. In particular, choose a Data Window option according to the output requirements from among those documented.

3. Convert a VRIMG file to EXR

VRIMG is a Chaos format that can store the V-Ray Frame Buffer render image together with its existing render elements. V-Ray Image to EXR Converter is available as both a graphical interface and a command-line tool for converting these files to OpenEXR. The tool can process a single file or multiple files, and can also display information about the channels in a VRIMG.

1. Open the converter. For a single file, select the input VRIMG file; for batch processing, select the input folder. For batch jobs, you can also set the output folder, filename search pattern, and whether to scan subfolders.

2. Review the channels to be written to EXR in the Available Channels list. As needed, you can write each channel to a separate file or keep them as separate parts in a single EXR.

3. Select 16-bit or 32-bit floating-point data under Pixel type. When choosing a compression type, consider the converter’s lossless and lossy options. ZIP is used unless another method is specified.

4. Start the conversion with Convert. The tool does not delete the source VRIMG file during processing.

Note the difference between the interface and command-line memory settings. The interface documentation gives a default buffer of 500 MB; if the file does not fit within this limit, it can be converted in several passes. The command-line documentation gives a default of 10 MB per channel and recommends a higher value for large files. On the command line, a larger buffer can speed up processing but uses more memory. These two defaults apply to different usage modes and should not be treated as interchangeable.

4. Impact on architectural visualization workflows

Interior and architectural visualization teams can use separate channels when they want to work on lighting, reflections, depth, or specific objects after rendering. Cryptomatte builds selection masks based on object name, material name, hierarchy, or layer. According to Chaos, this approach can reduce the need to prepare numerous Multimatte elements. Multichannel EXR also makes it possible to carry render elements in a single file.

In an office workflow, identifying the required elements before rendering can reduce the chance of having to reprocess a scene later because a channel is missing. Before delivery, it is important to open the output in the target application and check that the masks select the correct objects. Since the sources do not specify hardware requirements, licensing terms, or compatibility details across all applications, these should be verified separately based on the software in use and the office’s standards.

Common mistakes

• Adding Cryptomatte or another element without checking whether the required scene option is enabled. Some channels, such as Global Illumination and Caustics, depend on specific conditions.

• Expecting multichannel EXR without checking the Multichannel setting, or misinterpreting how Single File Output behaves.

• Sending a file to compositing without checking the channel list and 16-bit/32-bit selection in the converter.

• Assuming the graphical interface and command line have the same default buffer settings. Failing to account for the fact that a larger buffer can increase memory usage.

• Not testing channel availability in the target application before delivering the EXR output.