Best Practices For Exporting Render Passes To Exr In Blender

Understanding Render Passes

Render passes are separate outputs from Blender’s rendering engine that each contain specific elements of a scene. For example, common passes include diffuse color, reflections, shadows, emission, specular highlights, ambient occlusion, depth, normals, object id’s, and more.

Using multiple render passes provides flexibility in compositing by allowing you to adjust, combine, and layer different aspects of the rendered image separately. This helps improve realism, accentuate artistic style, or fix issues without having to rerender the entire scene.

The main differences between rendering with multiple passes versus a single render output are:

• Passes split up scene information into layered images allowing for more compositing control
• Adjustments can be made to passes individually without rerendering
• Render times may increase slightly due to outputting more data
• File storage is larger with multiple EXR pass files
• Compositing is required to combine passes into final image

Configuring Render Layers for Render Passes

Render layers in Blender allow you to configure multiple outputs from a scene while only having to render once. Here are some guidelines for setting up render layers to output useful passes:

• Diffuse Color – Enable this pass to output basic object colors and textures
• Glossy Direct/Indirect – Separates glossy reflections into direct and indirect lighting
• Transmission Direct/Indirect – Separates glass/transparent surfaces into direct and indirect
• AO – Useful for adjusting ambient occlusion intensity in the composite
• Vector Pass – Provides speed/motion vector data for motion blur effects

Try combining passes like diffuse, emission, shadows, specular, reflections, AO, and normals to gain extensive control while compositing. Or use object id’s, depth, and vector passes for post-process effects like depth of field, object masking, camera effects, particles, and more.

Exporting Passes to EXR Files

EXR files contain full scene data encoded in high dynamic range, unlike other image types like PNG or JPEG that may posterize or introduce compression artifacts. Benefits to outputting passes to EXR include:

• 32-bit float HDR values perfect for CGI compositing
• Compressed sizes similar to 8-bit images
• No quality loss from encoding compared to other formats
• Layer multi-part data for flexible compositing
• ASA OpenEXR libraries support complex HDR workflows

In the Properties panel under Output settings, ensure color depth is set to float (Full). Then in the File Format dropdown, select OpenEXR MultiLayer to output render passes.

Next, configure the output node in the compositor. Enable “Save Buffers” to automatically output enabled passes to EXR files. The “File Format” field will automatically switch to EXR when saving passes. That’s it – your layered passes will now save individually or combined in EXR format!

Compositing EXR Pass Layers

Importing the exported render passes into Blender’s compositor provides a full suite of tools to combine, adjust, and enhance your rendered elements. Some tips include:

• Use color balance and curves nodes to tweak values
• Overlay different math node operations like add, subtract, multiply to blend passes
• Use ID masks and depth data to separate objects and foreground vs. background
• Build complex node trees with prioritized layering and effects

Always break apart your large composites into organized sections that can feed a final composite node. Some sections could include:

• Beauty – Layer base colors, lighting, shadows
• Adjustments – Tweak hue, saturation, values
• Effects – Add depth of field, glows, flares
• Touch-ups – Small corrections with painting, cloning etc.

Structured pass compositing enables iteratively improving and troubleshooting your renders as needed.

Optimizing Render Times with Passes

Adding multiple render passes and compositing can quickly increase render times if not optimized. Here are some tips for maximizing efficiency:

• Use lower sample counts during look dev to save time
• Identify noise patterns and sample only noisy passes like indirect
• Render primary passes like diffuse together, then layer secondary passes
• Only re-render specific problematic passes instead of the entire scene
• Use Render Regions to test small areas faster
• Add passes one by one while compositing to identify issues early

Finding the right balance comes down to identifying which passes contribute the most to your final render look and quality. Deferred rendering architecture means passes render intersecting scene data, so order and technique can greatly minimize waste.

Troubleshooting Render Passes

Some common problems when working with render passes and how to solve them include:

• Black renders – Ensure passes are enabled in render layers before exporting. Missing AOVs result in black images.
• Missing layers – Each pass must have an output AOV node connected to the File Output in compositing before rendering.
• Transparency issues – Disable transparency in the display window before rendering to ensure proper pass sorting.
• Noisy passes – Smooth noise by increasing sample counts on specific passes as needed.
• Fireflies – Enable clamp direct and indirect lighting in render settings to eliminate hot pixels.

When diagnosing issues with passes, simplify the scene, reduce layers/effects, and test one pass at a time to identify the source of the problem. Then adjustments and corrections can be made accurately before returning complexity back.