Viewport Limitations When Compositing Emissive Alpha Overlays In Blender
What Causes Emissive Materials to Appear Black in Viewport
Emissive materials in Blender rely on scene exposure and color management settings to properly display emitting light in the viewport. However, limitations in the viewport’s ability to display a wide range of values can cause emissive materials to appear black rather than emitting light.
Specifically, the viewport clamps emissive material values to a displayable range based on the scene exposure. Very large emissive values that should result in glowing materials may have their emitting light completely clamped in the viewport.
Emissive Materials Rely on Scene Exposure and Color Management
For emissive materials to appear brightly emitting in the viewport, the scene exposure must be properly calibrated to the intensity of the emissive values. The color management controls map the scene values to the limited ranges displayable in the viewport, including emissive materials. If exposure is too low or color management is not accounting for intense emissives, the emitted light will never be mapped to visible ranges in the viewport.
Viewport Limitations Clamp Emissive Values
Even with adjusted exposure and color management, the emissive values are ultimately limited to what the viewport can reasonably display. Common graphics hardware has limitations in displaying high dynamic range (HDR) emissive values. Instead, these numbers get clamped to the maximum brightness representable in the viewport.
Additonally, limitations in the precision of floats displayed in the viewport can round off, for example, bright white light from an emissive down to complete black if its intensity exceeds the viewport’s capable display ranges.
Adjusting Viewport Settings to View Emissive Materials
To overcome viewport limitations when displaying emissive materials, the scene exposure and color management controls can be adjusted to make emitting light visible within the viewport’s restricted display ranges.
Increasing Viewport Exposure
Increasing the Scene Exposure value under Color Management expands the viewport display ranges, allowing higher emissive values to be mapped to visible intensities. However, increasing exposure too high can blow out diffuse shading – finding the right value is key for seeing emissives while preserving base rendering details.
Disabling Viewport Color Management
With color management disabled in the viewport under Scene Properties, the raw emissive values are directly mapped. While color accuracy suffers, this allows high numbers from emission shaders to actually produce visible light rather than clamping to black.
Compositing Emissive Passes in Rendered Image
Rather than relying on inaccurate viewport representations, emissive materials can be accurately rendered by separating out the emission pass. The final image is composited together in Blender’s compositor, mixing a bright emissive pass over the base renders.
Rendering Emissive Pass Separately
By rendering the emissive light pass separately from diffuse color and shadows, the full range of emitting light can be preserved. The key benefit is avoiding the viewport’s limitations on display intensities by deferring compositing until after render. This avoids losing detail that exceeds the viewport’s dynamic range.
Compositing Emissive Overlay in Blender’s Compositor
With render passes separated, the emissive light can then overlay as a transparent mix over the primary beauty render in Blender’s compositor. This allows proper View Transform controls to adjust gamma, exposure, and color management specifically for the emissive pass while retaining ideal settings for shading in the base render.
Example Node Setup for Emissive Material Compositing
Below outlines a node setup in Blender’s compositor that can separately handle rendering emissive materials for compositing as an overlay over a primary scene render with objects using standard shading.
Render Layers Node to Separate Emissive Pass
Using a Render Layers Node, the Emission pass can output just the emissive light from a scene render. This separated pass will retain all high frequency detail unlike attempts to view directly in the restrictive viewport.
Alpha Over Node to Overlay Emissive on Base Render
The Image node of the full beauty render excluding the emission pass is mixed with the Emissive pass using an Alpha Over node. This allows the emitting light to composite directly over the primary shaded objects while retaining perfect opacity and intensity of emissives.
With this node setup, restrictions in the viewport no longer limit the accuracy and visibility of intense emissive materials. Instead the values render in full and mix naturally with the rest of the scene during compositing. This workflow provides a precise view compared to the limited viewport preview.
