When using Substrate, applying a gradient or multi-colored texture to drive the Transmittance Color or Thickness (SSS MFP) of a translucent material via a single Volume Slab results in severe colored banding when rendered with the Path Tracer.
The issue is isolated to the Path Tracer; the Deferred renderer displays the gradient smoothly without banding. (Note: ruling out texture compression artifacts, the issue persists even when using a UV gradient to lerp between two vector colors).
Steps to Reproduce:
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Enable Substrate in Project Settings.
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Create a new Substrate material.
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Set the material up for forward shading and select the
TranslucentColoredTransmittance(orTranslucentGreyTransmittance) profile. -
Add a Substrate Volume Slab.
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Set up the Transmittance logic: Create a Linear Gradient (via TexCoord) and use it to Lerp between two Vector Parameter colors.
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Plug the Lerp result into the SSS MFP input on the Volume Slab (the bug occurs whether using the Substrate Transmittance to Mean Free Path helper function or plugging in directly).
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Plug a constant color into the Diffuse input.
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Apply the material to a basic cube mesh with a 1cm thickness.
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View the mesh in the viewport using Lit mode (Deferred) — observe a smooth gradient transition.
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Switch the viewport to Path Tracer.
Expected Result: The Path Tracer should render a smooth transition between the transmittance colors/thicknesses, matching the behavior of the Deferred renderer.
Actual Result: The Path Tracer displays severe, hard-edged colored banding along the gradient transition.
Additional Context & Workarounds:
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Current Workaround: The only functional workaround is to use a
Substrate Horizontal Blendto mix two separate Volume Slabs, using the gradient as the blend mask. However, this is unscalable for complex glass materials or assets requiring multi-colored transmittance textures. -
Secondary Observation: To achieve the same visual transmittance result between Deferred and Path Tracer, entirely different thickness values are currently required.
