Diffuse vs Specular Transmission in glTF

In glTF physically based rendering (PBR), light transmission describes how rays pass through a surface rather than being reflected or absorbed. The core distinction between diffuse transmission and specular transmission lies in how directional light rays behave as they traverse the material: specular transmission preserves directional coherence, producing transparent refractive effects like clear glass, whereas diffuse transmission completely scatters incoming light, creating translucent effects like paper or foliage. Understanding the specific roles of the KHR_materials_transmission and KHR_materials_diffuse_transmission extensions allows 3D artists and technical directors to accurately reproduce real-world transmissive phenomena in real-time engines.

Specular Transmission (KHR_materials_transmission)

Specular transmission models optical transparency. When light strikes a surface using standard glTF transmission, the light enters the medium and continues along a path governed by Snell's Law of refraction and the material's Index of Refraction (IOR), typically defined via the KHR_materials_ior extension.

Key characteristics include:

• Directional Preservation: Specular transmission preserves the shapes and details of objects behind the transmissive surface. Viewers can see through the geometry.
• Roughness Dependency: The sharpness of the transmitted view is determined by the material's roughness. A roughness value of zero yields perfectly clear glass or water, while higher roughness values simulate frosted or etched glass via microfacet refraction.
• Volume Integration: Specular transmission frequently pairs with KHR_materials_volume to define light attenuation (Beer-Lambert Law) and internal color tinting over a specified physical thickness.

Diffuse Transmission (KHR_materials_diffuse_transmission)

Diffuse transmission models translucency, specifically designed for thin surfaces where light penetrates the boundary, undergoes intense internal scattering, and exits uniformly from the opposite side.

Key characteristics include:

• Lambertian Scattering: Once light passes through the surface, its original direction is completely randomized. The exiting light follows a Lambertian diffuse distribution on the back side of the geometry.
• No Image Clarity: Objects placed behind a diffusely transmissive material cannot be resolved as distinct images. Only blurred light intensities and shadows pass through.
• Thin-Walled Approximation: Unlike volumetric subsurface scattering (SSS), glTF diffuse transmission is tailored for thin, non-refractive structures where computational expense must remain low.
• Target Materials: It accurately represents materials such as leaves, paper sheets, fabric curtains, lampshades, and thin plastic membranes.

Direct Comparison

Implementation Considerations

When authoring glTF assets, choosing between these two extensions depends entirely on the degree of internal light scattering:

• Use specular transmission when rendering solid clear optics, lenses, glass containers, liquids, or any material where light rays remain largely parallel as they enter the body.
• Use diffuse transmission when light is meant to illuminate the reverse side of a thin object without producing any see-through transparency, simulating back-lit subsurface luminance efficiently across real-time viewers.