Transmission Vs. Transparency: Understanding Light Path Settings In Blender

Understanding Light Transport in Blender

In Blender, multiple types of light rays are used to simulate real-world lighting and materials. Rays start from the camera, bounce around the scene with reflections and refractions, and some rays ultimately reach light sources to calculate illumination. The path that rays take depends on object properties and render settings.

Materials in Blender can either block light completely (opaque), allow partial pass-through (transmissive), bend light (refractive), reflect light diffusely or specularly, or emit light themselves as light sources. The Blender Render and Cycles rendering engines each process light differently but share some common concepts for how rays interact with surfaces.

Ray Types that Control Light Behavior

In Cycles, various ray types are responsible for specific light interactions:

Table of Contents

Camera Rays: Rays starting from the camera that bounce around the scene.
Shadow Rays: Rays sent from surfaces to light sources to test for shadows.
Glossy Rays: Rays originating from shiny surfaces that cause reflections.
Transmission Rays: Rays passing through transparent surfaces while losing intensity.
Reflection Rays: Rays bouncing off surfaces at precise reflection angles.
Refraction Rays: Rays bending as they pass from one transparent medium to another.

In Blender Internal renderer, ray tracing is less flexible but materials have basic ray trace settings for reflections, refractions, and shadows that determine which types of rays are produced.

Transmission Settings for Surface Light Pass-Through

Transmission in Blender refers to light passing directly through a transparent surface. For transmission to work, a material’s surface must not block rays entirely.

In Cycles, the Transmission parameter controls how much incident light passes through a shader. A value of 0 means no light passes through and 1.0 allows all light through. Glass, gemstones, and other transparent materials would have high transmission values. Additionally, the transmission depth limit caps how many consecutive surface transitions a ray can make, to avoid excessive ray bouncing.

In Blender Internal, the alpha transparency setting is used instead of transmission, along with ray trace settings. Alpha=1 means fully transparent for maximum light pass-through, while alpha=0 signifies fully opaque. Ray tracing must be enabled to get any transmission-style effects.

Refraction Settings for Bending Light Rays

Refraction is distinct from transmission in that it causes light rays to bend at an angle as they pass between mediums, usually based on differences in density. This effect makes partially submerged objects appear bent or offset.

In both Cycles and Blender Internal, the IOR (index of refraction) property adjusts how much refractive bending occurs. For example, air has IOR=1.0 while water is 1.33. The higher the IOR, the more deviation occurs. Fresnel settings also affect the refraction angle depending on surface angle relative to the camera.

To enable refraction, Cycles materials must have non-zero Transmission values. In Blender Internal, refraction ray tracing must be enabled for materials that also have alpha transparency turned on.

Rendering Complex Light Interactions

For photoreal renders, having random rays bounce around a scene can be inefficient. Fortunately, Blender offers advanced light path node editors to precisely control ray behavior for different shader nodes.

By connecting shader nodes to Light Path input nodes in the Node Editor, you can determine how rays interact with materials based on where they’ve been and where they’re headed. For example, limiting glossy reflections only to rays coming directly from the camera.

Optimizing Your Scene’s Light Performance

Complex scenes with many transparent and reflective surfaces can quickly bog down render times. Light paths grow exponentially as rays bounce around more. Fortunately, light paths can be optimized in several key ways.

Strategies include:

Lowering the max transmission bounce limit
Reducing refraction roughness to avoid noisy refractive caustics
Limiting reflections only to direct camera rays
Disabling glossy rays entirely for flat reflections
Baking indirect illumination instead of calculating in real-time

Optimized light paths keep render times quick without sacrificing too much visual quality.

Examples of Light Path Node Setups

Some example Cycles node configurations using Light Path inputs are:

Selective Reflections – Connect the Is Reflection Ray input to the emission strength of a glossy shader to only enable reflections for rays coming directly from other surfaces.
Fresnel Transmission – Mix a transparent and diffuse shader based on the dot product between the surface normal and camera vector using Layer Weight node.
Falloff Transparency – Modulate the alpha channel mixer of a Holdout shader using the Ray Length input to fade object visibility farther from camera.

By mixing and matching light path data with shader nodes, very advanced material response to rays is possible.

Frequently Asked Questions

Is transmission the same as transparency in Blender?

No, transmission and transparency are actually different effects in Cycles and other physically based renderers. Transmission means allowing direct pass-through of light rays without diffusion or coloration. Transparency can include transmission but also relates to alpha channel visibility, refractive ray bending, diffusion, and other light behaviors in transparent materials.

Should my glass material use transmission or transparency?

For photoreal glass, the Cycles Glass Shader node is recommended with a Transmission of 1.0. Some reflection and glossiness should be enabled too for the most realistic glass look. For non-photoreal graphics, transparent shaders work fine.

How do I fix dark spots in refractive/transparent materials?

Dark spots usually occur when not enough light rays bounce around the scene and sample the volume under refracting surfaces. Try increasing refractive caustics or using portals and custom light path settings to allow more bouncing between the surface and environment.