Lit Translucency

Document Summary: An overview of how to use statically lit translucency in the Unreal Engine.

Document Changelog: Created by Dave Burke^?.

Overview

Unreal Engine 3 includes support for statically-lit translucency. Meshes with translucent materials that use lit lighting models will receive the effects of static level lighting. Dynamic lights cannot affect translucent materials. Furthermore, the level lighting does not take into account surface translucency when computing static shadows.

Creating a lit translucent material

The important take-away points of using lit translucency are:

1. Author the lighting on your material with the BLEND_Opaque blend mode, then switch to a translucent blend mode and preview the material in the level.
2. Level lighting must be built for lit translucency to be visible

To create a lit translucent material in the material editor, begin as you would when creating a lit opaque material (BLEND_Opaque, MLM_Phong). In the example below, we have a green specular surface that's 50% transparent and uses the normal map of the default material:

Applied to a static mesh placed in a level, the material appears as follows:

To make the material lit translucent, simply change the blend mode on the material from BLEND_Opaque to one of the translucent blend modes (BLEND_Translucent, BLEND_Additive or BLEND_Modulate). In the example below, we've chosen BLEND_Translucent:

Notice in the above image how the material preview on the left has turned black. This is because the material preview scene contains no static lighting -- it's light by a single dynamic point light.

Recompile the material by clicking on the green 'Apply Changes' checkmark button. The preview mesh in the material editor will not change, but you'll see the material on the level-placed static mesh update.

Assuming level lighting is built, as is the case in the above image, the translucent material will be correctly lit. If level lighting is not up-to-date, the mesh will appear black in the level, as it did in the material editor preview window.

Changing the blend mode to BLEND_Add and recompiling the material, the level-placed mesh updates again:

With BLEND_Modulate: