Translucent materials with prerendered backgrounds

Development Programming & Scripting
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2

I suggest having a look here: Post Process Materials | Unreal Engine 4.27 Documentation. It states that: PostProcessInput1 has the SeparateTranslucency (Alpha is mask).

Here is a reference to the link above.

Blendable Location Description
Before Tonemapping All lighting is provided in HDR with scene color when Scene Texture expression’s Post Process Input 0 is used. It fixes issues with temporal anti-aliasing (TAA) and GBuffer lookups. For example, issues that can happen when using depth and normals.
After Tonemapping This option indicates that post processing will take place after tonemapping and color grading has been completed. It is the preferred location for performance since the color is LDR and requires less precision and bandwidth. When this option is selected, the SceneTexture expression’s Post Process Inputs 2 and 3 are used to control where Scene Color is in the pipeline. Input 2 applies scene color before tonemapping. Input 3 applies scene color after tonemapping.
Before Translucency This is even earlier in the pipeline than ‘Before Tonemapping’ before translucency was combined with the scene color. Note that SeparateTranslucency is composited later than normal translucency.
Replacing the Tonemapper PostProcessInput0 provides the HDR scene color, PostProcessInput1 has the SeparateTranslucency (Alpha is mask), PostprocessInput2 has the low resolution bloom input.

If you ever wondered what PostProcessInput 1, 2, 3, 4… was used for, have a look here:

chongdashu

May '20

Based on the UE source code: https://github.com/EpicGames/UnrealE…Material.h#L20 (Accurate as of 4.24)

/** Named post process material slots. Inputs are aliased and have different semantics
* based on the post process material blend point, which is documented with the input.
*/
enum class EPostProcessMaterialInput : uint32
{
// Always Active. Color from the previous stage of the post process chain.
SceneColor = 0,

// Always Active.
SeparateTranslucency = 1,

// Replace Tonemap Only. Half resolution combined bloom input.
CombinedBloom = 2,

// Buffer Visualization Only.
PreTonemapHDRColor = 2,
PostTonemapHDRColor = 3,

// Active if separate velocity pass is used--i.e. not part of base pass; Not active during Replace Tonemap.
Velocity = 4
};