Unreal 5.8 - Materials Masterclass - Chapter 6: Intro to Transparency

Topic: Unreal Engine 5.8 Materials Masterclass — Intro to Transparency (Chapter 6)

Core idea of transparency in materials

A transparent material is defined as a surface that allows some or all light to pass through.

Different types of transparency (for example, paper-thin, solid glass, smoke, or chain-link) require different math and rendering behavior so the material matches how light interacts in the real world.

---

Two main controls in the Material Details panel

1) Shading Model (how light interacts with the surface)

Common shading models mentioned include:

• Default Lit: Standard physically-based reflection for materials like metals, wood, and plastic.
• Unlit: Ignores game lighting and outputs only the original color (useful for neon / TV screens that emit light).
• Subsurface / Subsurface Profile: Light scattering inside (e.g., wax, jade, realistic human skin).
• Pre-integrated skin: Cheaper skin approximation for less detailed characters (often mobile/NPCs).
• Two-sided foliage: Tuned for thin geometry like leaves, grass, and paper.
• Clear coat: Adds a smooth transparent top layer (e.g., resin, varnish, paint).
• Hair: Handles anisotropic and strand-based lighting.
• Eye: Supports refraction through the cornea/iris.
• Cloth: Simulates micro-fiber pile (e.g., velvet, denim, silk).
• Thin translucent (newer): Physically accurate tinted glass, including correct color shadows and background refraction (e.g., stained glass).

2) Blending Mode (how the material’s pixel blends with the background)

Options covered:

• Opaque: Fully solid; no light passes through.
• Mask: Binary transparency (100% visible or invisible). Cheap to render.
• Translucent: Partial transparency; more expensive due to blending and additional processing.
• Additive: Adds material color to the background → always brightens (e.g., fire, sparks, lasers).
• Modulate: Multiplies with the background (often for dark decals/shadows).
• Alpha Composite: Uses textures with pre-multiplied alpha (specialized FX/UI workflows).
• Alpha Holdout: “Punches a hole” in the scene (e.g., AR portals and technical compositing effects).

---

Practical guide/examples: choosing Mask vs Translucent

Masked transparency (performance-focused workflow)

Key advantage

Masked mode emphasizes very cheap rendering compared to true translucency.

Why it’s cheaper

Masked materials avoid costly depth sorting and pixel blending required by translucent rendering.

Editor feature: Transparency Mask Clip

Enabling masked mode activates the Transparency Mask Clip on the material node.

• Example behavior shown:
  • Default clip is around ~0.33
  • Values below the threshold become invisible
  • Values above become fully visible

Real texture example (cutouts)

• A brick wall base color + a black/white polka-dot texture into Opacity Mask yields:
  • White = visible
  • Black = hidden

Mask manipulation nodes

• 1 minus node (hotkey O): flips what’s visible/invisible.
• Panner node (hotkey P): animates the mask by scrolling UVs.
• Double-sided can allow viewing through masked holes.

Shadow behavior (important)

Masked materials:

• Project correct shadows, including shadows cast by masked geometry (useful for leaf shadows on canopies).

---

“Fake translucent” technique using Mask + dithering (pro trick)

To simulate soft transparency without true translucent rendering:

• Use masked materials (fast), but create an illusion of a gradient.

Workflow

1. Use Radial Gradient Exponential to create a near-binary circular threshold.
2. Use Dither Temporal AA to turn a smooth alpha into a fine pattern, then let anti-aliasing smooth it visually.

Key inputs:

• Alpha Threshold: the soft gradient source (e.g., radial gradient)
• Random: controls whether the noise repeats or varies (default often enabled)

Result

• Appears like a semi-transparent gradient
• Achieves masked-material performance cost

Limitation

• Can be noisy
• Sometimes you still need real translucent materials for quality.

---

True translucent materials (quality-focused workflow)

Switching to Translucent mode

When Blending Mode becomes Translucent:

• The opacity mask inputs disappear
• Standard Opacity becomes available
• Opacity controls how much light passes through with a smooth transition

---

Translucency lighting modes and constraints

Unreal changes which inputs are available because translucent rendering is handled differently.

Modes described

• Volumetric Non-Directional (default/cheapest): Renders a full volume without directional light interaction (common for dust/smoke).

• Volumetric Directional: Adds a Normal input (direction matters), but still lacks full metallic/roughness options.

• Surface Translucency Volume and Surface Forward Shading: More expensive, but most accurate. More connectors (like metallic/roughness) become available.

Note: The tutorial emphasizes that even if Forward Lit was selected earlier, translucent materials still require these dedicated translucency settings.

---

Sorting issues with layered transparency

A demonstrated artifact occurs when using multiple transparent layers on a mesh (reflections can merge/cut off).

Attempted mitigations:

• Disable bidirectionality and related options

However, Unreal can still struggle sorting multiple transparent layers—especially when layers originate from the same mesh/topology.

---

Reflection dependency on opacity (without ray tracing)

In a “glass ball” example:

• Setting opacity to 0 makes the object disappear because “fake reflections” depend on opacity.
• Raising opacity to around ~0.25 restores reflections while still appearing transparent.
• Adding correct refraction makes the material look more “real.”

---

Physically-based refraction (key concept + tutorial steps)

Enabling refraction in Unreal

Refraction is disabled by default for performance reasons.

Enable it from the material details/refraction section by using:

• Refractive Index (IOR) (physically accurate approach)

IOR values provided

• Air: 1.0
• Water: ~1.33
• Glass: ~1.5–1.52

---

Animated “portal/warp” refraction demo

• Use a Lerp node (hotkey L) between air IOR and glass IOR
• Drive the blend alpha with a changing value over time (shown via time-like debug behavior)
• Result: smooth transition between no refraction and strong refraction, approximating warp/portal motion

---

Adding detail with normals to refract properly

A normal map is connected to the material’s Normal output to create refractive distortion.

Observed effects:

• Refraction > 1 with a water normal texture → wavy distortion (like old glass/raindrops)
• Refraction around ~1.33 with a larger normal scale → finer detail, resembling frosted glass
• Optionally animate via panning UVs to create water-like movement while preserving refraction behavior

---

Building production-ready translucent materials (two complete examples)

Material 1: Thermal shimmer / hot air distortion over sand or stove

Base setup

• Translucent, Standard Lit
• Surface Direct shading
• Refraction via Refractive Index

Mask construction

• Uses texture coordinate gradients + math nodes (component masks, multiply/add/subtract, abs, invert)
• Builds a mask that:
  • is strongest at the bottom
  • fades outward toward edges/top

Normal setup

• Uses water normal 01 with panner-driven motion
• Combined via a normal flattening approach (referencing earlier episodes)

Result

• Vertical scrolling distortion
• Fades smoothly with no sharp refraction edge.

---

Material 2: Raindrops on glass (edge dirt + drop normal detail)

Base configuration

Reuses the translucent base and refraction configuration.

Textures used

• Dirt/grunge texture from the Fab Marketplace (not distributable)
• An edge-weighted noise mask (noise only around edges), provided via video description links
• A raindrop normal map created in Substance Designer (random small/medium dots + vertical stripes)

Construction details

• Base color: dirt texture × dark green constant → tints the glass
• Opacity:
  • dirt/grunge mask red channel × used to enhance edges
  • processed with Saturate
  • thresholded using Max(…, 0.2) to keep a minimum level
• Normal:
  • tile raindrop normal texture (4x each axis)
  • connected to the Normal output

Limitation hinted

Simple panning moves stripe patterns undesirably. A later episode promises encoding multiple behaviors into one texture for more randomized drop distribution.

---

Future topics foreshadowed

Next lessons will cover:

• Reusable material functions across projects
• Parameters to generate many material variants without duplicating full node graphs

---

Main speakers / sources

• Primary speaker/instructor: the YouTube creator (name not specified in subtitles)
• Engine/source: Epic Games / Unreal Engine 5.8 (material system behavior and settings described)