How I built a realistic grass shader in UE5! Shader driven bend, clump, tilt, wind and more!

Summary of Technological Concepts / Features (UE5 Grass Shader)

3-Part Series Context

• Video #2 builds on:
  • Part 1: Blender + UE5/Unreal Engine setup, including:
    • Niagara-driven 2D wind system
    • Dynamic tiling wind textures
    • Simple grass animation and projection tricks
  • Notes that prior “industry-grade” grass techniques were discussed in an earlier related video.

Goal

Create realistic, fluffy animated grass using primarily World Position Offset (WPO), with extensive control via:

• shader math
• texture-driven masks
• per-blade/per-card randomization

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Grass Mesh + Data Setup (Blender)

Grass Mesh Uses Flat Undeformed Cards

• Cards are intentionally flat
• All cards are oriented toward the positive X-axis
• Uses UV map #1 for grass textures

Baked Per-Card Data (via Blender Add-on)

A “data baker” Blender addon is used to bake into UVs:

• Pivot/baked pivot data into UVs
• Normalized height baked into UV (vertical axis)
• Random value per card baked into UV (horizontal axis)

Vertex-Stored Random Direction

This workflow relies heavily on storing random per-card directions in:

• vertex normals
• vertex colors

So each grass card can be oriented differently without expensive per-pixel logic.

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World Position Offset Reconstruction: Location, Scale, Rotation

Reconstruct World Position From Scratch

Uses a common WPO trick:

• subtract WorldPosition from 0 (collapsing vertices)
• then re-add:
  • baked pivot offsets
  • instance transforms (scale/rotation/position)

Distance-Based Instance Fading

• Uses PerInstanceFadeAmount for smooth card presence across distance.

Per-Instance Scale Control

• Assumes uniform instance scaling
• Extracts scale (e.g., from instance scale Z).

Rotation via 3×3 Matrix

Random rotation is performed by:

• building a local coordinate system per card
• applying a 3×3 rotation matrix

Landscape Alignment

• Card “up” / orientation can align to the landscape normal at each instance position.
• Requires cross products to build a stable basis.
• Includes handling for non-orthogonal vectors (orthogonalizing basis vectors to avoid deformation).

Tilt Clumps

• Tilt/clump shifting is implemented by modifying the “Z vector” basis using clump/tilt textures.

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Clumping System (Custom “Clump Map” Tool)

Purpose

Generate a texture that drives clumps by pulling each card’s “Z vector” toward a local direction/point.

Tool: UE Editor Utility Widget

• Uses a render target + dynamic brush material
• Paints procedural clump textures
• Outputs a tileable static texture saved to the UE asset directory

Key Shader Logic in the Tool

• Brush uses radial direction from brush center in UV space
• Adds noise for organic flow
• Uses a mask (subtitled as a “billionaire sphere mask,” likely a sphere-mask concept) to fade effects near the center and avoid bad intersections
• Brush is drawn many times and made tileable via repeated/offset drawing

Shader Application

• Avoids unstable per-vertex sampling from absolute world space by sampling the clump map using:
  • baked pivots in object space
• Clump texture coordinates are randomized using each card’s stored random 2D direction.
• Additional controls include:
  • Lerp-driven position offset (clump influence over blade bending)
  • A shared “Z vector” framework used for wind sampling and bending

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Wind Interaction (Directional Modulation + Bending)

Wind Sampled from 2D Wind Texture

• Samples the 2D wind texture to build a wind velocity vector

Wind Direction Matching

• Normalized wind velocity is dotted with the card’s facing/forward direction.
• Grass cards act like flat “sails,” reacting more when aligned with wind direction.

Curvature Along Blade

• Uses stored top gradient/UV data
• Offsets are faded along blade length to create curvature rather than uniform bending

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Rest Pose Bending (Mathematical Curve)

Mathematical “Bend Modifier”

• Applies a bend using sign + cosine-style math (similar to a bend modifier shown earlier)
• Mesh export is optimized because cards start aligned to X-axis, making bending predictable.

Wind Affects Rest Pose

• Wind magnitude reduces/modifies the bend amount so grass appears:
  • more curved at rest
  • straighter under stronger wind

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Camera-Facing + Density Illusion (Billboard-like Without Classic Billboards)

Problem Tackled

• With WPO cards you may get vertex cost without enough pixel coverage
• Classic camera-facing billboards maximize coverage but can look overly “billboard-y”

Solution

• Build a matrix to rotate cards toward the camera before bending.
• Discusses transform space options:
  • View-space (shadow pass mismatch from light POV)
  • Camera-space (shadow stability, different behavior)
• Final fix addresses order-of-operations complexity:
  • bending expects X-axis-aligned meshes
  • camera-facing rotation must be composed carefully
  • uses an adjustment: un-rotate the view vector via the inverse of card rotation so camera-facing remains correct after random rotation

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Normal Reconstruction (Most Complex Part)

Why Normals Are Hard

• Multiple WPO operations distort geometry
• Transforming existing normals becomes messy.

Approach: Rebuild World-Space Normals From Scratch

• Reconstruct normals using:
  • bend direction from WPO
  • planar normal logic derived from the rebuilt framework
• Mentions “schools”:
  • Stylized: often use consistent upward normals
  • Realistic: may use two-sided foliage shading and correct normals

Testing Setup

• Uses a small debug mesh (four cards)
• Helps identify issues like wrong-way rotation for normals.

Wind/Tilt Influence on Normals

• Workaround:
  • lerp the world normal toward tilt direction
  • ensures lighting responds consistently regardless of forward/up normal selection.

Normal Map Remapping

• Since normal maps are usually tangent-space, remaps/swizzles them into the reconstructed world-space basis built from:
  • bend orientation
  • tilt direction
  • rebuilt coordinate frame (via cross products)

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Opacity, LODs, and “Fluffy” Edges

LOD and Dithering

• Uses dither LOD fading for smoother transitions.

Distance-Based Opacity via Mask

• Uses a custom desert mask opacity approach to achieve fluffy edges.

Mip-map Issues

• Packed textures may become too “mushy” at distance, causing unintended transparency.

Two Mitigation Methods

1. Adjust mip generation / texture settings (e.g., ensuring values remain consistent across the mip chain).
2. Manual mip editing:
   • build custom mip chain layers in Photoshop
   • export to DDS
   • import while preserving mip levels.

Shadow-Pass Tricks (Advanced)

• Uses Shadow Pass Switch to alter behavior in the shadow pass:
  • change card opacity to adjust perceived density and shadow coverage
• Notes potential tradeoffs:
  • LOD transitions
  • shadow stability

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Pixel Depth Offset & Depth Illusion

Pixel Depth Offset

• Uses a per-card random value to push pixels away in camera space
• Creates a depth illusion and reduces flat-card artifacts
• Caveat:
  • can cause self-shadowing artifacts with dynamic lighting
• It’s mitigated by fading via mip handling.

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Performance Notes / Technology Choices

Vertex Cost Awareness

Highlights that performance depends on:

• keeping vertex count low
• aggressive LODs
• minimizing quad overdraw

Not Using Nanite or Virtual Shadow Maps

• WPO compatibility concerns with:
  • Nanite
  • Virtual Shadow Maps
• The goal is WPO-first grass on more “solid” classic meshes plus traditional shadow maps.

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Project Availability + Tools Mentioned

• Project files available as Patreon tier 3.
• Tools available as Patreon tier 1:
  • Landscape layer capture tool
  • Clump map tool
• Mentions a third video soon:
  • workflow for creating foliage assets in Blender
  • an addon to facilitate it

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Main Speakers / Sources

Main Speaker

• The YouTube creator (unnamed in subtitles) leading the UE5 grass shader build and demonstrating the tools.

Software/Engine Sources Referenced

• Epic (indirect references to grass workflows; also mentions Lumen / two-sided foliage shading considerations)
• Ghost of Tsushima (inspiration for grass rendering approach)
• Unreal Engine / UE5 material & shader nodes
  • WPO
  • PerInstanceFadeAmount
  • ShadowPassSwitch
  • etc.
• Niagara (from Part 1: wind system + tiling wind textures)