Gradient in UE5 Shaders

2026-01-13 10:37 UTC gpt-5-2 Open in ChatGPT ↗

You

Is there a way to find the GRADIENT of a black and whgite texture in UE5 using shaders?

ChatGPT

Yes — you can get a “gradient” (i.e., direction + strength of change) from a black/white texture in UE5 materials, but it depends what you mean by gradient:

1) Most common: edge/transition direction (∇texture) in UV space

You can approximate the texture’s gradient by sampling it a few times around the current UV and taking finite differences.

Material graph idea (no custom HLSL required)

1. Get your UVs (e.g., TextureCoordinate).
2. Compute a tiny UV offset roughly equal to one texel in UV units:
   • Use the node Texture Size (or GetDimensions equivalent) to get width/height.
   • du = 1 / width, dv = 1 / height
3. Sample your grayscale texture at:
   • T0 = Sample(UV)
   • Tx = Sample(UV + float2(du, 0))
   • Ty = Sample(UV + float2(0, dv))
4. Differences:
   • dx = Tx - T0
   • dy = Ty - T0
5. The gradient vector (in UV space) is:
   • grad = float2(dx, dy)
6. Useful outputs:
   • Magnitude: length(grad) (edge strength)
   • Direction: normalize(grad) (direction of increasing brightness)

Notes

• For cleaner results, use central differences (more samples, better quality):
  • dx = Sample(UV+du) - Sample(UV-du)
  • dy = Sample(UV+dv) - Sample(UV-dv)
  • Then multiply by 0.5.

This is basically what Sobel-like edge detection starts from (Sobel just uses a 3×3 weighted kernel).

2) If you literally mean “distance-to-edge style gradient”

If your texture is a mask (0/1-ish) and you want a smooth ramp away from edges, you’re talking about a distance field (SDF). That’s not something you can cheaply compute per-pixel in a regular material without lots of taps/iterations.

Better options:

• Author/bake an SDF texture offline (or generate a signed distance field in DCC tools).
• Use UE’s distance field workflows if applicable (mesh distance fields are for geometry, not arbitrary 2D textures).

3) If you mean “screen-space gradient”

UE materials also have derivatives (DDX, DDY) which can give you the rate of change across the screen. That’s fast, but it’s screen-space, not UV-space, and changes with distance/scale/LOD. Great for effects like foam, outlines, stylized shading; less great if you need stable texture-space direction.

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Practical tips

• Mipmaps matter: sampling at different mips changes gradients. If you need stable results, consider controlling mip bias (or accept the smoothing).
• Gamma: make sure the mask/texture is imported as linear if you’re doing math on it (sRGB off for data textures).
• Texel size: using 1/TextureSize offsets is key; a fixed tiny number will break when the texture resolution changes.

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If you tell me which of these you want:

1. edge direction/strength,
2. distance-from-edge ramp, or
3. screen-space change,

…I can give you an exact node-by-node setup (and whether to use forward/central differences or Sobel).

You

is there a way to lerp directions? called Slerp

ChatGPT

Yes 👍 — spherical linear interpolation (SLERP) is a thing for directions, but there are a few important caveats in UE5 materials.

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Short answer

• There is no built-in “Slerp” node in UE5 materials.
• For most shader use-cases, a normalized LERP (NLERP) is preferred and visually indistinguishable.
• True SLERP can be implemented in a Custom HLSL node if you really need it.

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1️⃣ The recommended approach: NLERP (cheap & stable)

For interpolating direction vectors (normals, gradients, flow directions):

Result = normalize( lerp(A, B, Alpha) )

Why this is usually enough

• Much cheaper than SLERP
• No trig functions
• Visually identical unless angles are very large (>120°)
• Used widely in real-time rendering

Material graph

1. Lerp (A, B, Alpha)
2. Normalize

That’s it.

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2️⃣ True SLERP (Custom node, HLSL)

If you must preserve constant angular velocity (e.g. precise rotations or extreme angles), you can implement SLERP manually.

SLERP formula

$ \text{slerp}(a,b,t) = \frac{\sin((1-t)\theta)}{\sin\theta} a + \frac{\sin(t\theta)}{\sin\theta} b $

Where: $ \theta = \arccos(\text{dot}(a,b)) $

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Custom HLSL node code

Inputs

• A (float3)
• B (float3)
• Alpha (float)

Output

• float3

float3 a = normalize(A);
float3 b = normalize(B);

float dotAB = saturate(dot(a, b));
float theta = acos(dotAB);

if (theta < 1e-4)
{
    return a;
}

float sinTheta = sin(theta);
float w1 = sin((1 - Alpha) * theta) / sinTheta;
float w2 = sin(Alpha * theta) / sinTheta;

return w1 * a + w2 * b;

⚠️ Cost warning

• acos + sin are expensive
• Avoid per-pixel usage unless absolutely necessary

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3️⃣ Hybrid approach (best practice)

Use NLERP most of the time, but fall back to SLERP only when angles are large.

Example idea:

• Compute dot(A,B)
• If dot > 0.95 → NLERP
• Else → SLERP

(This is common in animation and shading systems.)

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4️⃣ When you should NOT use SLERP

• Normal blending
• Gradient blending
• Flow maps
• Any per-pixel effect that runs on large surfaces

In those cases:

normalize(lerp())

is the correct solution.

---

Summary

Method	Use case	Cost
NLERP	Almost all shader direction blends	⭐ Cheap
SLERP	Exact angular interpolation	❌ Expensive
Lerp only	Vectors that don’t need normalization	⭐ Cheapest

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If you want, I can:

• Build a pure-node NLERP graph
• Optimize SLERP for gradient blending
• Show how to combine this with your texture-derived gradient

Just tell me what you’re blending and where (vertex vs pixel).

You

Ok. great.

I have implemented a stamping method using double-buffering. I’m addiing each new stamp with lerp, so there’s a trail left behind which is nice.

I now want to add a 2D Wave Equation so that there’s rippling as well

ChatGPT

[Unsupported Content]

ChatGPT

[Unsupported Content]

ChatGPT

Nice — you’re already 90% of the way there. A real-time 2D wave sim on a texture is a perfect fit for ping-pong / double buffering.

Key point

A 2D wave equation update needs two time states: h(t) and h(t-Δt) to compute h(t+Δt).
If you only have one buffer right now (just accumulating stamps), you can still do it with double-buffering by packing two states into channels.

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The standard discrete wave update (what you want)

Let:

• h = height field (your texture values)
• prev = previous height field
• laplacian(h) = neighbor curvature

Then:

$ next = (2\cdot h - prev) + c^2 \cdot laplacian(h) - damping\cdot(h - prev) + injection $

A super common form in shaders:

next = (2 - damp)*h - (1 - damp)*prev + waveK * laplacian(h) + injection

Where:

• damp is small (0.01–0.1-ish depending on scale)
• waveK controls speed / wavelength (acts like c²Δt²/Δx²)

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How to do it with ping-pong + 2 channels

Use your render targets like this:

• RT A stores: R = h(t), G = h(t-Δt)
• RT B stores the same (next frame’s result)

Each update pass:

1. Read h and prev from the source RT
2. Compute next
3. Write out:
   • R = next
   • G = h (shift current into previous)

That way you only need your existing ping-pong pair.

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Laplacian (4-tap)

In UV space:

hC = sample(UV).r
hL = sample(UV - float2(du,0)).r
hR = sample(UV + float2(du,0)).r
hD = sample(UV - float2(0,dv)).r
hU = sample(UV + float2(0,dv)).r

lap = (hL + hR + hD + hU - 4*hC)

Where du = 1/width, dv = 1/height (texel size).

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Injection: turn your stamp into a “kick”

Instead of (or in addition to) lerping stamps into the buffer, add energy:

• simplest: injection = stamp * strength
• better “impulse” feel: add to velocity (difference between h and prev)

Two good options:

A) Add to height directly

next += stamp * strength;

B) Add to velocity (often looks more like a ripple)

Velocity is roughly h - prev, so:

next += stamp * strength;        // adds displacement
prev -= stamp * strength * velK; // optional: makes it punchier

If you want “trail + ripples”, keep your existing trail stamp as a slow blend, but ALSO add a smaller impulse injection.

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Stability (so it doesn’t explode)

This matters. With the 4-neighbor laplacian, keep waveK modest.

Rule of thumb:

• start with waveK = 0.2
• damp = 0.02–0.08
• if it “blows up” (goes NaN/white), reduce waveK or increase damping.

(Under the hood this is the Courant condition; the “safe” region is fairly small in explicit solvers.)

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UE5 Material implementation sketch

In the update material (the one you draw into the next RT):

1. Texture Sample (source RT)
   • h = R
   • prev = G
2. 4 more samples for neighbors (R channel)
3. compute lap
4. compute next
5. output RGBA like:
   • R = next
   • G = h
   • B = 0 (or store extra data)
   • A = 1

Then your visual material reads R as height.

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Rendering ripples as normals (optional but common)

From height, compute a normal:

dx = hR - hL
dy = hU - hD
N = normalize(float3(-dx * normalStrength, -dy * normalStrength, 1))

Use that for refraction, lighting, foam, etc.

---

If you tell me one detail—are you doing the update pass via a “Draw Material to Render Target” / RDG / Niagara RT, and what resolution—I’ll give you an exact node-by-node setup (and the best default parameter ranges for that res).

You

Draw Material to Render Target, 512x512 textures. I’m also calculating outwards “force” via point SDF

ChatGPT

Got it. With Draw Material to Render Target at 512×512, you can do a very solid 2D ripple sim, and your point SDF “outward force” can plug in nicely as the injection term (or as a directional advection term if you want the ripples to “push” outward).

Here’s a concrete setup that works well in UE5 materials.

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1) Pack wave state into RG

Use your ping-pong RTs like:

• R = h(t) (current height)
• G = h(t-Δt) (previous height)

Each update pass writes:

• R = next
• G = h (shift)

This lets you keep only the two RTs you already have.

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2) The update material (the one you draw into the RT)

Parameters (start values)

• WaveK = 0.18
• Damping = 0.04
• StampStrength = whatever you like (try 0.5)
• TexelSize = (1/512, 1/512) = (0.001953125, 0.001953125)

Samples

In the update material:

1. Sample source RT at UV → C (float4)

   • h = C.r
   • prev = C.g

2. Sample neighbors (R channel):

• hL = Sample(UV - float2(du,0)).r
• hR = Sample(UV + float2(du,0)).r
• hD = Sample(UV - float2(0,dv)).r
• hU = Sample(UV + float2(0,dv)).r

3. Laplacian:

• lap = (hL + hR + hD + hU - 4*h)

Wave step (stable, common form)

Compute:

• next = (2 - Damping)*h - (1 - Damping)*prev + WaveK * lap

That’s your ripple propagation.

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3) Inject your stamp / SDF “force”

You mentioned you’re already computing an outward force via a point SDF. In practice you’ll want a scalar injection (adds displacement/energy) and optionally use the direction for advection later.

A) Easiest: SDF-based impulse into height

Let your stamp mask be S in 0..1 (white where you stamp). Then:

• inj = S * StampStrength

Add it:

• next += inj

This yields nice circular ripples.

B) Better “punch”: inject into velocity

Velocity is roughly h - prev. A quick trick:

• vel = h - prev
• vel += S * StampStrength
• next = h + vel + WaveK*lap - Damping*vel

If you want it in the earlier form without re-deriving, the cheap approximation is:

• next += S * StampStrength
• prev -= S * StampStrength * 0.5 (makes the kick sharper)

This often looks more like a splash.

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4) Using your outward force direction

If your SDF gives you a gradient direction dir = normalize(∇SDF) pointing outward, you have two useful options:

Option 1: directional injection (anisotropic splash)

Make the impulse stronger in the outward direction by biasing the height injection with the direction field you’re already computing (still scalar in the end):

• inj = S * StampStrength
• next += inj

…and separately compute a normal from height for visuals (below). This alone often “reads” as outward motion if the source is radial.

Option 2 (cooler): advection (push the height field outward)

This is optional and more “force-like”:

• UV_adv = UV - dir * AdvectStrength * S
• sample h_adv = Sample(UV_adv).r instead of h (or blend with it)

So you do:

• h_used = lerp(h, h_adv, S * AdvectAmount)

Then use h_used in the wave update.

This makes ripples get “pushed” outward by your force field, which sounds like what you’re after.

Start tiny:

• AdvectStrength = 0.5 * du (so about 0.000976)
• AdvectAmount = 0.25

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5) Keep your trail without killing the sim

Your current “lerp to accumulate” trail will act like heavy damping if you apply it to the wave state.

Best pattern:

• Maintain trail separately (another RT or another channel), OR
• Keep trail as a very small bias added to h (not overwriting the state).

If you want to keep everything in one RT:

• Put trail in B channel and don’t involve it in the wave step.
  • B = trail(t)
  • Update: trailNext = lerp(trail, stamp, TrailBlend) or trailNext = max(trail * fade, stamp)
  • Output: RGB = (next, h, trailNext)

Then in your display material you can combine:

• visualHeight = next + trailScale * B

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6) Rendering the ripples (normal from height)

In your display material (not the update pass), compute normals from height:

• dx = hR - hL
• dy = hU - hD
• N = normalize(float3(-dx * NormalStrength, -dy * NormalStrength, 1))

Use that for:

• refraction offset
• specular highlights
• foam mask = saturate(length(float2(dx,dy))*FoamK)

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7) Practical UE5 “Draw Material to RT” tips

• Use Clamp addressing on the RT sampler to avoid edge wrapping weirdness.
• Disable sRGB on the RT (it’s data).
• Use a float format RT if you can (R16f/RG16f ideal). If you’re using RGBA8, it still works but you’ll get quantization and faster damping.

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Minimal node recipe summary

Update pass output:

• R = next
• G = h
• B = trailNext (optional)
• A = 1

Where:

• next = (2 - Damping)*h - (1 - Damping)*prev + WaveK*lap + inj

---

If you tell me how you currently compute the SDF “outward force” in the material (do you already have dir as a float2/float3?), I’ll map it directly into either the impulse or the advection variant with exact node operations.

You

I also need to make a 2d diffusion pass in another material. always in UE5. provide the hlsl code to put into the shade

ChatGPT

Here’s a solid 2D diffusion pass you can drop into a UE5 Material “Custom” node (for your Draw Material to Render Target diffusion material). It’s a standard explicit diffusion (heat equation) step using a 4-neighbor Laplacian.

What it does

$ u_{t+1} = u_t + D \cdot \nabla^2 u_t $

• u can be your mask/ink/trail/etc.
• D controls diffusion amount per step
• optional Damping fades it over time

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Custom Node HLSL (single-channel diffusion)

Custom node inputs you should create:

• Tex (Texture2D) — your source RT/texture
• TexSampler (SamplerState) — the sampler for Tex
• UV (float2)
• TexelSize (float2) — set to (1/512, 1/512) for 512², or pass in 1/TextureSize
• Diffusion (float) — start around 0.10 to 0.25
• Damping (float) — start around 0.00 to 0.02 (optional)

Output type: float4 (so you can write RGBA to your RT)

// Read center + 4-neighbors (R channel)
float c  = Tex.Sample(TexSampler, UV).r;

float l  = Tex.Sample(TexSampler, UV + float2(-TexelSize.x, 0)).r;
float r  = Tex.Sample(TexSampler, UV + float2( TexelSize.x, 0)).r;
float d  = Tex.Sample(TexSampler, UV + float2(0, -TexelSize.y)).r;
float u  = Tex.Sample(TexSampler, UV + float2(0,  TexelSize.y)).r;

// 4-neighbor Laplacian
float lap = (l + r + d + u - 4.0 * c);

// Explicit diffusion step
float next = c + Diffusion * lap;

// Optional fade (helps prevent endless residual)
next *= (1.0 - Damping);

// Output to RT (pack into R, keep others if you want)
return float4(next, 0, 0, 1);

Good starting values (512×512, one update per frame)

• Diffusion = 0.15
• Damping = 0.005

If it “smears” too fast, lower Diffusion. If it barely spreads, raise it.

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If you want to diffuse RG (two-channel)

Useful if you’re diffusing a vector field (like direction/force):

float2 c  = Tex.Sample(TexSampler, UV).rg;

float2 l  = Tex.Sample(TexSampler, UV + float2(-TexelSize.x, 0)).rg;
float2 r  = Tex.Sample(TexSampler, UV + float2( TexelSize.x, 0)).rg;
float2 d  = Tex.Sample(TexSampler, UV + float2(0, -TexelSize.y)).rg;
float2 u  = Tex.Sample(TexSampler, UV + float2(0,  TexelSize.y)).rg;

float2 lap = (l + r + d + u - 4.0 * c);

float2 next = c + Diffusion * lap;
next *= (1.0 - Damping);

return float4(next, 0, 1);

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Important stability note

Explicit diffusion has a stability limit. With a 4-neighbor stencil, a safe-ish range is:

• Diffusion roughly <= 0.25 per step (often less, depending on scaling)

So if you see ringing or weird stepping, keep it ≤ 0.2.

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UE5 setup reminders (so this behaves correctly)

• Your RT should be linear (sRGB off).
• Prefer R16f / RG16f RT formats if possible.
• Sampler address: Clamp unless you want wrap diffusion.

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If you tell me what you’re diffusing (your trail mask? the outward force field? height?) and what channel layout you’re using (R only vs RGBA), I can tailor the code so it preserves the other channels and only diffuses the one you want.

You

ok, provide copy/paste code for a custom node in material bluperint

ChatGPT

Here’s copy/paste-ready code for a UE5 Material Custom node diffusion pass.

Custom Node settings

• Output Type: CMOT Float 4
• Inputs (exact names + types):
  • Tex (Texture2D)
  • TexSampler (SamplerState)
  • UV (float2)
  • TexelSize (float2) ← for 512×512 use (0.001953125, 0.001953125)
  • Diffusion (float) ← start 0.15
  • Damping (float) ← start 0.005

---

Diffuse R channel (single channel)

float c  = Tex.Sample(TexSampler, UV).r;

float l  = Tex.Sample(TexSampler, UV + float2(-TexelSize.x, 0.0)).r;
float r  = Tex.Sample(TexSampler, UV + float2( TexelSize.x, 0.0)).r;
float d  = Tex.Sample(TexSampler, UV + float2(0.0, -TexelSize.y)).r;
float u  = Tex.Sample(TexSampler, UV + float2(0.0,  TexelSize.y)).r;

float lap = (l + r + d + u - 4.0 * c);

float next = c + Diffusion * lap;
next *= (1.0 - Damping);

return float4(next, 0.0, 0.0, 1.0);

---

If you need to preserve existing GBA from the texture

(only diffuse R, pass-through GBA)

float4 center = Tex.Sample(TexSampler, UV);

float c = center.r;

float l  = Tex.Sample(TexSampler, UV + float2(-TexelSize.x, 0.0)).r;
float r  = Tex.Sample(TexSampler, UV + float2( TexelSize.x, 0.0)).r;
float d  = Tex.Sample(TexSampler, UV + float2(0.0, -TexelSize.y)).r;
float u  = Tex.Sample(TexSampler, UV + float2(0.0,  TexelSize.y)).r;

float lap = (l + r + d + u - 4.0 * c);

float next = c + Diffusion * lap;
next *= (1.0 - Damping);

return float4(next, center.gba);

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Quick hookup notes (Material → Draw Material to Render Target)

• Plug Texture Object Parameter (your source RT) into Tex
• Use a SamplerState node (or just wire the texture sample’s sampler) into TexSampler
• UV = TextureCoordinate (or whatever UVs you’re simulating in)
• TexelSize = constant2vector (1/512, 1/512)
• Output of the Custom node → Emissive Color (for RT drawing)

If you tell me your exact channel packing (what’s in R/G/B/A right now), I’ll give you the version that diffuses only the channel you want and keeps the rest intact.

You

I can’t find a SamplerState object in unreal

ChatGPT

That’s expected 👍 — UE materials do NOT expose a SamplerState node directly like raw HLSL does. This trips up a lot of people.

In UE5 Material Custom nodes, the sampler is automatically generated for any Texture2D input.

So the fix is simple: remove the SamplerState input entirely and use UE’s implicit sampler.

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✅ Correct way in UE5 (copy/paste)

Custom Node settings

• Output Type: CMOT Float 4
• Inputs:
  • Tex (Texture2D)
  • UV (float2)
  • TexelSize (float2)
  • Diffusion (float)
  • Damping (float)

❌ Do NOT add a SamplerState input

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✅ Diffusion code (UE-compatible)

float4 center = Tex.Sample(TexSampler, UV);
float c = center.r;

float l = Tex.Sample(TexSampler, UV + float2(-TexelSize.x, 0.0)).r;
float r = Tex.Sample(TexSampler, UV + float2( TexelSize.x, 0.0)).r;
float d = Tex.Sample(TexSampler, UV + float2(0.0, -TexelSize.y)).r;
float u = Tex.Sample(TexSampler, UV + float2(0.0,  TexelSize.y)).r;

float lap = (l + r + d + u - 4.0 * c);

float next = c + Diffusion * lap;
next *= (1.0 - Damping);

return float4(next, center.gba);

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🔑 Why this works

• When you declare a Texture2D input named Tex, UE auto-injects a sampler called:

TexSampler

• You must use that exact name: TexSampler
• You never create it manually in the graph

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Important UE5 gotchas (very relevant for sims)

1. Texture Object Parameter, NOT Texture Sample
   • Your Tex input must come from a Texture Object / Texture Object Parameter
2. Sampler Source
   • Set the Texture Object’s sampler source to From Texture Asset
3. RT settings
   • sRGB OFF
   • Prefer R16f / RG16f
4. UVs
   • Use TextureCoordinate, not ScreenPosition

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Sanity check

If this compiles, you’re wired correctly:

return Tex.Sample(TexSampler, UV);

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Extra (edge behavior)

If you want clamped edges (recommended for diffusion):

• Set the Texture Object → Address X/Y = Clamp
  (or set it on the Render Target asset)

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If you want, next I can:

• Combine diffusion + wave in one pass
• Add directional diffusion (along your SDF gradient)
• Optimize taps (5 → 3 samples)

Just tell me 👍

You

i get random errors about raytracing. what the fuck

[SM6] Shader debug info dumped to: “C:\Users\GeorgeAdamon\Documents\Unreal Projects\Position_Stamping\Saved\ShaderDebugInfo\PCD3D_SM6\Diffusion_937e5f197bf01141\Default\FLocalVertexFactory\TMaterialCHSFNoLightMapPolicy\0” C:\Program Files\Epic Games\UE_5.5\Engine\Shaders\Private\RayTracing\RayTracingMaterialHitShaders.usf(0): Shader TMaterialCHSFNoLightMapPolicy, Permutation 0, VF FLocalVertexFactory: error: validation errors [SM6] C:\Program Files\Epic Games\UE_5.5\Engine\Binaries\Win64\RayTracingMaterialHitShaders.dxil(3993,11): Shader TMaterialCHSFNoLightMapPolicy, Permutation 0, VF FLocalVertexFactory: RayTracingMaterialHitShaders.dxil:3993:11: error: Opcode Sample not valid in shader model lib_6_6(closesthit). [SM6] C:\Program Files\Epic Games\UE_5.5\Engine\Binaries\Win64\RayTracingMaterialHitShaders.dxil(3992,11): Shader TMaterialCHSFNoLightMapPolicy, Permutation 0, VF FLocalVertexFactory: RayTracingMaterialHitShaders.dxil:3992:11: error: Opcode Sample not valid in shader model lib_6_6(closesthit). [SM6] C:\Program Files\Epic Games\UE_5.5\Engine\Binaries\Win64\RayTracingMaterialHitShaders.dxil(3991,11): Shader TMaterialCHSFNoLightMapPolicy, Permutation 0, VF FLocalVertexFactory: RayTracingMaterialHitShaders.dxil:3991:11: error: Opcode Sample not valid in shader model lib_6_6(closesthit). [SM6] C:\Program Files\Epic Games\UE_5.5\Engine\Binaries\Win64\RayTracingMaterialHitShaders.dxil(3990,11): Shader TMaterialCHSFNoLightMapPolicy, Permutation 0, VF FLocalVertexFactory: RayTracingMaterialHitShaders.dxil:3990:11: error: Opcode Sample not valid in shader model lib_6_6(closesthit).

ChatGPT

[Unsupported Content]

ChatGPT

[Unsupported Content]

ChatGPT

That error is Unreal compiling your material for DXR ray tracing hit shaders (closesthit). In that shader stage, regular texture sampling (.Sample) isn’t allowed because it relies on screen-space derivatives (ddx/ddy) for mip selection — and ray tracing hit shaders don’t have those.

So nothing is “random” here: your material is being compiled for a ray tracing permutation, and your Custom node uses .Sample, which triggers:

Opcode Sample not valid in shader model lib_6_6(closesthit)

Fix (the correct one): use SampleLevel (explicit LOD)

In your Custom node, replace every Tex.Sample(TexSampler, ...) with Tex.SampleLevel(TexSampler, ..., 0).

Copy/paste diffusion (UE5 Custom node) — works in RT permutations too

// Center
float4 center = Tex.SampleLevel(TexSampler, UV, 0);
float c = center.r;

// Neighbors (explicit LOD 0)
float l = Tex.SampleLevel(TexSampler, UV + float2(-TexelSize.x, 0.0), 0).r;
float r = Tex.SampleLevel(TexSampler, UV + float2( TexelSize.x, 0.0), 0).r;
float d = Tex.SampleLevel(TexSampler, UV + float2(0.0, -TexelSize.y), 0).r;
float u = Tex.SampleLevel(TexSampler, UV + float2(0.0,  TexelSize.y), 0).r;

// 4-neighbor Laplacian
float lap = (l + r + d + u - 4.0 * c);

// Diffusion step
float next = c + Diffusion * lap;
next *= (1.0 - Damping);

// Diffuse only R, preserve GBA
return float4(next, center.gba);

That should stop the ray tracing compiler from freaking out.

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Two other ways to avoid this entirely (optional)

Option A: Make this material not compile for ray tracing

If this material is only for Draw Material to Render Target, you can keep it out of the ray tracing pipeline by:

• Using Material Domain = User Interface (often works great for RT drawing materials), or
• Ensuring this material is not used on any mesh that might be ray traced.

(If it’s assigned to a mesh anywhere, UE will try to compile RT hit shaders for it when ray tracing is enabled.)

Option B: Disable ray tracing in the project (if you don’t need it)

Project Settings → Rendering → Ray Tracing (off).
But I’d still recommend the SampleLevel fix since it’s robust.

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Quick sanity check

If you’re also doing the wave equation update in a Custom node, you’ll need the same change there too (all samples → SampleLevel).

If you paste your wave update Custom code, I’ll convert it to ray-tracing-safe sampling in one go.

You

ok, I now want to put this in a loop, to apply it many times during a frame. But I cant implement double buffering with UE5 BeginDraw / End Draw, because Begin Draw is outside the loop, and doesnt get informed about the buffer swaps

ChatGPT

Yep — you’ve hit a real engine constraint: BeginDrawCanvasToRenderTarget / EndDrawCanvasToRenderTarget (the Canvas path) is not designed for iterative ping-pong inside a single frame. It “locks” the target for the duration of the draw, so swapping RTs mid-loop doesn’t integrate cleanly, and even if you hack it, you’ll often end up sampling stale data because the GPU work hasn’t completed between iterations.

Here are the practical UE5 ways to do N simulation steps per frame with ping-pong.

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Option 1 (Blueprint-friendly, correct): multiple Draw Material to Render Target calls in a loop

This is the one that maps cleanly to ping-pong in a single frame.

Why it works

Each call is a separate render pass. You can swap RT references in Blueprint between calls.

Blueprint pattern

Variables:

• RT_A, RT_B
• bUseAasSource (or just swap references)
• StepsPerFrame (int)

Loop i = 0..StepsPerFrame-1:

1. Set your update material parameter SourceTex = RT_A
2. DrawMaterialToRenderTarget(Target = RT_B, Material = UpdateMat)
3. Swap: (RT_A, RT_B) = (RT_B, RT_A)

Critical: your update material must read from a Texture Object Parameter (e.g. SourceTex) and you set that to the current source RT before each draw.

✅ This avoids Canvas Begin/End entirely.

Reality check: this can be expensive if StepsPerFrame is big, but it’s the simplest and most reliable.

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Option 2 (Fast + scalable): do the loop on the GPU in one pass using a compute shader / RDG

If you want 10–100+ iterations per frame, Blueprint looping draw calls becomes heavy. The “right” solution is a compute shader that runs N iterations in one dispatch (or a few dispatches), using UAV ping-pong buffers.

Best UE5 paths

• C++ Global Shader + RDG (Render Dependency Graph) (clean, modern, fastest)
• Plugin compute framework (if you already have one)

This gives you:

• true per-frame substepping
• proper barriers between iterations
• no “draw call” overhead per step

If you’re staying Blueprint-only, skip this for now.

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Option 3 (Hacky but sometimes OK): “fake multiple steps” with a larger timestep

Not a real loop: you crank WaveK / Diffusion as if Δt is larger.

• For diffusion: increasing Diffusion behaves like multiple steps… until stability breaks
• For wave: larger Δt tends to explode or look wrong fast

This is only good for small multipliers.

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Why Canvas Begin/End fails for ping-pong loops

Canvas is a batching API. The render target binding is established at BeginDraw, and the draw list is submitted at EndDraw. Your mid-loop “swap” happens on the CPU, but the GPU doesn’t see separate passes with barriers—so your “next” read can still read the old contents, plus the RT binding isn’t reconfigured the way you need.

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What you should do (given your setup)

Use Option 1: a Blueprint loop around DrawMaterialToRenderTarget.

A couple of gotchas (important)

1. Make sure the material samples from a parameter:
   • TextureObjectParameter named SourceTex
   • In Custom HLSL: sample SourceTex (and SourceTexSampler)
2. Don’t read and write the same RT in the same pass
3. Use SampleLevel(..., 0) in Custom nodes (as you already found)

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Bonus: reduce iteration count with better numerics

If you’re doing diffusion and wave together, you can often get away with:

• StepsPerFrame = 2..4
• and tune coefficients for stability/feel

Also, diffusion can be accelerated with a larger-radius kernel (still stable) so you need fewer steps.

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Blueprint pseudo-logic (exact)

• On Tick:
  • LocalSrc = RT_A
  • LocalDst = RT_B
  • ForLoop (0 → StepsPerFrame-1):
    • SetTextureParameterValue(UpdateMID, "SourceTex", LocalSrc)
    • DrawMaterialToRenderTarget(LocalDst, UpdateMID)
    • Swap LocalSrc/LocalDst
  • After loop, store back which RT is “current”

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If you tell me whether you’re doing this in:

• Actor Blueprint Tick
• Material Parameter Collection
• Niagara / BP function library

…I’ll describe the exact nodes to use and the cleanest way to implement the swap without spaghetti.

You

how to apply a 2d velocity map as force to niagara particles

ChatGPT

[Unsupported Content]

ChatGPT

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ChatGPT

You can treat a 2D velocity map as a flow field and sample it per particle each tick, then add it as Acceleration (force) or directly add to Velocity.

The clean Niagara way is: GPU emitter + Texture Data Interface.

1) Prepare the velocity texture

• Store your flow in RG (X,Y), ideally in linear space (sRGB OFF).
• If it’s coming from a Render Target, use RT format like RG16f / RGBA16f if possible.
• Decide what RG means:
  • 0..1 encoded (common): 0.5 = zero, <0.5 negative, >0.5 positive
  • or signed float (if using float RT): RG already in -1..1 or whatever scale you wrote

Decode examples

• If encoded 0..1: vel = (RG * 2 - 1)
• If already signed: vel = RG

2) Make a Niagara System (GPU)

Texture sampling is best/expected on GPU sim.

• In the emitter: set Sim Target = GPUCompute Sim

3) Expose the texture to Niagara

In Niagara System/User Params:

• Add a User Parameter of type Texture2D (e.g. User.FlowTex)
• Assign your velocity map texture or render target to it in the component/system.

4) Sample the texture using particle position → UV

You need to convert particle world position into UVs that match the texture.

Typical mapping:

• Define a rectangle in world space: MinXY, MaxXY
• Convert:
  • uv = (Particle.Position.xy - MinXY) / (MaxXY - MinXY)
  • clamp/saturate to 0..1

You can store MinXY/MaxXY as User.Vector2D params.

5) Apply as force (Acceleration) or velocity

Create a Scratch Pad module in Particle Update (or use existing modules if you have them):

Particle Update steps

1. Build UV from position
2. Sample velocity map at UV
3. Decode & scale
4. Apply:
   • Force style: Particles.Acceleration += float3(flow.x, flow.y, 0) * Strength
   • Velocity style: Particles.Velocity += float3(flow.x, flow.y, 0) * Strength * DeltaTime

Force style is usually nicer/physically consistent.

Scratch-pad pseudo logic

• Inputs: User.FlowTex, User.MinXY, User.MaxXY, Strength
• uv = saturate((Position.xy - MinXY) / (MaxXY - MinXY))
• sample = SampleTexture(User.FlowTex, uv) (returns RGBA)
• flow = sample.rg * 2 - 1 (if encoded)
• Acceleration += float3(flow, 0) * Strength

6) Common gotchas

• Flip Y if it’s backwards: uv.y = 1 - uv.y
• If particles “stick” at borders: clamp UVs and/or fade strength near edges.
• If the flow feels too weak/strong, it’s usually a units mismatch:
  • If your map is “units per frame”, convert to per-second: multiply by 1/DeltaTime or switch to velocity style.
• If you need sub-stepping, do it in Niagara (Solve Forces and Velocity handles that well).

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Alternative: use a Vector Field (if you don’t need RT updates)

Niagara supports Vector Fields, but they’re 3D volumes. For a purely 2D map, sampling a texture is usually simpler (and works with dynamic render targets).

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If you tell me how your velocity map is encoded (0..1 with 0.5 center, or signed float), and what plane you want it on (XY? XZ?), I’ll write the exact Scratch Pad module graph (inputs/outputs) so you can rebuild it quickly.

You

my velocity map has negative as well as positive values 16F

ChatGPT

Perfect — if it’s RG16F (or RGBA16F) and you’re writing real signed values, you can skip any 0..1 decoding. You’ll just sample RG and treat it as a signed 2D vector.

Below is a clean Niagara setup that works well for a 2D flow field force.

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1) Niagara requirements

• Use a GPU emitter (recommended for texture sampling performance):

  • Emitter → Sim Target = GPUCompute Sim

• Expose your RT/texture as a user param:

  • Niagara System → Parameters → User → add User.FlowTex (Texture2D)

Assign your Render Target to User.FlowTex on the Niagara Component.

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2) Map particle world position to UV

Add user params to define the world-space rectangle covered by the texture:

• User.FlowMin (Vector2D) // bottom-left in world XY
• User.FlowMax (Vector2D) // top-right in world XY
• User.FlowStrength (float)

Then compute UV per particle: $ uv = saturate((pos.xy - FlowMin)/(FlowMax-FlowMin)) $

If your sim is on XZ instead of XY, use pos.xz.

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3) Apply as a force (Acceleration) in Particle Update

Create a Scratch Pad Module in Particle Update (call it “Flow Map Force”).

Module logic (node-by-node)

1. Read Particles.Position

2. Choose plane:

   • pos2 = float2(Position.x, Position.y) (XY plane)

3. Compute UV:

   • uv = (pos2 - User.FlowMin) / (User.FlowMax - User.FlowMin)
   • uv = Saturate(uv)
   • (optional) flip Y if needed: uv.y = 1 - uv.y

4. Sample texture

   • Use Niagara’s Sample Texture 2D node
   • Texture = User.FlowTex
   • UV = uv
   • Result = float4 sample

5. Extract flow:

   • flow = sample.rg (already signed because 16F)

6. Force/accel:

   • Particles.Acceleration += float3(flow.x, flow.y, 0) * User.FlowStrength

That’s the simplest “force field”.

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4) If you want it to behave like a velocity field (often nicer)

Instead of adding acceleration, push velocity toward the field:

Velocity injection (frame-rate stable):

• Particles.Velocity += float3(flow.x, flow.y, 0) * User.FlowStrength * DeltaTime

or steer toward field (smoother):

• targetVel = float3(flow.x, flow.y, 0) * User.FlowStrength
• Particles.Velocity = lerp(Particles.Velocity, targetVel, 1 - exp(-FollowRate * DeltaTime))

Where FollowRate is like 2–20.

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5) Important practical notes (so it doesn’t look “wrong”)

Units / scaling

Your flow values from the RT are just numbers. Decide what they mean:

• If your RT stores units per second, you can do:
  • Velocity += flow * Strength * DeltaTime (Strength dimensionless)
• If your RT stores units per frame, then do:
  • Velocity += flow * Strength (no DeltaTime)

Most people store “per second-ish” values, so multiplying by DeltaTime is the safe default.

Filtering & mipmaps

For a Render Target flow map:

• You usually want no mips and linear filtering (smooth flow).
• If you see shimmering / weird sampling, check the RT texture settings and sampler filtering.

Edge behavior

Particles at the edge will clamp to the border and can “stick”. Common fix: fade force near edges:

• edge = smoothstep(0, EdgeWidth, uv) * smoothstep(0, EdgeWidth, 1-uv)
• multiply strength by edge.x * edge.y

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6) Quick checklist (most common failure points)

• ✅ Emitter is GPUCompute
• ✅ User.FlowTex is assigned at runtime
• ✅ UV mapping rectangle matches your world area
• ✅ You’re sampling the correct plane (XY vs XZ)
• ✅ Your RT is linear (sRGB off), float format

---

If you tell me:

• are particles in XY or XZ, and
• does your velocity map use world units (meters) or some normalized scale,

…I’ll give you exact recommended default values for FlowStrength / FollowRate, and an edge-fade expression that fits your 512×512 domain.

You

i dont even know how to expose the texture parameter

ChatGPT

No worries — this part is not obvious in Niagara. I’ll walk it step-by-step, exactly how to expose and use a Texture2D parameter in Niagara.

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Step-by-step: expose a Texture2D to Niagara

1️⃣ Create the User Texture parameter

1. Open your Niagara System
2. In the Parameters panel (left side):
   • Expand User
   • Click ➕
   • Choose Texture → Texture2D
3. Name it something like:

   User.FlowTex

That’s it — you’ve now exposed a texture slot on the system.

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2️⃣ Assign the texture (or Render Target)

1. Select the Niagara System Component in your level
2. In the Details panel → User Parameters
3. You’ll see:

   FlowTex : None

4. Assign:
   • your Render Target (velocity map), or
   • a regular Texture2D

✅ Niagara can sample Render Targets directly.

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3️⃣ Use the texture inside the emitter (Scratch Pad)

Now you need to sample it.

Create the module

1. Open the Emitter
2. Go to Particle Update
3. Click ➕ Add Module
4. Choose New Scratch Pad Module
5. Name it e.g.:

   Flow Map Force

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4️⃣ Sample the texture (this is the non-obvious node)

Inside the Scratch Pad graph:

Add node:

• Sample Texture 2D

This node:

• Works only on GPU emitters
• Accepts Texture2D parameters
• Outputs RGBA

Wire it:

• Texture → User.FlowTex
• UV → your computed UVs (next step)

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5️⃣ Build UVs from particle position

Add nodes:

1. Particles.Position
2. Break Vector
3. Make Vector2D (X,Y or X,Z)

Then:

UV = (Pos2D - User.FlowMin) / (User.FlowMax - User.FlowMin)
UV = Saturate(UV)

(Optional: UV.Y = 1 - UV.Y if flipped)

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6️⃣ Apply as force

From Sample Texture 2D:

• Take R,G
• Make float3(R, G, 0)
• Multiply by User.FlowStrength
• Add to:
  • Particles.Acceleration (force)
  • OR Particles.Velocity (direct push)

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7️⃣ Common mistakes (VERY common)

❌ Trying to use textures in CPU emitters
→ Won’t work

❌ Looking for “Texture Sample” instead of Sample Texture 2D
→ Niagara uses a different node

❌ Expecting the texture param to appear in the emitter
→ User params are set on the System / Component

❌ Texture is sRGB
→ Flow math breaks

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8️⃣ Minimal sanity test (do this first)

To confirm it’s wired correctly:

In the Scratch Pad:

• Sample User.FlowTex
• Set:

Particles.Color = float4(sample.r, sample.g, 0, 1)

If particles change color based on the texture → everything is hooked up correctly.

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Mental model (important)

• Materials: texture parameters live in the material
• Niagara: texture parameters live on the system (User params)
• The Scratch Pad is just reading them

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If you want, next I can:

• Give you a screenshot-level node layout
• Provide a copy-paste Niagara Scratch Pad graph (node list)
• Show how to sync your flow RT bounds with Niagara automatically

Just tell me 👍