Suppress lightning ghosting in the cloud temporal smoother

Flashes embedded into the 0.92 history EMA and outlived themselves
3-10x, so a camera move dragged a reprojected imprint of the lit deck
across the sky. A CPU-tracked fade (live flash + 0.25 s tail) now
collapses the history weight to ~0.18 while a flash could still be in
history, so it tracks near-live and never embeds. Inert when no
lightning is active. Details in docs/fork-touchpoints.md.

Author: Kim2091

docs/fork-touchpoints.md

@@ -2408,3 +2408,16 @@ Look-check driven ("thunderstorm looks like a normal day — the sky is blue, th
   *Stale "neutral in every preset" gate comment updated in `writeBlendedToDerivedLayer`.*
 
 ---
+
+## Workstream — Lightning ghost suppression (fork — 2026-07-14)
+
+A lightning flash embedded into the cloud temporal smoother's ~1 s EMA (fixed 0.92 history weight in `evalSkyRadiance`) outlived itself 3–10x; on camera move the reprojected history dragged a sharp-edged "old frame" imprint of the flash-lit deck across the sky. The strike scheduler now tracks a ghost-suppression signal — 1 while a flash is live, decaying over ~0.25 s after — and the temporal blend collapses its history weight by `x(1 - 0.8 * fade)` (0.92 → ~0.18 at full fade, ~2-frame convergence), so the flash never embeds. The jitter the EMA normally hides is masked by the flash itself; inert at fade 0. Validated in-game.
+
+- **`src/dxvk/shaders/rtx/pass/atmosphere/atmosphere_args.h`** — fork-owned change.
+  *Reclaims the `pad_cloudSunsetWarmth` slot as `lightningHistoryFade`; CB layout unchanged.*
+- **`src/dxvk/rtx_render/rtx_atmosphere.h` / `rtx_atmosphere.cpp`** — fork-owned change.
+  *`m_lightningHistoryFade` tracked at the end of `advanceLightning` (max of envelope and a tau-0.25 s decay); published by `getAtmosphereArgs`; zeroed in `normalizeForSkyLutCache`.*
+- **`src/dxvk/shaders/rtx/pass/atmosphere/atmosphere_sky.slangh`** — fork-owned change.
+  *Temporal-smoothing block scales `kHistoryWeight` by the fade before the history lerp.*
+
+---

src/dxvk/rtx_render/rtx_atmosphere.cpp

@@ -368,6 +368,7 @@ namespace {
     args.lightningStrikePosKm        = vec3(0.0f, 0.0f, 0.0f);
     args.lightningFlashIntensity     = 0.0f;
     args.lightningEnvelope           = 0.0f;
+    args.lightningHistoryFade        = 0.0f;
   }
 
   // Quantize one direction-vector component to the granularity step.
@@ -681,6 +682,7 @@ AtmosphereArgs RtxAtmosphere::getAtmosphereArgs() const {
     args.lightningEnvelope       = m_lightningEnvelope;
     args.lightningFlashIntensity = m_lightningEnvelope * std::max(RtxOptions::lightningFlashIntensity(), 0.0f);
     args.lightningColor          = RtxOptions::lightningColor();
+    args.lightningHistoryFade    = m_lightningHistoryFade;
   }
 
   // Cloud volumetric / appearance enhancements
@@ -1698,6 +1700,7 @@ void RtxAtmosphere::requestLightningStrike() {
 void RtxAtmosphere::advanceLightning(float dt) {
   if (!RtxOptions::lightningEnable()) {
     m_lightningEnvelope = 0.0f;
+    m_lightningHistoryFade = 0.0f;
     m_lightningPulsesLeft = 0;
     s_lightningStrikeRequested.store(false);  // don't bank a Test Strike while disabled
     return;
@@ -1766,6 +1769,21 @@ void RtxAtmosphere::advanceLightning(float dt) {
     m_lightningPulsesLeft = static_cast<int>(rand01() * 3.0f);  // 0-2 restrikes
     m_lightningTimeToPulse = 0.04f + 0.11f * rand01();
   }
+
+  // Ghost-suppression window (fork — 2026-07-14). Tracks the envelope but
+  // decays ~3.5x slower, so it covers the whole flicker PLUS the frames right
+  // after, while any flash residue could still be sitting in the cloud
+  // temporal history. evalSkyRadiance collapses its EMA history weight by
+  // this factor — without it a 100-300 ms flash embeds into the ~1 s history
+  // and a camera move drags a reprojected "old frame" imprint of the lit
+  // deck across the sky. Computed AFTER the fire block so a fresh strike
+  // raises it the same frame its envelope peaks.
+  constexpr float kHistoryFadeTau = 0.25f;
+  m_lightningHistoryFade = std::max(std::min(m_lightningEnvelope, 1.0f),
+                                    m_lightningHistoryFade * std::exp(-dt / kHistoryFadeTau));
+  if (m_lightningHistoryFade < 1e-3f) {
+    m_lightningHistoryFade = 0.0f;
+  }
 }
 
 void RtxAtmosphere::dispatchCloudRender(Rc<DxvkContext> ctx) {

src/dxvk/rtx_render/rtx_atmosphere.h

@@ -443,6 +443,7 @@ class RtxAtmosphere : public CommonDeviceObject {
   // the envelope / restrike / inter-arrival model.
   Vector3  m_lightningStrikePosKm       { 0.0f, 0.0f, 0.0f };
   float    m_lightningEnvelope          { 0.0f };   // raw flicker envelope [0..~1.2]
+  float    m_lightningHistoryFade       { 0.0f };   // ghost-suppression window (outlasts the envelope)
   int      m_lightningPulsesLeft        { 0 };      // restrike pulses of the active flash
   float    m_lightningTimeToPulse       { 0.0f };   // seconds to the next restrike pulse
   uint32_t m_lightningRngState          { 0x9E3779B9u };  // xorshift32 state

src/dxvk/shaders/rtx/pass/atmosphere/atmosphere_args.h

@@ -255,7 +255,13 @@ struct AtmosphereArgs {
 
   float cloudThickness;        // Cloud-slab vertical depth, km
   float cloudLayer2TypeSpread; // [0,1] cloud-type variation for layer 2 (independent of layer 1)
-  float pad_cloudSunsetWarmth; // was cloudSunsetWarmth (removed 2026-06-21 — no shader consumer)
+  float lightningHistoryFade;  // [0..1] lightning ghost-suppression signal (fork — 2026-07-14):
+                               // 1 while a flash is live, decaying over ~0.25 s after it ends.
+                               // evalSkyRadiance collapses the cloud temporal-history weight by
+                               // this factor so the flash never embeds into the ~1 s EMA (the
+                               // reprojected "old frame" ghost on camera move). Reuses the former
+                               // pad_cloudSunsetWarmth slot; CB layout unchanged. Zeroed in
+                               // normalizeForSkyLutCache (per-frame animated, never feeds a bake).
   uint cloudViewSamples;       // Ray-march steps through cloud slab
 
   // ----- Spatial variation fields (Nubis-style weather) -----

src/dxvk/shaders/rtx/pass/atmosphere/atmosphere_sky.slangh

@@ -1022,7 +1022,18 @@ vec3 evalSkyRadiance(
         // a stable signal in ~30 frames at 60Hz (~0.5s) while still strongly
         // suppressing the per-frame jitter variance DLSS sees.
         const float kHistoryWeight = 0.92f;
-        vec4 blended = lerp(currentPremul, history, kHistoryWeight);
+        // Lightning ghost suppression (fork — 2026-07-14). A 100-300 ms flash
+        // embedded into this ~1 s EMA outlives itself 3-10x, and on camera
+        // move the reprojected history drags a bright "old frame" imprint of
+        // the lit deck across the sky (sharp-edged where reprojection leaves
+        // the old viewport). While a flash is live — plus a ~0.25 s trailing
+        // window (CPU-side lightningHistoryFade) — collapse the history
+        // weight so the flash tracks near-live and never embeds. The jitter
+        // noise the EMA normally hides is masked by the flash itself; weight
+        // returns to 0.92 as the fade expires. Inert at fade 0.
+        float historyWeight = kHistoryWeight
+                            * (1.0f - 0.8f * saturate(args.lightningHistoryFade));
+        vec4 blended = lerp(currentPremul, history, historyWeight);
         smoothedCloud = vec4(blended.rgb, blended.a);
       } else {
         smoothedCloud = currentPremul;