From 53af39b68e2dbc2f89096bbd987f1d0b8a12ace5 Mon Sep 17 00:00:00 2001
From: Andrey Kleshchev <117672381+akleshchev@users.noreply.github.com>
Date: Wed, 4 Mar 2026 01:56:46 +0200
Subject: #1625 Blinding white screen when changing environments

---
 indra/llinventory/llsettingssky.cpp | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

(limited to 'indra/llinventory/llsettingssky.cpp')

diff --git a/indra/llinventory/llsettingssky.cpp b/indra/llinventory/llsettingssky.cpp
index 4957cf3c02..6f7510fef2 100644
--- a/indra/llinventory/llsettingssky.cpp
+++ b/indra/llinventory/llsettingssky.cpp
@@ -653,14 +653,39 @@ void LLSettingsSky::blend(LLSettingsBase::ptr_t &end, F64 blendf)
             }
         }
 
-        mHasLegacyHaze |= lerp_legacy_float(mHazeHorizon, mLegacyHazeHorizon, other->mHazeHorizon, other->mLegacyHazeHorizon, 0.19f, (F32)blendf);
         mHasLegacyHaze |= lerp_legacy_float(mHazeDensity, mLegacyHazeDensity, other->mHazeDensity, other->mLegacyHazeDensity, 0.7f, (F32)blendf);
         mHasLegacyHaze |= lerp_legacy_float(mDistanceMultiplier, mLegacyDistanceMultiplier, other->mDistanceMultiplier, other->mLegacyDistanceMultiplier, 0.8f, (F32)blendf);
-        mHasLegacyHaze |= lerp_legacy_float(mDensityMultiplier, mLegacyDensityMultiplier, other->mDensityMultiplier, other->mLegacyDensityMultiplier, 0.0001f, (F32)blendf);
         mHasLegacyHaze |= lerp_legacy_color(mAmbientColor, mLegacyAmbientColor, other->mAmbientColor, other->mLegacyAmbientColor, LLColor3(0.25f, 0.25f, 0.25f), (F32)blendf);
-        mHasLegacyHaze |= lerp_legacy_color(mBlueHorizon, mLegacyBlueHorizon, other->mBlueHorizon, other->mLegacyBlueHorizon, LLColor3(0.4954f, 0.4954f, 0.6399f), (F32)blendf);
         mHasLegacyHaze |= lerp_legacy_color(mBlueDensity, mLegacyBlueDensity, other->mBlueDensity, other->mLegacyBlueDensity, LLColor3(0.2447f, 0.4487f, 0.7599f), (F32)blendf);
 
+        if (mLegacyHazeHorizon == mLegacyDensityMultiplier == mLegacyBlueHorizon)
+        {
+            // mHazeHorizon coupled with mDensityMultiplier, mDistanceMultiplier and
+            // drastic blue horizon changes can result in a very bright sky during
+            // the transition. Purpose of this code is to calculate a 'fake level'
+            // and use it to even out brightness change.
+            //
+            // Example values that make lerp-based individual transition painfully bright:
+            // Start: 3 Haze Horiz, 0.1 Density, 6.54 Distance, white ambient, white blue horizon
+            // End: 0.03 Haze Horiz, 0.775 Density, 90.95 Distance, black ambient, black blue horizon
+            F32 strt_level = mHazeHorizon * mDensityMultiplier * mBlueHorizon.length();
+            F32 end_level = other->mHazeHorizon * other->mDensityMultiplier * other->mBlueHorizon.length();
+            mHasLegacyHaze |= lerp_legacy_float(mHazeHorizon, mLegacyHazeHorizon, other->mHazeHorizon, other->mLegacyHazeHorizon, 0.19f, (F32)blendf);
+            mHasLegacyHaze |= lerp_legacy_float(mDensityMultiplier, mLegacyDensityMultiplier, other->mDensityMultiplier, other->mLegacyDensityMultiplier, 0.0001f, (F32)blendf);
+            mHasLegacyHaze |= lerp_legacy_color(mBlueHorizon, mLegacyBlueHorizon, other->mBlueHorizon, other->mLegacyBlueHorizon, LLColor3(0.4954f, 0.4954f, 0.6399f), (F32)blendf);
+
+            // lerp the fake level instead of density multiplier to avoid brightening the sky too much.
+            // This makes density multiplier non linear.
+            F32 new_level = lerp(strt_level, end_level, (F32)blendf);
+            mDensityMultiplier = new_level / (mHazeHorizon * mBlueHorizon.length());
+        }
+        else
+        {
+            // default values are used, so we should lerp settings independently
+            mHasLegacyHaze |= lerp_legacy_float(mHazeHorizon, mLegacyHazeHorizon, other->mHazeHorizon, other->mLegacyHazeHorizon, 0.19f, (F32)blendf);
+            mHasLegacyHaze |= lerp_legacy_float(mDensityMultiplier, mLegacyDensityMultiplier, other->mDensityMultiplier, other->mLegacyDensityMultiplier, 0.0001f, (F32)blendf);
+            mHasLegacyHaze |= lerp_legacy_color(mBlueHorizon, mLegacyBlueHorizon, other->mBlueHorizon, other->mLegacyBlueHorizon, LLColor3(0.4954f, 0.4954f, 0.6399f), (F32)blendf);
+        }
         parammapping_t defaults = other->getParameterMap();
         stringset_t skip = getSkipInterpolateKeys();
         stringset_t slerps = getSlerpKeys();
-- 
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