From b2457b1865cc5db987259efd6374892bff2fdb1a Mon Sep 17 00:00:00 2001
From: Nils Schimmelmann <nschimme@users.noreply.github.com>
Date: Tue, 20 Oct 2026 09:48:40 -0700
Subject: [PATCH] optimize atmosphere fragment shader for Intel GPUs (#260)

- Replace dynamic textureSize() lookups with WEATHER_INV_NOISE_SIZE constant multiplier
- Fully unroll the 4-octave fbm() function
---
 src/display/Textures.cpp                      |  4 +--
 src/opengl/Weather.h                          |  2 ++
 src/opengl/legacy/ShaderUtils.cpp             |  7 +++++-
 .../legacy/weather/atmosphere/frag.glsl       | 25 +++++++++++--------
 4 files changed, 24 insertions(+), 14 deletions(-)

diff --git a/src/display/Textures.cpp b/src/display/Textures.cpp
index 11e6473cc..4362e1420 100644
--- a/src/display/Textures.cpp
+++ b/src/display/Textures.cpp
@@ -480,8 +480,8 @@ void MapCanvas::initTextures()
         textures.white_pixel = MMTexture::alloc(std::vector<QImage>{whitePixel}, true);
     }
     {
-        // weather noise is 256
-        QImage noiseImage = createTileableValueNoiseImage(256);
+        // weather noise texture
+        QImage noiseImage = createTileableValueNoiseImage(WeatherConstants::WEATHER_NOISE_SIZE);
         textures.noise = MMTexture::alloc(std::vector<QImage>{noiseImage}, false);
     }
 
diff --git a/src/opengl/Weather.h b/src/opengl/Weather.h
index cf3757f11..44a631b8b 100644
--- a/src/opengl/Weather.h
+++ b/src/opengl/Weather.h
@@ -40,6 +40,8 @@ constexpr float WEATHER_EXTENT = 28.0f;
 constexpr float WEATHER_MASK_RADIUS_OUTER = 12.0f;
 constexpr float WEATHER_MASK_RADIUS_INNER = 8.0f;
 constexpr float TRANSITION_DURATION = 2.0f;
+constexpr int WEATHER_NOISE_SIZE = 256;
+constexpr float WEATHER_INV_NOISE_SIZE = 1.0f / static_cast<float>(WEATHER_NOISE_SIZE);
 } // namespace WeatherConstants
 
 /**
diff --git a/src/opengl/legacy/ShaderUtils.cpp b/src/opengl/legacy/ShaderUtils.cpp
index 0fe8ad8dd..4edf8cf03 100644
--- a/src/opengl/legacy/ShaderUtils.cpp
+++ b/src/opengl/legacy/ShaderUtils.cpp
@@ -239,13 +239,18 @@ NODISCARD static GLuint compileShader(Functions &gl, const GLenum type, const So
                                          + text_utils::formatFloatClassic(
                                              WeatherConstants::WEATHER_MASK_RADIUS_INNER)
                                          + "\n";
+    std::string defineWeatherInvNoiseSize = "#define WEATHER_INV_NOISE_SIZE "
+                                            + text_utils::formatFloatClassic(
+                                                WeatherConstants::WEATHER_INV_NOISE_SIZE)
+                                            + "\n";
 
-    std::array<const char *, 8> ptrs = {gl.getShaderVersion(),
+    std::array<const char *, 9> ptrs = {gl.getShaderVersion(),
                                         defineNamedColors.c_str(),
                                         defineWeatherRadius.c_str(),
                                         defineWeatherExtent.c_str(),
                                         defineWeatherMaskOuter.c_str(),
                                         defineWeatherMaskInner.c_str(),
+                                        defineWeatherInvNoiseSize.c_str(),
                                         "#line 1\n",
                                         source.source.c_str()};
     gl.glShaderSource(shaderId, static_cast<GLsizei>(ptrs.size()), ptrs.data(), nullptr);
diff --git a/src/resources/shaders/legacy/weather/atmosphere/frag.glsl b/src/resources/shaders/legacy/weather/atmosphere/frag.glsl
index c895941c0..92488cc5d 100644
--- a/src/resources/shaders/legacy/weather/atmosphere/frag.glsl
+++ b/src/resources/shaders/legacy/weather/atmosphere/frag.glsl
@@ -11,7 +11,7 @@ layout(std140) uniform NamedColorsBlock
 layout(std140) uniform CameraBlock
 {
     mat4 uViewProj;
-    vec4 uPlayerPos;        // xyz, w=zScale
+    vec4 uPlayerPos; // xyz, w=zScale
 };
 
 layout(std140) uniform TimeBlock
@@ -32,22 +32,22 @@ uniform sampler2D uTexture;
 in vec3 vWorldPos;
 out vec4 vFragmentColor;
 
+// Avoid dynamic textureSize queries in fragment loop on Intel GPUs; WEATHER_INV_NOISE_SIZE is injected at shader compile time
 float get_noise(vec2 p)
 {
-    vec2 size = vec2(textureSize(uTexture, 0));
-    return texture(uTexture, p / size).r;
+    return texture(uTexture, p * WEATHER_INV_NOISE_SIZE).r;
 }
 
 float fbm(vec2 p)
 {
-    float v = 0.0;
-    float a = 0.5;
     vec2 shift = vec2(100.0);
-    for (int i = 0; i < 4; ++i) {
-        v += a * get_noise(p);
-        p = p * 2.0 + shift;
-        a *= 0.5;
-    }
+    float v = 0.5 * get_noise(p);
+    p = p * 2.0 + shift;
+    v += 0.25 * get_noise(p);
+    p = p * 2.0 + shift;
+    v += 0.125 * get_noise(p);
+    p = p * 2.0 + shift;
+    v += 0.0625 * get_noise(p);
     return v;
 }
 
@@ -56,7 +56,10 @@ void main()
     vec3 worldPos = vWorldPos;
 
     float distToPlayer = distance(worldPos.xy, uPlayerPos.xy);
-    float localMask = 1.0 - smoothstep(WEATHER_MASK_RADIUS_INNER, WEATHER_MASK_RADIUS_OUTER, distToPlayer);
+    float localMask = 1.0
+                      - smoothstep(WEATHER_MASK_RADIUS_INNER,
+                                   WEATHER_MASK_RADIUS_OUTER,
+                                   distToPlayer);
 
     float uCurrentTime = uTime.x;
     float uWeatherStartTime = uConfig.x;