From fac6acf7daf3efd872814af34a8faac11a0a5c9d Mon Sep 17 00:00:00 2001
From: salmonik <salmonik@tuta.io>
Date: Wed, 4 Nov 2026 00:50:59 +0100
Subject: Skybox image: match HDR sky colour decode

The HDR "sky" shader samples the compressed texture as rgb *= alpha,
multiplies by 8, then converts linear->sRGB for display. Feeding it the raw
sRGB image produced the blown-out / oversaturated sky.

When the map uses the HDR sky shader, pre-convert the image to linear and set
alpha to 1/8 so the 8x scale cancels and the final linear->sRGB conversion
restores the original colours. For LDR (UnlitGeneric) materials, create the
texture with TEXTUREFLAGS_SRGB so the normal sRGB sampling is used.

The first sky material's shader name is logged for confirmation.
---
 src/world.cpp | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 61 insertions(+), 5 deletions(-)

diff --git a/src/world.cpp b/src/world.cpp
index eac31d9..ddd047a 100644
--- a/src/world.cpp
+++ b/src/world.cpp
@@ -18,6 +18,7 @@
 #include "hl2h.h"
 #include <cstdio>
 #include <cstdlib>
+#include <cmath>
 #include <cstring>
 #include <ctime>
 #include <dlfcn.h>
@@ -386,7 +387,7 @@ int paeth(int a, int b, int c) {
 }
 
 bool load_png_rgba(const char *path, std::vector<unsigned char> &pixels,
-                   int &out_w, int &out_h) {
+                   int &out_w, int &out_h, bool hdr_compensation) {
   FILE *f = fopen(path, "rb");
   if (!f) {
     logf("skybox: cannot open %s", path);
@@ -609,6 +610,24 @@ bool load_png_rgba(const char *path, std::vector<unsigned char> &pixels,
     }
   }
 
+  // HDR sky materials use the "sky" shader. It samples the compressed HDR
+  // texture as rgb *= alpha, multiplies by 8, then converts linear->sRGB for
+  // display. Cancel the 8x with alpha = 1/8 and feed linear values so the
+  // final conversion restores the original sRGB image.
+  if (hdr_compensation) {
+    for (size_t i = 0; i + 3 < pixels.size(); i += 4) {
+      float rgb[3];
+      for (int c = 0; c < 3; c++) {
+        float v = (float)pixels[i + (size_t)c] / 255.0f;
+        rgb[c] = (v <= 0.04045f) ? v / 12.92f
+                                 : powf((v + 0.055f) / 1.055f, 2.4f);
+        pixels[i + (size_t)c] = (unsigned char)(rgb[c] * 255.0f + 0.5f);
+      }
+      pixels[i + 3] = 32; // 1/8 * 255
+    }
+    logf("skybox: HDR sky shader, using linear colours with 1/8 alpha");
+  }
+
   out_w = (int)w;
   out_h = (int)h;
   return true;
@@ -826,6 +845,39 @@ void restore_sky_texture() {
   g_sky_applied = false;
 }
 
+bool sky_uses_hdr_shader() {
+  void *ms = material_system();
+  if (!ms)
+    return false;
+  auto invalid = vfunc<MatCalls::InvalidFn>(ms, MATSYS_InvalidMaterial);
+  auto first = vfunc<MatCalls::FirstFn>(ms, MATSYS_FirstMaterial);
+  auto next = vfunc<MatCalls::NextFn>(ms, MATSYS_NextMaterial);
+  auto get = vfunc<MatCalls::GetFn>(ms, MATSYS_GetMaterial);
+  auto count = vfunc<MatCalls::CountFn>(ms, MATSYS_GetNumMaterials);
+  int total = count(ms);
+  if (total <= 0 || total > 200000)
+    return false;
+  using ShaderFn = const char *(*)(void *);
+  unsigned short inv = invalid(ms);
+  int seen = 0;
+  for (unsigned short h = first(ms); h != inv; h = next(ms, h)) {
+    if (++seen > total + 16)
+      break;
+    void *mat = get(ms, h);
+    if (!mat)
+      continue;
+    const char *name = vfunc<MatCalls::NameFn>(mat, MAT_GetName)(mat);
+    const char *group =
+        vfunc<MatCalls::GroupFn>(mat, MAT_GetTextureGroupName)(mat);
+    if (!is_sky_material(name, group))
+      continue;
+    const char *shader = vfunc<ShaderFn>(mat, 49)(mat);
+    if (shader && strcmp(shader, "sky") == 0)
+      return true;
+  }
+  return false;
+}
+
 void load_sky_texture() {
   g_sky_tex_tried = true;
   void *ms = material_system();
@@ -836,7 +888,8 @@ void load_sky_texture() {
   std::vector<unsigned char> pixels;
   int w = 0, h = 0;
   const char *path = config_file_cstr("skybox.png");
-  if (!load_png_rgba(path, pixels, w, h)) {
+  bool hdr = sky_uses_hdr_shader();
+  if (!load_png_rgba(path, pixels, w, h, hdr)) {
     if (!g_sky_fail_logged) {
       g_sky_fail_logged = true;
       toastf(TOAST_ERROR, "Skybox image missing or unreadable: %s", path);
@@ -851,9 +904,12 @@ void load_sky_texture() {
   // is called on Download() and again on device restore.
   using CreateProcFn = void *(*)(void *, const char *, const char *, int, int,
                                  int, int);
-  g_sky_tex = vfunc<CreateProcFn>(ms, 81)(
-      ms, "hl2hake/skybox", GROUP_SKYBOX, w, h, 0 /* RGBA8888 */,
-      0x10C /* CLAMPS|CLAMPT|NOMIP */);
+  // 0x40 = TEXTUREFLAGS_SRGB: the LDR sky path decodes sRGB from the texture
+  // flags, the HDR path forces sRGB reads off and does its own conversion.
+  int tex_flags = 0x10C /* CLAMPS|CLAMPT|NOMIP */ | (hdr ? 0 : 0x40);
+  g_sky_tex = vfunc<CreateProcFn>(ms, 81)(ms, "hl2hake/skybox",
+                                          GROUP_SKYBOX, w, h, 0 /*RGBA8888*/,
+                                          tex_flags);
   if (!g_sky_tex) {
     logf("skybox: CreateProceduralTexture failed for %dx%d", w, h);
     return;
-- 
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