Gamma: clamp slider to the engine's 2.6 cap, defer readback

The engine clamps mat_monitorgamma at 2.6, so the 2.8..3.0 end of the slider
did nothing. Read the value back one tick later (ClientCmd is queued) and
interpose SDL_SetWindowGammaRamp to see whether the engine pushes a ramp at
all.

4 files changed, 46 insertions, 13 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index 4df44bd..fc73214 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -239,11 +239,16 @@ vocabulary / decisions.
   migrate to the stock value).
 - `viewmodel_fov` and `mat_monitorgamma` are client cvars; applied through
   `ClientCmd` when their slider changes. Gamma is stored as x10
-  (`[view] gamma=22` = 2.2). The gamma value is read back through
-  `ICvar::FindVar` and logged (`view: gamma request ..., cvar reports ...`)
-  to distinguish a rejected command from an OS gamma ramp the compositor
-  ignores. Both live in the always-on `Camera / display` card
-  (`[view] fov/vmfov/gamma`).
+  (`[view] gamma=22` = 2.2) and the slider stops at 26 because the engine
+  clamps `mat_monitorgamma` at 2.6. The value is read back through
+  `ICvar::FindVar` on the next tick (ClientCmd is queued) and logged
+  (`view: gamma request ..., cvar reports ...`) to distinguish a rejected
+  command from an OS gamma ramp the compositor ignores. `SDL_SetWindowGammaRamp`
+  is also interposed (first 12 calls log `gamma: SDL_SetWindowGammaRamp
+  mid=...`) to see whether the engine ever pushes a ramp after the slider
+  moves; if it does and the screen does not change, the compositor is
+  discarding it. Both live in the always-on
+  `Camera / display` card (`[view] fov/vmfov/gamma`).
 
 ## Map transitions
 
diff --git a/src/gui.cpp b/src/gui.cpp
index 88e5521..e8bcac7 100644
--- a/src/gui.cpp
+++ b/src/gui.cpp
@@ -419,8 +419,9 @@ static void build_cards() {
              "Weapon model field of view (20-120; stock 54).", 20, 120,
              &g_fov_viewmodel);
   add_slider(view, "Gamma x10",
-             "mat_monitorgamma x10: 16 = 1.6, 30 = 3.0, stock is 22.", 16, 30,
-             &g_gamma);
+             "mat_monitorgamma x10: 16 = 1.6, 26 = 2.6 (engine max), stock "
+             "is 22.",
+             16, 26, &g_gamma);
 
   add_card(2, &g_fullbright, "Fullbright",
            "Ignore world lighting so dark areas are visible.", true);
@@ -2240,6 +2241,28 @@ SDL_WarpMouseGlobal(int x, int y) {
     real_warp_global(x, y);
 }
 
+// Diagnostic: tell whether mat_monitorgamma reaches the SDL gamma ramp at all.
+static int (*real_set_gamma_ramp)(void *, const unsigned short *,
+                                  const unsigned short *,
+                                  const unsigned short *) = nullptr;
+
+extern "C" __attribute__((visibility("default"))) int
+SDL_SetWindowGammaRamp(void *window, const unsigned short *r,
+                       const unsigned short *g, const unsigned short *b) {
+  if (!real_set_gamma_ramp)
+    real_set_gamma_ramp = (int (*)(void *, const unsigned short *,
+                                   const unsigned short *,
+                                   const unsigned short *))dlsym(
+        RTLD_NEXT, "SDL_SetWindowGammaRamp");
+  static int logs = 0;
+  if (logs < 12) {
+    logs++;
+    logf("gamma: SDL_SetWindowGammaRamp mid=%u/%u/%u", r ? r[128] : 0,
+         g ? g[128] : 0, b ? b[128] : 0);
+  }
+  return real_set_gamma_ramp ? real_set_gamma_ramp(window, r, g, b) : -1;
+}
+
 static unsigned int (*real_get_rel_mouse)(int *, int *) = nullptr;
 static int (*real_poll_event)(void *) = nullptr;
 
diff --git a/src/hl2h.cpp b/src/hl2h.cpp
index f7106cd..1fc7096 100644
--- a/src/hl2h.cpp
+++ b/src/hl2h.cpp
@@ -674,13 +674,18 @@ static void view_settings_apply(void *ent) {
     last_vm = g_fov_viewmodel;
   }
   static int last_gamma = 22;
+  static int pending_gamma = -1;
   if (g_gamma != last_gamma) {
     char cmd[48];
     snprintf(cmd, sizeof(cmd), "mat_monitorgamma %d.%d", g_gamma / 10,
              g_gamma % 10);
     console_cmd(cmd);
-    // Read back through ICvar to tell an ignored command from an OS gamma
-    // ramp that the compositor discards.
+    pending_gamma = g_gamma;
+    last_gamma = g_gamma;
+  }
+  if (pending_gamma >= 0) {
+    // ClientCmd is queued, so read the cvar on the next tick to see the value
+    // the engine actually accepted (it clamps mat_monitorgamma at 2.6).
     float got = -1.0f;
     if (g_iface.cvar) {
       using FindFn = void *(*)(void *, const char *);
@@ -689,9 +694,9 @@ static void view_settings_apply(void *ent) {
       if (v)
         got = *(float *)((uintptr_t)v + 0x2c); // ConVar m_Value.f
     }
-    logf("view: gamma request %d.%d, cvar reports %.2f", g_gamma / 10,
-         g_gamma % 10, got);
-    last_gamma = g_gamma;
+    logf("view: gamma request %d.%d, cvar reports %.2f", pending_gamma / 10,
+         pending_gamma % 10, got);
+    pending_gamma = -1;
   }
 }
 
diff --git a/src/menu.cpp b/src/menu.cpp
index ead5f2d..248aa48 100644
--- a/src/menu.cpp
+++ b/src/menu.cpp
@@ -407,7 +407,7 @@ static bool config_load() {
         } else if (strcmp(key, "gamma") == 0) {
           dst = &g_gamma;
           lo = 16;
-          hi = 30;
+          hi = 26; // the engine clamps mat_monitorgamma at 2.6
           stock = 22;
         }
         if (dst) {