Improve perceived display brightness (#21884)

* tweak light sensor function

* CIE 1931 compensation

* before filter

* C3 scaling

* C2 is also linear

* whitespace

* scaling and min brightness

selfdrive/ui/ui.cc

@@ -185,15 +185,18 @@ static void update_state(UIState *s) {
   if (sm.updated("roadCameraState")) {
     auto camera_state = sm["roadCameraState"].getRoadCameraState();
 
-    float max_lines = Hardware::EON() ? 5408 : 1757;
-    float gain = camera_state.getGain();
+    float max_lines = Hardware::EON() ? 5408 : 1904;
+    float max_gain = Hardware::EON() ? 1.0: 10.0;
+    float max_ev = max_lines * max_gain;
 
-    if (Hardware::TICI()) {
-      // Max gain is 4 * 2.5 (High Conversion Gain)
-      gain /= 10.0;
+    // C3 camera only uses about 10% of available gain at night
+    if (Hardware::TICI) {
+      max_ev /= 10;
     }
 
-    scene.light_sensor = std::clamp<float>((1023.0 / max_lines) * (max_lines - camera_state.getIntegLines() * gain), 0.0, 1023.0);
+    float ev = camera_state.getGain() * float(camera_state.getIntegLines());
+
+    scene.light_sensor = std::clamp<float>(1.0 - (ev / max_ev), 0.0, 1.0);
   }
   scene.started = sm["deviceState"].getDeviceState().getStarted() && scene.ignition;
 }
@@ -335,9 +338,19 @@ void Device::setAwake(bool on, bool reset) {
 }
 
 void Device::updateBrightness(const UIState &s) {
-  float brightness_b = 10;
-  float brightness_m = 0.1;
-  float clipped_brightness = std::min(100.0f, (s.scene.light_sensor * brightness_m) + brightness_b);
+  // Scale to 0% to 100%
+  float clipped_brightness = 100.0 * s.scene.light_sensor;
+
+  // CIE 1931 - https://www.photonstophotos.net/GeneralTopics/Exposure/Psychometric_Lightness_and_Gamma.htm
+  if (clipped_brightness <= 8) {
+    clipped_brightness = (clipped_brightness / 903.3);
+  } else {
+    clipped_brightness = std::pow((clipped_brightness + 16.0) / 116.0, 3.0);
+  }
+
+  // Scale back to 10% to 100%
+  clipped_brightness = std::clamp(100.0f * clipped_brightness, 10.0f, 100.0f);
+
   if (!s.scene.started) {
     clipped_brightness = BACKLIGHT_OFFROAD;
   }