ash: Expand ambient EQ temperature range

Originally we remapped temperatures to a range [5700, 7500) and then
computed RGB scale factors using linear functions.

The table that remaps temperatures has been updated with
crrev.com/c/2001254 but we didn't update the code to accept the new range
[4500, 7500).

This CL updates the accepted range and also recomputes GB coefficients
used for the linear regression in the interval [4500, 6500].

Bug: 1045647
Test: ash_unittests --gtest_filter=AmbientTemperature.AmbientTemperatureToRGBScaleFactors
Change-Id: Ifa19820cd8d6b076dbf6c85841f126bca7ea4585
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2022246Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736498}

ash/system/night_light/night_light_controller_impl.cc

@@ -99,7 +99,7 @@ constexpr int kNightLightAnimationFrameRate = 30;
 
 // The following are color temperatues in Kelvin.
 // The min/max are a reasonable range we can clamp the values to.
-constexpr float kMinColorTemperatureInKelvin = 5700;
+constexpr float kMinColorTemperatureInKelvin = 4500;
 constexpr float kNeutralColorTemperatureInKelvin = 6500;
 constexpr float kMaxColorTemperatureInKelvin = 7500;
 
@@ -455,8 +455,8 @@ float NightLightControllerImpl::GetNonLinearTemperature(float temperature) {
 float NightLightControllerImpl::RemapAmbientColorTemperature(
     float temperature_in_kelvin) {
   // This function maps sensor input temperatures to other values since we want
-  // to avoid extreme color temperatures (e.g: temperatures below 5700 and
-  // above 7450 are too extreme.)
+  // to avoid extreme color temperatures (e.g: temperatures below 4500 and
+  // above 7500 are too extreme.)
   // The following table was created with internal user studies.
   constexpr struct {
     int32_t input_temperature;
@@ -516,8 +516,8 @@ NightLightControllerImpl::ColorScalesFromRemappedTemperatureInKevin(
     float temperature_decrement =
         (kNeutralColorTemperatureInKelvin - temperature_in_kelvin) /
         (kNeutralColorTemperatureInKelvin - kMinColorTemperatureInKelvin);
-    green = 1.f - temperature_decrement * 0.0368f;
-    blue = 1.f - temperature_decrement * 0.0882f;
+    green = 1.f - temperature_decrement * 0.1211f;
+    blue = 1.f - temperature_decrement * 0.2749f;
   }
   return {red, green, blue};
 }

ash/system/night_light/night_light_controller_unittest.cc

@@ -1487,8 +1487,8 @@ TEST(AmbientTemperature, AmbientTemperatureToRGBScaleFactors) {
             allowed_difference);
   // Warm
   vec =
-      NightLightControllerImpl::ColorScalesFromRemappedTemperatureInKevin(5800);
-  EXPECT_LT((vec - gfx::Vector3dF(1.0f, 0.968f, 0.924f)).Length(),
+      NightLightControllerImpl::ColorScalesFromRemappedTemperatureInKevin(4500);
+  EXPECT_LT((vec - gfx::Vector3dF(1.0f, 0.8816f, 0.7313f)).Length(),
             allowed_difference);
   // Daylight
   vec =