From 244daaeddc80aaca2e2f323e81df8bed0632f782 Mon Sep 17 00:00:00 2001
From: Daniel Solomon <solomondaniel@google.com>
Date: Sun, 17 Mar 2019 17:30:41 -0700
Subject: Change Display White Balance to track the Planckian locus

Currently when DWB adjusts the display's white point it uses standard
Illuminant D to convert CCT to CIE xy coordinates. This causes a slight
green tint, detectable by some users. Change this behavior to calculate
CIE xy based on the Planckian locus.

Bug: 128712066
Test:
a. Manual, by overriding target DWB CCT and observing resulting target
xy coordinates
b. atest FrameworksServicesTests:ColorDisplayServiceTest, making sure
there are no new failures (existing failures tracked in a separate bug)

Change-Id: I6fa336ee694730df45d5eacceb442aa556b5718e
---
 graphics/java/android/graphics/ColorSpace.java | 39 ++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

(limited to 'graphics/java/android')

diff --git a/graphics/java/android/graphics/ColorSpace.java b/graphics/java/android/graphics/ColorSpace.java
index 0d5233880674..9c4b5e8b0165 100644
--- a/graphics/java/android/graphics/ColorSpace.java
+++ b/graphics/java/android/graphics/ColorSpace.java
@@ -1808,6 +1808,45 @@ public abstract class ColorSpace {
         return mul3x3(inverse3x3(matrix), mul3x3Diag(LMS, matrix));
     }
 
+    /**
+     * <p>Computes the chromaticity coordinates of a specified correlated color
+     * temperature (CCT) on the Planckian locus. The specified CCT must be
+     * greater than 0. A meaningful CCT range is [1667, 25000].</p>
+     *
+     * <p>The transform is computed using the methods in Kang et
+     * al., <i>Design of Advanced Color - Temperature Control System for HDTV
+     * Applications</i>, Journal of Korean Physical Society 41, 865-871
+     * (2002).</p>
+     *
+     * @param cct The correlated color temperature, in Kelvin
+     * @return Corresponding XYZ values
+     * @throws IllegalArgumentException If cct is invalid
+     *
+     * @hide
+     */
+    @NonNull
+    @Size(3)
+    public static float[] cctToXyz(@IntRange(from = 1) int cct) {
+        if (cct < 1) {
+            throw new IllegalArgumentException("Temperature must be greater than 0");
+        }
+
+        final float icct = 1e3f / cct;
+        final float icct2 = icct * icct;
+        final float x = cct <= 4000.0f ?
+            0.179910f + 0.8776956f * icct - 0.2343589f * icct2 - 0.2661239f * icct2 * icct :
+            0.240390f + 0.2226347f * icct + 2.1070379f * icct2 - 3.0258469f * icct2 * icct;
+
+        final float x2 = x * x;
+        final float y = cct <= 2222.0f ?
+            -0.20219683f + 2.18555832f * x - 1.34811020f * x2 - 1.1063814f * x2 * x :
+            cct <= 4000.0f ?
+            -0.16748867f + 2.09137015f * x - 1.37418593f * x2 - 0.9549476f * x2 * x :
+            -0.37001483f + 3.75112997f * x - 5.8733867f * x2 + 3.0817580f * x2 * x;
+
+        return xyYToXyz(new float[] {x, y});
+    }
+
     /**
      * <p>Computes the chromaticity coordinates of a CIE series D illuminant
      * from the specified correlated color temperature (CCT). The specified CCT
-- 
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