color: add Bradford matrix computation

This function is required for media-relative rendering intents.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

include/libweston/colorimetry.h

@@ -152,6 +152,9 @@ weston_normalized_primary_matrix_init(struct weston_mat3f *npm,
 				      const struct weston_color_gamut *gamut,
 				      enum weston_npm_direction dir);
 
+struct weston_mat3f
+weston_bradford_adaptation(struct weston_CIExy from, struct weston_CIExy to);
+
 /** Color primaries known by libweston */
 enum weston_color_primaries {
 	WESTON_PRIMARIES_CICP_SRGB = 0,

libweston/color.c

@@ -1038,3 +1038,35 @@ weston_normalized_primary_matrix_init(struct weston_mat3f *npm,
 
 	return true;
 }
+
+/** Compute linearized Bradford transformation
+ *
+ * \param from Source adapted white point.
+ * \param to Destination adapted white point.
+ * \return Full adaptation matrix.
+ *
+ * Based on ICC.1:2022 (ICC v4.4), annex E.
+ */
+WL_EXPORT struct weston_mat3f
+weston_bradford_adaptation(struct weston_CIExy from, struct weston_CIExy to)
+{
+	static const struct weston_mat3f bradford = WESTON_MAT3F(
+		 0.8951,  0.2664, -0.1614,
+		-0.7502,  1.7135,  0.0367,
+		 0.0389, -0.0685,  1.0296
+	);
+	struct weston_mat3f inv;
+	struct weston_vec3f from_cr;
+	struct weston_vec3f to_cr;
+	struct weston_vec3f r;
+	struct weston_mat3f tmp;
+
+	weston_m3f_invert(&inv, bradford);
+	from_cr = weston_m3f_mul_v3f(bradford, CIExy_to_XYZ(from));
+	to_cr = weston_m3f_mul_v3f(bradford, CIExy_to_XYZ(to));
+	r = WESTON_VEC3F(to_cr.x / from_cr.x,
+			 to_cr.y / from_cr.y,
+			 to_cr.z / from_cr.z);
+	tmp = weston_m3f_mul_m3f(weston_m3f_diag(r), bradford);
+	return weston_m3f_mul_m3f(inv, tmp);
+}

tests/color-math-test.c

@@ -356,3 +356,21 @@ TEST_P(npm, npm_test_cases)
 
 	return RESULT_OK;
 }
+
+/* https://www.color.org/chadtag.xalter */
+TEST(bradform_adaptation_D65_D50)
+{
+	const struct weston_CIExy D65 = { 0.3127, 0.3290 };
+	const struct weston_CIExy D50 = { 0.3457, 0.3585 };
+	const struct weston_mat3f ref = WESTON_MAT3F(
+		1.04790738171017, 0.0229333845542104, -0.0502016347980104,
+		0.0296059594177168, 0.990456039910785, -0.01707552919587,
+		-0.00924679432678241, 0.0150626801401488, 0.751791232609078
+	);
+	struct weston_mat3f M;
+
+	M = weston_bradford_adaptation(D65, D50);
+	test_assert_f32_ge(diff_precision(M, ref), 13);
+
+	return RESULT_OK;
+}