color-lcms: implement media white point scaling

Implement media white point scaling according to ICC.1:2022 (profile
v4.4), section 6.3.2.2 "Translation between media-relative colorimetric
data and ICC-absolute colorimetric data".

This is a lot of code for something that, under the current
circumstances, is a no-change. However, realizing that chromatic
adaptation and media white point scaling are two separate adjustments
was a revelation to me. I want to document in code how they work, even
if the net result is no-op.

The PCS white point is taken from ICC, it does not matter what non-zero
point it is, the result will be the same.

I did skip a bit the media white point scaling. One should first scale
from in to PCS, and then from PCS to out. The code is mathematically
equivalent. If some new operation needs to be done in between, this step
than be split in two, just like I split the chromatic adaptation step in
two in this patch.

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

libweston/color-lcms/color-transform.c

@@ -1432,6 +1432,7 @@ init_blend_to_parametric(struct cmlcms_color_transform *xform)
 struct rendering_intent_flags {
 	bool black_point_compensation;
 	bool chromatic_adaptation;
+	bool media_white_point_scaling;
 	bool perceptual;
 	bool saturate;
 };
@@ -1447,18 +1448,22 @@ rendering_intent_to_flags(enum weston_render_intent intent)
 		break;
 	case WESTON_RENDER_INTENT_RELATIVE:
 		flags.chromatic_adaptation = true;
+		flags.media_white_point_scaling = true;
 		break;
 	case WESTON_RENDER_INTENT_RELATIVE_BPC:
 		flags.chromatic_adaptation = true;
+		flags.media_white_point_scaling = true;
 		flags.black_point_compensation = true;
 		break;
 	case WESTON_RENDER_INTENT_PERCEPTUAL:
 		flags.chromatic_adaptation = true;
+		flags.media_white_point_scaling = true;
 		flags.black_point_compensation = true;
 		flags.perceptual = true;
 		break;
 	case WESTON_RENDER_INTENT_SATURATION:
 		flags.chromatic_adaptation = true;
+		flags.media_white_point_scaling = true;
 		flags.black_point_compensation = true;
 		flags.saturate = true;
 		break;
@@ -1475,6 +1480,19 @@ rgb_to_rgb_matrix(struct weston_mat4f *mat,
 		  char **errmsg)
 {
 	struct rendering_intent_flags flags = rendering_intent_to_flags(intent);
+	static const struct weston_CIExy PCS_wp = { 0.3457, 0.3585 }; /* D50 */
+
+	/*
+	 * "A viewer, who is viewing the display defined by the resulting image
+	 * description (the viewing environment included), is assumed to be fully
+	 * adapted to the primary color volume's white point."
+	 *
+	 * - wp_image_description_creator_params_v1
+	 */
+	struct weston_vec3f in_medium_wp = weston_CIExy_to_XYZ(in->primaries.white_point);
+	struct weston_CIExy in_adapted_wp = in->primaries.white_point;
+	struct weston_vec3f out_medium_wp = weston_CIExy_to_XYZ(out->primaries.white_point);
+	struct weston_CIExy out_adapted_wp = out->primaries.white_point;
 
 	/**
 	 * The matrix input is optical where RGB 0,0,0 corresponds to
@@ -1525,8 +1543,42 @@ rgb_to_rgb_matrix(struct weston_mat4f *mat,
 	p2p = npm_in;
 	if (flags.chromatic_adaptation) {
 		struct weston_mat3f chad =
-			weston_bradford_adaptation(in->primaries.white_point,
+			weston_bradford_adaptation(in_adapted_wp, PCS_wp);
-						   out->primaries.white_point);
+		p2p = weston_m3f_mul_m3f(chad, p2p);
+
+		/* Media white points must be chromatically adapted too. */
+		in_medium_wp = weston_m3f_mul_v3f(chad, in_medium_wp);
+		chad = weston_bradford_adaptation(out_adapted_wp, PCS_wp);
+		out_medium_wp = weston_m3f_mul_v3f(chad, out_medium_wp);
+
+		/*
+		 * If the adapted white point is the same as the medium white point,
+		 * and they are as seen at the top of this function, then the
+		 * medium white point adapted to PCS is by definition the PCS
+		 * white point. IOW, the above could be replaced with
+		 *
+		 * in_medium_wp = out_medium_wp = weston_CIExy_to_XYZ(PCS_wp);
+		 *
+		 * This is equivalent to the ICC Display class profile
+		 * requirement of setting mediaWhitePointTag to D50.
+		 */
+	}
+	if (flags.media_white_point_scaling) {
+		struct weston_vec3f s = WESTON_VEC3F(
+			out_medium_wp.x / in_medium_wp.x,
+			out_medium_wp.y / in_medium_wp.y,
+			out_medium_wp.z / in_medium_wp.z);
+		p2p = weston_m3f_mul_m3f(weston_m3f_diag(s), p2p);
+
+		/*
+		 * If the adapted white point is the same as the medium white
+		 * point for each of in and out profiles, and chromatic
+		 * adaptation is used, then this step is a no-op.
+		 */
+	}
+	if (flags.chromatic_adaptation) {
+		struct weston_mat3f chad =
+			weston_bradford_adaptation(PCS_wp, out_adapted_wp);
 		p2p = weston_m3f_mul_m3f(chad, p2p);
 	}
 	p2p = weston_m3f_mul_m3f(npm_out_inv, p2p);