info: add di_info_get_default_color_primaries()

Add a high-level function to retrieve color primaries. Wayland
compositors will be interested in this.

A high-level function may later be extended to support stand-alone
DisplayID and other sources.

The high-level data is derived from low-level API calls in
di_info_parse_edid(). As we stash the results in struct di_info to avoid
requiring the callers to explicitly free the high-level structures, this
allows keeping struct di_info deeply const in high-level API
implementation. We also avoid some dynamic allocations.

https://github.com/linuxhw/EDID.git database contains 20399 samples for
which edid-decode complains:
  Basic Display Parameters & Features: sRGB is signaled, but the chromaticities do not match.

Therefore I assume that the sRGB flag is more correct when set.

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

include/info.h

@@ -7,10 +7,20 @@
 
 #include <libdisplay-info/info.h>
 
+/**
+ * All information here is derived from low-level information contained in
+ * struct di_info. These are exposed by the high-level API only.
+ */
+struct di_derived_info {
+	struct di_color_primaries color_primaries;
+};
+
 struct di_info {
 	struct di_edid *edid;
 
 	char *failure_msg;
+
+	struct di_derived_info derived;
 };
 
 #endif

include/libdisplay-info/info.h

@@ -2,6 +2,7 @@
 #define DI_INFO_H
 
 #include <stddef.h>
+#include <stdbool.h>
 
 /**
  * libdisplay-info's high-level API.
@@ -102,4 +103,52 @@ di_info_get_model(const struct di_info *info);
 char *
 di_info_get_serial(const struct di_info *info);
 
+/** CIE 1931 2-degree observer chromaticity coordinates */
+struct di_chromaticity_cie1931 {
+	float x;
+	float y;
+};
+
+/** Display color primaries and default white point */
+struct di_color_primaries {
+	/* Are primary[] given */
+	bool has_primaries;
+
+	/* Is default_white given */
+	bool has_default_white_point;
+
+	/* Chromaticities of the primaries in RGB order
+	 *
+	 * Either all values are given, or they are all zeroes.
+	 */
+	struct di_chromaticity_cie1931 primary[3];
+
+	/* Default white point chromaticity
+	 *
+	 * If non-zero, this should be the display white point when it has
+	 * been reset to its factory defaults. Either both x and y are given,
+	 * or they are both zero.
+	 */
+	struct di_chromaticity_cie1931 default_white;
+};
+
+/**
+ * Get display color primaries and default white point
+ *
+ * Get the parameters of the default RGB colorimetry mode which is always
+ * supported. Primaries for monochrome displays might be all zeroes.
+ *
+ * These primaries might not be display's physical primaries, but only the
+ * primaries of the default RGB colorimetry signal when using IT Video Format
+ * (ANSI/CTA-861-H, Section 5).
+ *
+ * The returned pointer is owned by the struct di_info passed in. It remains
+ * valid only as long as the di_info exists, and must not be freed by the
+ * caller.
+ *
+ * This function does not return NULL.
+ */
+const struct di_color_primaries *
+di_info_get_default_color_primaries(const struct di_info *info);
+
 #endif

info.c

@@ -11,6 +11,61 @@
 const char *
 pnp_id_table(const char *key);
 
+static void
+derive_edid_color_primaries(const struct di_edid *edid,
+			    struct di_color_primaries *cc)
+{
+	const struct di_edid_chromaticity_coords *cm;
+	const struct di_edid_misc_features *misc;
+
+	/* Trust the flag more than the fields. */
+	misc = di_edid_get_misc_features(edid);
+	if (misc->srgb_is_primary) {
+		/*
+		 * https://www.w3.org/Graphics/Color/sRGB.html
+		 * for lack of access to IEC 61966-2-1
+		 */
+		cc->primary[0].x = 0.640f; /* red */
+		cc->primary[0].y = 0.330f;
+		cc->primary[1].x = 0.300f; /* green */
+		cc->primary[1].y = 0.600f;
+		cc->primary[2].x = 0.150f; /* blue */
+		cc->primary[2].y = 0.060f;
+		cc->has_primaries = true;
+		cc->default_white.x = 0.3127f; /* D65 */
+		cc->default_white.y = 0.3290f;
+		cc->has_default_white_point = true;
+
+		return;
+	}
+
+	cm = di_edid_get_chromaticity_coords(edid);
+
+	/*
+	 * Broken EDID might have only partial values.
+	 * Require all values to report anything.
+	 */
+	if (cm->red_x > 0.0f &&
+	    cm->red_y > 0.0f &&
+	    cm->green_x > 0.0f &&
+	    cm->green_y > 0.0f &&
+	    cm->blue_x > 0.0f &&
+	    cm->blue_y > 0.0f) {
+		cc->primary[0].x = cm->red_x;
+		cc->primary[0].y = cm->red_y;
+		cc->primary[1].x = cm->green_x;
+		cc->primary[1].y = cm->green_y;
+		cc->primary[2].x = cm->blue_x;
+		cc->primary[2].y = cm->blue_y;
+		cc->has_primaries = true;
+	}
+	if (cm->white_x > 0.0f && cm->white_y > 0.0f) {
+		cc->default_white.x = cm->white_x;
+		cc->default_white.y = cm->white_y;
+		cc->has_default_white_point = true;
+	}
+}
+
 struct di_info *
 di_info_parse_edid(const void *data, size_t size)
 {
@@ -38,6 +93,8 @@ di_info_parse_edid(const void *data, size_t size)
 	else
 		free(failure_msg_str);
 
+	derive_edid_color_primaries(info->edid, &info->derived.color_primaries);
+
 	return info;
 
 err_edid:
@@ -194,3 +251,9 @@ di_info_get_serial(const struct di_info *info)
 	memory_stream_cleanup(&m);
 	return NULL;
 }
+
+const struct di_color_primaries *
+di_info_get_default_color_primaries(const struct di_info *info)
+{
+	return &info->derived.color_primaries;
+}