tag: Adds GST_TAG_IMAGE_ORIENTATION tag

Adds a new tag to inform about the image orientation and how
to rotate and flip it before display.

Note that this tag is a string with a predefined set of
possible values.

API: GST_TAG_IMAGE_ORIENTATION

Fixes #619508

docs/gst/gstreamer-sections.txt

@@ -2202,6 +2202,7 @@ GST_TAG_GROUPING
 GST_TAG_USER_RATING
 GST_TAG_DEVICE_MANUFACTURER
 GST_TAG_DEVICE_MODEL
+GST_TAG_IMAGE_ORIENTATION
 
 gst_tag_register
 gst_tag_merge_use_first

gst/gsttaglist.c

@@ -349,6 +349,9 @@ _gst_tag_initialize (void)
   gst_tag_register (GST_TAG_DEVICE_MODEL, GST_TAG_FLAG_META, G_TYPE_STRING,
       _("device model"),
       _("Model of the device used to create this media"), NULL);
+  gst_tag_register (GST_TAG_IMAGE_ORIENTATION, GST_TAG_FLAG_META, G_TYPE_STRING,
+      _("image orientation"),
+      _("How the image should be rotated or flipped before display"), NULL);
 }
 
 /**

gst/gsttaglist.h

@@ -944,6 +944,32 @@ gboolean     gst_tag_list_get_buffer_index  (const GstTagList * list,
  * Since: 0.10.30
  */
 #define GST_TAG_DEVICE_MODEL                      "device-model"
+/**
+ * GST_TAG_IMAGE_ORIENTATION:
+ *
+ * Represents the 'Orientation' tag from EXIF. Defines how the image
+ * should be rotated and mirrored for display. (string)
+ *
+ * This tag has a predefined set of allowed values:
+ *   "rotate-0"
+ *   "rotate-90"
+ *   "rotate-180"
+ *   "rotate-270"
+ *   "flip-rotate-0"
+ *   "flip-rotate-90"
+ *   "flip-rotate-180"
+ *   "flip-rotate-270"
+ *
+ * The naming is adopted according to a possible transformation to perform
+ * on the image to fix its orientation, obviously equivalent operations will
+ * yield the same result.
+ *
+ * Rotations indicated by the values are in clockwise direction and
+ * 'flip' means an horizontal mirroring.
+ *
+ * Since: 0.10.30
+ */
+#define GST_TAG_IMAGE_ORIENTATION            "image-orientation"
 
 G_END_DECLS