Remove any support for relative paths with field lists and add a note

Author: ItayZiv

README.md

@@ -28,7 +28,7 @@ Users hosting documentation on Read The Docs *do not* need to set any of the fol
 * `ogp_site_name`
     * This is not required. Name of the site. This is displayed above the title.
 * `ogp_image`
-    * This is not required. Link to image to show.
+    * This is not required. Link to image to show. Note that all relative paths are converted to be relative to the root of the html output.
 * `ogp_image_alt`
     * This is not required. Alt text for image. Defaults to using `ogp_site_name` or the document's title as alt text, if available. Set to `False` if you want to turn off alt text completely.
 * `ogp_use_first_image`
@@ -78,7 +78,7 @@ These are some overrides that can be used, you can actually override any tag and
 * `:og:type:`
   * Override the type of the page, for the list of available types take a look at https://ogp.me/#types.
 * `:ogp:image:`
-  * Set the image for the page.
+  * Set the image for the page. Note: Relative file paths are ***not*** currently supported when using field lists.
 * `:ogp:image:alt:`
   * Sets the alt text. Will be ignored if there is no image set.
 
@@ -92,12 +92,16 @@ Remember that the fields **must** be placed at the very start of the file. You c
 
 Page contents
 =============
+```
 
 ### Arbitrary Tags
 Additionally, you can use field lists to add any arbitrary OpenGraph tag not supported by the extension. The syntax for arbitrary tags is the same with `:og:tag: content`. For Example:
 
+Note: Relative file paths are ***not*** currently supported for images, videos and audio when using field lists.
+
 ```rst
 :og:video: http://example.org/video.mp4
 
 Page contents
 =============
+```

sphinxext/opengraph/__init__.py

@@ -113,6 +113,13 @@ def get_tags(
         image_url = config["ogp_image"]
         ogp_use_first_image = config["ogp_use_first_image"]
         ogp_image_alt = fields.get("og:image:alt", config["ogp_image_alt"])
+
+        if image_url:
+            image_url_parsed = urlparse(image_url)
+            if not image_url_parsed.scheme:
+                # Relative image path detected. Make absolute.
+                image_url = urljoin(config["ogp_site_url"], image_url_parsed.path)
+
     fields.pop("og:image:alt", None)
 
     if ogp_use_first_image:
@@ -125,10 +132,6 @@ def get_tags(
             ogp_image_alt = first_image.get("alt", None)
 
     if image_url:
-        image_url_parsed = urlparse(image_url)
-        if not image_url_parsed.scheme:
-            # Relative image path detected. Make absolute.
-            image_url = urljoin(config["ogp_site_url"], image_url_parsed.path)
         tags["og:image"] = image_url
 
         # Add image alt text (either provided by config or from site_name)