[PATCH] Fix ox-md remote image links export

[Jay Kamat <jaygkamat@gmail.com>]

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Hi!

I discovered a bug in the ox-md exporter and have created a patch to fix it:

Previously, this image link:
[[https://git-scm.com/images/logo@2x.png]]

would export to:
![img](//git-scm.com/images/logo@2x.png)

which is an invalid markdown image, and won't display properly in
browsers and other places.

A workaround (which I have been using extremely heavily), is:
[[file:https://git-scm.com/images/logo@2x.png]]

which exports to
![img](https://git-scm.com/images/logo@2x.png)

Which is the proper (unbroken) image link.

I have attached a patch which fixes the first example, so it exports
the same link as the workaround (and is no longer broken).

One of my concerns with the attached patch is whether there's a better
way to determine if a link is 'remote'. Maybe I should make a constant
in the file listing 'remote' types and use that everywhere.
I'm also not sure if I broke any other type of image link (but the
simple local case seems to work fine).

My papers are currently being processed, but I thought I would post
this patch early to get feedback, since ideally I would like this bug
to be fixed as soon as possible :). Hopefully they'll be done before
the review is completed.

Let me know if you see anything wrong, and thanks again for hacking org mode :D.

-Jay

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From b4768baf2c6ea234c92f24540ed927cf8473e999 Mon Sep 17 00:00:00 2001
From: Jay Kamat <jaygkamat@gmail.com>
Date: Sat, 5 Aug 2017 19:14:44 -0700
Subject: [PATCH] ox-md.el: Fix exporting of remote image links

* lisp/ox-md.el (org-md-link): Check if a link is a remote, and if so,
  use the raw-link property of the link, rather than the path.

A workaround for this bug is to do this:
[[file:http://orgmode.org/img/org-mode-unicorn-logo.png]]

This commit lets the workaround continue working, while the proper method:
[[http://orgmode.org/img/org-mode-unicorn-logo.png]]
is fixed.
---
 lisp/ox-md.el | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lisp/ox-md.el b/lisp/ox-md.el
index 08e3852823..a4e1d6749b 100644
--- a/lisp/ox-md.el
+++ b/lisp/ox-md.el
@@ -430,8 +430,14 @@ a communication channel."
 	       (format "[%s](#%s)"
 		       description
 		       (org-export-get-reference destination info))))))))
+     ;; Generate a md image link
      ((org-export-inline-image-p link org-html-inline-image-rules)
-      (let ((path (let ((raw-path (org-element-property :path link)))
+       ;; Get the path and caption to insert into the image
+       (let ((path (let ((raw-path
+			   ;; types of links which we just use the raw link
+			   (if (member type '("http" "https" "ftp"))
+			     (org-element-property :raw-link link)
+			     (org-element-property :path link))))
 		    (if (not (file-name-absolute-p raw-path)) raw-path
 		      (expand-file-name raw-path))))
 	    (caption (org-export-data
-- 
2.11.0