If requested, use external image converters for exotic formats

* doc/lispref/display.texi (Defining Images): Document it.

* lisp/image.el (convert-images-externally): New variable.
(image-type): Use it.
(create-image): Convert images.

* lisp/image/image-converter.el (image-converter--convert): New file.

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index fd6820897f3..ec288b1c47d 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -6044,6 +6044,13 @@ properties---for example,
 (create-image "foo.xpm" 'xpm nil :heuristic-mask t)
 @end example
 
+@vindex convert-images-externally
+If Emacs doesn't have native support for the image format in question,
+and @code{convert-images-externally} is non-@code{nil}, Emacs will try
+to determine whether there are external utilities that can be used to
+transform the image in question to @acronym{PNG} before displaying.
+GraphicsMagick, ImageMagick and ffmpeg are currently supported.
+
 The function returns @code{nil} if images of this type are not
 supported.  Otherwise it returns an image descriptor.
 @end defun