Add support for viewing "images" such as Krita (.kra) files

* doc/emacs/files.texi (Image Mode): Document it.

* lisp/image/image-converter.el (image-converter-add-handler): New
function (bug#48415).
(image-convert): Use it.

doc/emacs/files.texi

@@ -2355,6 +2355,25 @@ can be used to transform the image in question to @acronym{PNG} before
 displaying.  GraphicsMagick, ImageMagick and @command{ffmpeg} are
 currently supported for image conversions.
 
+@findex image-converter-add-handler
+  In addition, you may wish to add special handlers for certain image
+formats.  These can be added with the
+@code{image-converter-add-handler} function.  For instance, to allow
+viewing Krita files as simple images, you could say something like:
+
+@lisp
+(image-converter-add
+ "kra"
+ (lambda (file)
+   (call-process "unzip" nil t nil
+                 "-qq" "-c" "-x" file "mergedimage.png"))))
+@end lisp
+
+The function takes two parameters, where the first is a file name
+suffix, and the second is a function to do the ``conversion''.  This
+function takes one parameter, the file name, and should output an
+image in @code{image-convert-to-format} format in the current buffer.
+
 @findex thumbs-mode
 @cindex mode, Thumbs
   The Image-Dired package can also be used to view images as

lisp/image/image-converter.el

@@ -68,6 +68,8 @@ not, conversion will fail."
     (imagemagick :command "convert" :probe ("-list" "format")))
   "List of supported image converters to try.")
 
+(defvar image-converter--extra-converters (make-hash-table :test #'equal))
+
 (defun image-converter-initialize ()
   "Determine the external image converter to be used.
 This also determines which external formats we can parse."
@@ -123,15 +125,21 @@ converted image data is returned as a string."
     (error "IMAGE-FORMAT should be a symbol like `image/png'"))
   (with-temp-buffer
     (set-buffer-multibyte nil)
-    (when-let ((err (image-converter--convert
+    (let* ((source (if (listp image)
-                     image-converter
+                       (plist-get (cdr image) :file)
-                     (if (listp image)
+                     image))
-                         (plist-get (cdr image) :file)
+           (format (if (listp image)
-                       image)
+                       (plist-get (cdr image) :data-p)
-                     (if (listp image)
+                     image-format))
-                         (plist-get (cdr image) :data-p)
+           (type (if format
-                       image-format))))
+                     (image-converter--mime-type format)
-      (error "%s" err))
+                   (file-name-extension source)))
+           (extra-converter (gethash type image-converter--extra-converters)))
+      (if extra-converter
+          (funcall extra-converter source)
+        (when-let ((err (image-converter--convert
+                         image-converter source format)))
+          (error "%s" err))))
     (if (listp image)
         ;; Return an image object that's the same as we were passed,
         ;; but ignore the :type value.
@@ -298,6 +306,18 @@ Only suffixes that map to `image-mode' are returned."
                                           "-")))))
       "ffmpeg error when converting")))
 
+(defun image-converter-add-handler (suffix converter)
+  "Make Emacs use CONVERTER to parse image files that end with SUFFIX.
+CONVERTER is a function with one parameter, the file name.  The
+converter should output the image in the current buffer,
+converted to `image-convert-to-format'."
+  (cl-pushnew suffix image-converter-file-name-extensions :test #'equal)
+  (setq image-converter-file-name-extensions
+        (sort image-converter-file-name-extensions #'string<))
+  (setq image-converter-regexp
+        (concat "\\." (regexp-opt image-converter-file-name-extensions) "\\'"))
+  (setf (gethash suffix image-converter--extra-converters) converter))
+
 (provide 'image-converter)
 
 ;;; image-converter.el ends here