Add support for image flipping

* lisp/image.el (image-map): Keybindings for flipping functions.
(image-flip-horizontally): New function that toggles image flipping
property.
(image-flip-vertically): New function that toggles image flipping
property and rotates image 180 degrees.
* src/image.c (syms_of_image): Add property.
(image_set_transform): Modify image rotation code to also horizontally
flip the image when the property is set.
* etc/NEWS: Add description.
* doc/lispref/display.texi (Image Descriptors): Document :flip
(bug#47095).

doc/lispref/display.texi

@@ -5717,6 +5717,12 @@ are supported, unless the image type is @code{imagemagick}.  Positive
 values rotate clockwise, negative values counter-clockwise.  Rotation
 is performed after scaling and cropping.
 
+@item :flip @var{flip}
+If this is @code{t}, the image will be horizontally flipped.
+Currently it has no effect if the image type is @code{imagemagick}.
+Vertical flipping can be achieved by rotating the image 180 degrees
+and toggling this value.
+
 @item :transform-smoothing @var{smooth}
 If this is @code{t}, any image transform will have smoothing applied;
 if @code{nil}, no smoothing will be applied.  The exact algorithm used

etc/NEWS

@@ -1545,6 +1545,10 @@ This controls whether or not to show a message when opening certain
 image formats saying how to edit it as text.  The default is to show
 this message for SVG and XPM.
 
++++
+*** New commands: 'image-flip-horizontally' and 'image-flip-vertically'.
+These commands horizontally and vertically flip the image under point.
+
 ** Image-Dired
 
 +++

lisp/image.el

@@ -177,6 +177,8 @@ or \"ffmpeg\") is installed."
   "+" #'image-increase-size
   "r" #'image-rotate
   "o" #'image-save
+  "h" #'image-flip-horizontally
+  "v" #'image-flip-vertically
   "C-<wheel-down>" #'image-mouse-decrease-size
   "C-<mouse-5>"    #'image-mouse-decrease-size
   "C-<wheel-up>"   #'image-mouse-increase-size
@@ -1288,6 +1290,22 @@ changing the displayed image size does not affect the saved image."
       (write-region (point-min) (point-max)
                     (read-file-name "Write image to file: ")))))
 
+(defun image-flip-horizontally ()
+  "Horizontally flip the image under point."
+  (interactive)
+  (let ((image (image--get-image)))
+    (image-flush image)
+    (setf (image-property image :flip)
+          (not (image-property image :flip)))))
+
+(defun image-flip-vertically ()
+  "Vertically flip the image under point."
+  (interactive)
+  (let ((image (image--get-image)))
+    (image-rotate 180)
+    (setf (image-property image :flip)
+          (not (image-property image :flip)))))
+
 (provide 'image)
 
 ;;; image.el ends here

src/image.c

@@ -2587,6 +2587,11 @@ image_set_transform (struct frame *f, struct image *img)
   double rotation = 0.0;
   compute_image_rotation (img, &rotation);
 
+  /* Determine flipping.  */
+  bool flip;
+  Lisp_Object m = image_spec_value (img->spec, QCflip, NULL);
+  flip = !NILP (m);
+
 #ifndef HAVE_HAIKU
 # if defined USE_CAIRO || defined HAVE_XRENDER || defined HAVE_NS
   /* We want scale up operations to use a nearest neighbor filter to
@@ -2626,14 +2631,25 @@ image_set_transform (struct frame *f, struct image *img)
   /* Perform rotation transformation.  */
 
   int rotate_flag = -1;
-  if (rotation == 0)
+  if (rotation == 0 && !flip)
     rotate_flag = 0;
   else
     {
 # if (defined USE_CAIRO || defined HAVE_XRENDER \
       || defined HAVE_NTGUI || defined HAVE_NS)
       int cos_r, sin_r;
-      if (rotation == 90)
+      if (rotation == 0)
+	{
+	  /* FLIP is always true here.  As this will rotate by 0
+	     degrees, it has no visible effect.  Applying only
+	     translation matrix to the image would be sufficient for
+	     horizontal flipping, but writing special handling for
+	     this case would increase code complexity somewhat.  */
+	  cos_r = 1;
+	  sin_r = 0;
+	  rotate_flag = 1;
+	}
+      else if (rotation == 90)
 	{
 	  width = img->height;
 	  height = img->width;
@@ -2674,9 +2690,14 @@ image_set_transform (struct frame *f, struct image *img)
 	  matrix3x3 v;
 	  matrix3x3_mult (rot, u, v);
 
-	  /* 3. Translate back.  */
+	  /* 3. Translate back.  Flip horizontally if requested.  */
 	  t[2][0] = width * -.5;
 	  t[2][1] = height * -.5;
+	  if (flip)
+	    {
+	      t[0][0] = -t[0][0];
+	      t[2][0] = -t[2][0];
+	    }
 	  matrix3x3_mult (t, v, matrix);
 #  else
 	  /* 1. Translate so (0, 0) is in the center of the image.  */
@@ -2694,9 +2715,10 @@ image_set_transform (struct frame *f, struct image *img)
 	  matrix3x3 v;
 	  matrix3x3_mult (u, rot, v);
 
-	  /* 3. Translate back.  */
+	  /* 3. Translate back.  Flip horizontally if requested.  */
 	  t[2][0] = width * .5;
 	  t[2][1] = height * .5;
+	  if (flip) t[0][0] = -t[0][0];
 	  matrix3x3_mult (v, t, matrix);
 #  endif
 	  img->width = width;
@@ -11940,6 +11962,7 @@ non-numeric, there is no explicit limit on the size of images.  */);
   DEFSYM (QCtransform_smoothing, ":transform-smoothing");
   DEFSYM (QCcolor_adjustment, ":color-adjustment");
   DEFSYM (QCmask, ":mask");
+  DEFSYM (QCflip, ":flip");
 
   /* Other symbols.  */
   DEFSYM (Qlaplace, "laplace");