From b58619431d6fa0f6b9f50a94aefc09be6121fa3e Mon Sep 17 00:00:00 2001
From: Timo <timo@taipalus.cc>
Date: Wed, 29 Dec 2021 10:55:06 +0200
Subject: [PATCH] Add support for image flipping (Bug#47095)

* 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 by 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.
---
 etc/NEWS      |  4 ++++
 lisp/image.el | 18 ++++++++++++++++++
 src/image.c   | 26 ++++++++++++++++++++++----
 3 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index c9466d0fef..ae894c5465 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -570,6 +570,10 @@ 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
 
 +++
diff --git a/lisp/image.el b/lisp/image.el
index 702985f41f..05e50cc4ae 100644
--- a/lisp/image.el
+++ b/lisp/image.el
@@ -181,6 +181,8 @@ image-map
     (define-key map [C-wheel-up]   'image-mouse-increase-size)
     (define-key map [C-mouse-4]    'image-mouse-increase-size)
     (define-key map "r" 'image-rotate)
+    (define-key map "h" 'image-flip-horizontally)
+    (define-key map "v" 'image-flip-vertically)
     (define-key map "o" 'image-save)
     map))
 
@@ -1249,6 +1251,22 @@ image-save
       (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
diff --git a/src/image.c b/src/image.c
index 1d83065cf7..5cbc85f357 100644
--- a/src/image.c
+++ b/src/image.c
@@ -2406,6 +2406,11 @@ 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
@@ -2445,14 +2450,20 @@ 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)
+	{
+          cos_r = 1;
+	  sin_r = 0;
+	  rotate_flag = 1;
+	}
+      else if (rotation == 90)
 	{
 	  width = img->height;
 	  height = img->width;
@@ -2493,9 +2504,14 @@ 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.  */
@@ -2513,9 +2529,10 @@ 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;
@@ -11363,6 +11380,7 @@ syms_of_image (void)
   DEFSYM (QCtransform_smoothing, ":transform-smoothing");
   DEFSYM (QCcolor_adjustment, ":color-adjustment");
   DEFSYM (QCmask, ":mask");
+  DEFSYM (QCflip, ":flip");
 
   /* Other symbols.  */
   DEFSYM (Qlaplace, "laplace");
-- 
2.32.0 (Apple Git-132)