bug#47095: [PATCH] Add support for image mirroring (Bug#47095)

[Timo Taipalus <timo@taipalus.cc>]

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It seems image-mode does not have capability for image mirroring, so
here's a patch for it.  Tested NS (macOS) and XRender/Cairo
implementations.  As it is, it has no effect if ImageMagick is
enabled.

I chose 'h' as the action key because 'm' and 'f' were reserved, and
'h' is easy to memorize ("horizontal").

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From 3e67734ceb1f710a6039824b8c224b97cdebcd44 Mon Sep 17 00:00:00 2001
From: Timo <timo@taipalus.cc>
Date: Sat, 25 Dec 2021 15:38:20 +0200
Subject: [PATCH] Add support for image mirroring (Bug#47095)

* lisp/image.el (image-map): Keybinding for mirroring function.
(image-mirror): New function that toggles image mirroring property.
* src/image.c (syms_of_image): Add property.
(image_set_transform): Modify image rotation code to also horizontally
mirror the image when the property is set.
* etc/NEWS: Add description.
---
 etc/NEWS      |  4 ++++
 lisp/image.el |  9 +++++++++
 src/image.c   | 26 ++++++++++++++++++++++----
 3 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index c9466d0fef..08cf624020 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 command 'image-mirror'.
+This command mirrors (horizontally flips) the image under point.
+
 ** Image-Dired
 
 +++
diff --git a/lisp/image.el b/lisp/image.el
index 702985f41f..c060c4c650 100644
--- a/lisp/image.el
+++ b/lisp/image.el
@@ -181,6 +181,7 @@ 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-mirror)
     (define-key map "o" 'image-save)
     map))
 
@@ -1249,6 +1250,14 @@ image-save
       (write-region (point-min) (point-max)
                     (read-file-name "Write image to file: ")))))
 
+(defun image-mirror ()
+  "Mirror (horizontally flip) the image under point."
+  (interactive)
+  (let ((image (image--get-image)))
+    (image-flush image)
+    (setf (image-property image :mirror)
+          (not (image-property image :mirror)))))
+
 (provide 'image)
 
 ;;; image.el ends here
diff --git a/src/image.c b/src/image.c
index 1d83065cf7..5b324637ce 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 mirroring.  */
+  bool mirroring;
+  Lisp_Object m = image_spec_value (img->spec, QCmirror, NULL);
+  mirroring = !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 && !mirroring)
     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. Apply mirroring if requested. */
 	  t[2][0] = width * -.5;
 	  t[2][1] = height * -.5;
+	  if (mirroring)
+	    {
+	       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. Apply mirroring if requested. */
 	  t[2][0] = width * .5;
 	  t[2][1] = height * .5;
+	  if (mirroring) 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 (QCmirror, ":mirror");
 
   /* Other symbols.  */
   DEFSYM (Qlaplace, "laplace");
-- 
2.32.0 (Apple Git-132)