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

[Timo Taipalus <timo@taipalus.cc>]

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On 2.1.2022 16:54, Alan Third wrote:
> That's actually what I'd expect to happen, tbh. :)
> 

> There's a bit of a problem with Emacs image manipulation, IMO, in that
> the transform order is baked in, so we end up with strange situations
> like when you set max-width, then rotate 90 degrees so now max-width
> affects the height of the image instead of the width.
> 
> So whatever, I don't feel very strongly about it.

In my experience the way flipping works now is how image viewers 
generally work. But I
guess neither is the one true way, the way it works now is my 
preference. If many do feel
strongly about it, then it can be changed I guess.

> 
> One more thing: do you feel up to adding a note in the manual about
> this new property? It should go in doc/lispref/display.texi. Look for
> rotation.
> 

I added documentation as requested plus code comments.

The paperwork was sent on 26th Dec to assign@gnu.org, so admins please 
check there.

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From e25c6211a88d0b05aad035c481f3a7cee3d0eb16 Mon Sep 17 00:00:00 2001
From: Timo Taipalus <timo@taipalus.cc>
Date: Thu, 6 Jan 2022 10:28:38 +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 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.
---
 doc/lispref/display.texi |  6 ++++++
 etc/NEWS                 |  4 ++++
 lisp/image.el            | 18 ++++++++++++++++++
 src/image.c              | 31 +++++++++++++++++++++++++++----
 4 files changed, 55 insertions(+), 4 deletions(-)

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index 1b08eda200..488af2a866 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -5596,6 +5596,12 @@ Image Descriptors
 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
diff --git a/etc/NEWS b/etc/NEWS
index 9c892b285d..231beae5d1 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -599,6 +599,10 @@ 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 c9165f7781..53358888c2 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 e7d347b782..61771ee4cf 100644
--- a/src/image.c
+++ b/src/image.c
@@ -2408,6 +2408,11 @@ 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
@@ -2447,14 +2452,25 @@ 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;
@@ -2495,9 +2511,14 @@ 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.  */
@@ -2515,9 +2536,10 @@ 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;
@@ -11365,6 +11387,7 @@ 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)