bug#69602: 29.1; Image :map should adjust with :scale and :rotation

[Eli Zaretskii]

> Cc: Stephen Berman <stephen.berman@gmx.net>, Juri Linkov <juri@linkov.net>
> Date: Wed, 06 Mar 2024 21:37:50 -0800
> From:  Joseph Turner via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Currently, when running `image-increase-size' or `image-decrease-size'
> on an image with a :map property, the image scales but the image map
> does not.  For example, run the following snippet:
> 
> (with-current-buffer (get-buffer-create "*image-properties-test*")
>   (let ((svg "<?xml version=\"1.0\" encoding=\"UTF-8\" 
> standalone=\"no\"?>\n<!DOCTYPE svg PUBLIC \"-//W3C//DTD SVG 1.1//EN\"\n 
> \"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd\";>\n<!-- Generated by 
> graphviz version 2.43.0 (0)\n -->\n<!-- Title: orggraphview Pages: 1 
> -->\n<svg width=\"128pt\" height=\"128pt\"\n viewBox=\"0.00 0.00 127.59 
> 127.59\" xmlns=\"http://www.w3.org/2000/svg\"; 
> xmlns:xlink=\"http://www.w3.org/1999/xlink\";>\n<g id=\"graph0\" 
> class=\"graph\" transform=\"scale(1 1) rotate(0) translate(4 
> 123.59)\">\n<title>orggraphview</title>\n<polygon fill=\"white\" 
> stroke=\"transparent\" points=\"-4,4 -4,-123.59 123.59,-123.59 123.59,4 
> -4,4\"/>\n<!-- a -->\n<g id=\"node1\" class=\"node\">\n<title>a</title>\n<g 
> id=\"a_node1\"><a xlink:href=\"1\" xlink:title=\"Hover me!\">\n<ellipse 
> fill=\"none\" stroke=\"black\" cx=\"59.79\" cy=\"-59.79\" rx=\"59.59\" 
> ry=\"59.59\"/>\n<text text-anchor=\"middle\" x=\"59.79\" y=\"-56.09\" 
> font-family=\"Times,serif\" font-size=\"14.00\" fill=\"#000000\">Hover 
> me!</text>\n</a>\n</g>\n</g>\n</g>\n</svg>\n")
>         (map '(((circle (85 . 85) . 80) "1" (help-echo "Surprise!"))))
>         (inhibit-read-only t))
>     (erase-buffer)
>     (insert-image (create-image svg 'svg t :map map))
>     (goto-char (point-min))
>     (pop-to-buffer (current-buffer))))
> 
> Hovering the circle alters the pointer style and displays the tooltip.
> 
> Now run `M-x image-increase-size' or press "i +".  While the image
> becomes larger, the area which activates the tooltip remains the same.
> 
> See earlier discussion here:
> 
> https://yhetil.org/emacs-devel/87r0gng41l.fsf@ushin.org/T/#t
> 
> While a proper solution perhaps belongs on the C side, the following
> workaround adds an :unscaled-map property to images and sets :map
> according to :unscaled-map and :scale whenever :scale changes.

The ELisp manual says about :map:

     Note that the map's coordinates should reflect the displayed image
     after all transforms have been done (rotation, scaling and so on),
     and also note that Emacs (by default) performs auto-scaling of
     images, so to make things match up, you should either specify
     ‘:scale 1.0’ when creating the image, or use the result of
     ‘image-compute-scaling-factor’ to compute the elements of the map.

Can this technique help?