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

[Joseph Turner via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Joseph Turner <joseph@breatheoutbreathe.in>
>> Cc: 69602@debbugs.gnu.org, stephen.berman@gmx.net, juri@linkov.net
>> Date: Wed, 06 Mar 2024 23:14:21 -0800
>>
>> Eli Zaretskii <eliz@gnu.org> writes:
>> > 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?
>>
>> Thank you for your help!
>>
>> When the user runs `image-increase-size', where should third-party code
>> recompute :map to fit the new image scale?
>>
>> There's no `image-after-change-size-hook' nor `image-after-rotate-hook'.
>
> I think the idea is to define the value of :map such that it runs
> image-compute-scaling-factor as part of computing the coordinates of
> the map.

Sorry, I don't understand.

When creating an image, we set its :map property according to the return
value of `image-compute-scaling-factor'.  Once the image is inserted into
the buffer, the user may run `image-increase-size' or `image-rotate',
which changes how the image is displayed but not its :map.

Now, we need to rerun `image-compute-scaling-factor' and recompute :map.
However, there is no hook which runs after the user runs those commands,
so AFAICT there's no way for our code to know when to recompute :map.

Thanks!

Joseph