bug#18797: 24.4; Unable to do image transforms even when emacs is compiled with Imagemagick

[Kaushal]

I have posted a complete solution of what worked for me on Emacs SE: http://emacs.stackexchange.com/a/2458/115

I modified the image-type-header-regexps list so that imagemagick is used to more more than just jpg files.

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Kaushal Modi

On Wed, Oct 22, 2014 at 2:50 PM, Kaushal <kaushal.modi@gmail.com> wrote:

I went deeper in the references bug posts and I found the answer in bug # 10112!

I had to add:

(add-to-list 'image-type-header-regexps '("\\`\377\330" . imagemagick))

But just for my knowledge, what is "\\`\377\330" ?

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Kaushal Modi

On Wed, Oct 22, 2014 at 2:45 PM, Kaushal <kaushal.modi@gmail.com> wrote:

Thanks for your reply.

I evaluated (setq image-type-header-regexps nil) and verified that it is actually set to nil by doing C-h v on that variable.

But (eq image-type 'imagemagick) is still returning nil and I am M-: (image-transform-set-scale 0.1) still does nothing.

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Kaushal Modi

On Wed, Oct 22, 2014 at 1:52 PM, Glenn Morris <rgm@gnu.org> wrote:

This message was over 1 MB. Please compress attachments in future.
No need to resend this one, since interested people can fetch the attachment
from http://debbugs.gnu.org/cgi/bugreport.cgi?bug=18797#5 .
(But I think it has no relevance anyway.)

Kaushal wrote:

> I have compiled emacs with Imagemagick (the config.log file is attached).
>
> But "M-x image-transform-set-scale" is not working for me. The image scale
> stays the same when I set the scale to 0.5. I tested it with bot png and
> jpg images.
>
> But the eimp package is working fine (which uses mogrify from Imagemagick).
>
> When an image is open in a buffer, evaluating "(eq image-type
> 'imagemagick)" returns "nil".

Right, so it's not an ImageMagick image, so you can't transform it.
Emacs uses libjpeg for jpgs by default.
Try testing it on a format that Emacs does't support otherwise, eg bmp.
You have to force the images to be created with type imagemagick to be
able to scale them. This remains poorly documented IMO;
http://debbugs.gnu.org/10746

> What am I missing in enabling the imagemagick detection for functions like
> image-transform-set-scale?