Scale images in PDF relative to its original size

[tedour]

Now all options to scale images are relative to the width of the content area (https://asciidoctor.org/docs/asciidoctor-pdf/#image-scaling).
I'd like to have an option to scale the image based of it's original size (1300px => 100% of the width, 650px => 50% of the width for example, 325px => 25%, etc)
Prawn seem to be able to do it (https://prawnpdf.org/docs/0.11.1/Prawn/Images.html) with scale argument.

[mojavelinux]

This would entail implementing support for the scale attribute in this converter.

[cod3licious]

Somewhat related to this, I was wondering, if it is possible to globally scale all images relative to their defined width as well.

For the html version, I've set a dimensionless width argument for all my images (e.g. width=340), which looks good in the html version, but feels a bit too large in the PDF.

It would be great if something like image-scale: 80% defined in the theme would automatically convert width=340 to width=0.8 * 340 and in case width is not set for an image, it could act on the original size of the image instead, thereby (I think) covering both use cases.

One important thing here is that this should only act on actual images, not mathematical formulas inserted as quasi-block images (I've noticed that setting image: width: 80% in the theme file didn't do anything for my images which already had a width attributed, but suddenly all my non-inline equations were really big, since apparently they are treated as normal images).

[mojavelinux]

I implemented this by adding support for the scale attribute (e.g., scale=0.5) as well as the iw units on the pdfwidth attribute (pdfwidth=0.5iw).