Relaxed image resizing (less params)

Feature
1

The current resizing syntax accepts a size argument in the form of |WxH[, perc%], where the WxH part appears always madatory. However, folks (read: I) occasionally want to only specify either the width/height (maintaining ratio, of course) or the percentage only. Will it be a good idea to relax these constraints?

Here’s a list of possible extensions, meant to duplicate the functionalities of MediaWiki’s image sizes:

  • WxH extensions
    • |W (e.g. ![foo|220]) can interpreted as a request to only add a width attribute to the img element generated.
    • |xH (e.g. ![foo|x200]) can interpreted as a request to only add a height attribute to the img element generated.
    • brainless scalng: |MWxH (e.g. ![foo|M200x200]) sets the maximum dimension of the image to 200x200px, but does not distort it when scaling. (Inline style?)
      • usage note: on Wikipedia this is most useful for fitting images into a box of a given size, centered and padded automatically. There aren’t many fixed-size boxes in discourse, so it might be more useful making it an img with margin/padding added to reach the target size. Sounds a bit too complicated though.
    • Most modern browsers does a proportional scaling with only one dimension given. Something along the lines of img { width: auto; height: auto } may be required for older stuff.
    • Only specifying one dimension obviously can cause abrupt layout changes as the image gets loaded. To avoid it, maybe auto-rewrite the expression into the full WxH form on save?
  • percent-only sizing (![|50%])
    • This is the same as [|WxH,perc%], where WxH are the real dimensions of the image.
    • Applying percentages to img's dimension attributes is awful, so this must get rewritten into the full WxH form.

Security considerations

  • Expanding the expression into a usual WxH form involves opening an image and reading its dimensions. If there’re some vulnerbilities in your image-handling libraries, you may be doomed.
  • Having the server fetch a remote image is a good way to expose the server IP, which might be hidden behind some sort of an anti-DDoS CDN. Maybe only rewrite the sizes on local imgs?
    • Discourse already fetches remote images to generate a metadata section.
2

No, because images that change unpredictably in dimension are anathema to infinite scroll.

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