Lack of support for Pixel Aspect Ratio (PAR) for images

[sdsnatcher]

I talked with Ian Hickson about this issue, and he directed me here.

I would like to bring to your attention that the Pixel Aspect Ratio (PAR) is still not rendered correctly by any of the current browsers I tested. This is a metadata present inside JPG, PNG and GIF files.

A lot of content was created in the 80's and 90's that had non-square pixels.

For example, many legacy pixel art based on non-square pixels (*1) are being destroyed, as the use case is still present and the users are doing workarounds by artificially stretching the images via resize to have the desired aspect ratio. Once the image is stretched like this, it's nearly impossible to restore it.

The same issue affects all images (stills and video) captured from SDTV. NTSC had an effective resolution of 720x480, which means that it's pixels aren't square. Just like pixel art, users have no alternative other than artificially stretch the image to adjust it for square pixels, creating artifacts that will be impossible to cleanup later.

Inspired by the HTML AcidTests, I created the Pixel Aspect Ratio Acid Test, that can be seen in the link below. It will check if the browser renders two sets of images correctly.

http://frs.badcoffee.info/PAR_AcidTest/

*1: Read C64, CGA, EGA, VGA (320x200), SNES, MSX, Amiga, PC88 and nearly the majority of devices from the 80s, when square pixels were still a luxury.

[annevk]

I suspect we cannot support this without an opt-in attribute as it might be that there's content out there that relies on this metadata being ignored.

cc @svgeesus @tabatkins

[sdsnatcher]

Humm. Albeit not prevalent, since cameras and image editors set the PAR correctly (or leave it unset) there might be some cases here and there with images generated by unusual tools.

But here's some food for thought:

Do you remember when the HTML AcidTests where released? A lot of content relied on distorted rendering done by one specific browser or another. There were even websites that explicitly required the use of that specific browser, so they could be "correctly" viewed.

I.e.: Photoshop is one application that forces the user to set the PAR correctly. It always render the images with the PAR informed by the file, unless the user specifically disable that option per image.

[annevk]

I don't remember the first Acid test, but I do remember that for subsequent tests the test sometimes changed because we found out what the test was asking for could not be shipped in browsers. In any event, it seems like this is a thing that browsers could measure the impact of, if there's sufficient interest.

[sdsnatcher]

Ok, I just wanted to raise the awareness about the issue.

There are some more modern use cases for the PAR that might get people interested. For example, it can be used as a form of lossy compression for animated GIFs, while still keeping a decent perceived image quality.

Also, subpixel rendering support for images (and not just fonts) becomes viable.

[annevk]

If you're still interested in pursuing this I recommend raising this with the https://wicg.io/ community. They might be able to help turn it into a proper proposal.