Apply correct colourspace conversion when generating on-the-fly preview

[theefer]

Not sure how to do this as it's a different conversion path (via nginx module), but let's see if we can think of a way.

See also #784

[paperboyo]

Unfortunately, depends on libgd/libgd#136 (https://bitbucket.org/libgd/gd-libgd/issues/136) :sadpanda:

[paperboyo]

Hmm, I wouldn’t know, but is this a possible replacement? ngx_http_image_filter_module not only messes up colour, but also chokes on big images. AWS alternative uses sharp which in turn uses libvips which we wanted to have a look at separately as a replacement for GM. From what I can tell, it supports colour management. And promises to be super-fast.

[paperboyo]

@akash1810 Because Firefox now supports WebP and because libgd/libgd#136 (comment), maybe we could ask for WebPs? This might have an additional advantage of not having to use optimisedPng intermediary asset for cutouts. WDYT?

[paperboyo]

It looks like we would need another nginx module: pagespeed.

Although they say that any conversion/resize removes colour profile. Let’s hope they have no idea what are they talking about (which a non-mention of profile conversion prior to profile removal luckily suggests) and that the above comment still holds.

[paperboyo]

Not sure, but maybe https://github.com/imazen/imageflow can help here? Or in #155. Or – both?

[paperboyo]

Another one based on VIPS: https://docs.imgproxy.net/#/generating_the_url_advanced?id=strip-color-profile

[paperboyo]

Also VIPS? https://github.com/weserv/images

[paperboyo]

Also VIPS? https://github.com/h2non/imaginary & https://github.com/humanmade/tachyon