Large thumbnails undershoot the requested size

[ara4n]

If you request a larger thumbnail size than the largest precalculated thumbnail, you'll get the largest precalculated thumbnail, which could significantly undershoot the requested size.

This will end up looking horrible and blurry as the client then scales up the thumbnail to the size it asked for.

In Riot/Web (as of matrix-org/matrix-react-sdk#2439) we explicitly ask for the original image if the thumbnail exceeds 800x600, but we might do better to handle this more intelligently. Perhaps the solution is just to pregenerate even bigger thumbnails? Or default to serving the original image?

[ara4n]

see also #1222

[deepbluev7]

Is there any progress on this? I just hit that issue in nheko, which relies on the server thumbnailing a cropped image of 128x128 for some avatars. Some servers only return 96x96 cropped (since that is the recommendation in the spec). From reading the spec, specifically "Servers MUST NOT return a smaller thumbnail than requested, unless the original content makes that impossible.", I thought I would just get the original image in the worst case. I'd say the current behavior is a bug in synapse, but if I'm wrong, I will have to implement a fallback in nheko... The current behavior seems to be quite a pitfall for clients in any case.

[bwindels]

Note that to be able to expect this from all HS implementations, this would require a spec change, currently it says:

Servers SHOULD produce thumbnails with the following dimensions and methods:

32x32, crop
96x96, crop
320x240, scale
640x480, scale
800x600, scale

[reivilibre]

It seems very clear that the dimensions must match or exceed those asked for by the client unless the original is smaller (this is also mentioned in the description of the width and height query string parameters.

Servers must never return thumbnails smaller than the client's requested dimensions, unless the content being thumbnailed is smaller than the dimensions. When the content is smaller than the requested dimensions, servers should return the original content rather than thumbnail it.

(emphasis mine)

As such, this seems like a Synapse defect.