I'm wondering if there are any studies - or even anecdotal evidence - on what the acceptable resolution of images is on high-resolution screens (e.g. phones and tablets).
Obviously, an image that's only 50% of a display's native resolution will look noticeably upscaled and blurry, and one that's 100% or above will look as crisp as possible.
But between those two cases, there are diminishing returns when increasing the image's resolution. An image that's 98% of native resolution will still look pretty darn crisp, despite the minor upscaling.
(Granted, the image's motif matters: Line art will likely show signs of scaling much more readily than a nature photograph or portrait.)
In my case, I'm dealing with an iOS/Android app that needs to show a lot of thumbnails and full-size images (all photographs; no line art), which are downloaded from a server. Obviously, the less there is to download the better, and the fewer separate resolutions the server has to prepare the better.
The server can of course just prepare, say, 4 different resolutions of an image, and just serve whichever resolution is exactly as large or larger than the native resolution of the client (the apps report their native resolution to the server when fetching content, so the server decides what resolution to send back).
But intentionally serving too-large images to mobile clients seems pretty inelegant and increases load times. What I'd like to be able to do is have the server send images that are at least X% of native resolution, where X% is whatever's "acceptable".
So I'm wondering if there's a rule of any sort for what is considered "near-enough" to native resolution. For instance, if an image is only 80% of native resolution, will it be noticeable enough to hurt the user experience? Can it be smaller? Should it be larger?
I'm well aware that it may be dependent on way too many factors to have a hard and fast rule, but I'm curious if anyone's tested it.