[StickyC]

Right now, when you grab metadata for a book, it'll search the various sources given for alternative covers. The results may or may not be a good match and may or may not be larger/better than the existing cover. The covers are sorted by size, so oftentimes a higher quality/size replacement is not the first on the list. This makes looking for larger covers require a lot of cognitive overhead.

While that part of Calibre that normally handles downloading/choosing covers is probably not really 'touchable' code (is it?). It shouldn't be too difficult to write a plugin that:

- Downloads alternative book covers for a book (probably need to throttle the requests to avoid getting blocked)
- Rejects any candidates smaller than the existing cover
- Generates a histogram of the existing cover image and remaining candidates
- Automatically replaces the existing cover with the downloaded match that is larger/better, or at least provides a list sorted by histogram similarity so that a likely better cover is shown first.

I have not actually tried doing cover comparisons this way, so I have no idea how effective it would actually be. I do know that a lot of "Duplicate image finder" apps use histograms to compare identical pics of different sizes.

Ideally, it'd be something you could run against a library or individual book with options for minimum size to seek replacements and/or minimum size increase and, of course, the ability to accept/decline. Possibly even a 'minimum match' for comparing histograms.

I've not looked into what it takes to interface with Calibre, but the histogram comparison element seems like it'd be fairly easy to implement.