I've picked a featured image of today for a small demonstration:
$ du -h Mansudae-Monument-Bow-2014.* | sort -n 2,0M Mansudae-Monument-Bow-2014.jpg 4,9M Mansudae-Monument-Bow-2014..progressiv.jpg * 6,5M Mansudae-Monument-Bow-2014.flif 16M Mansudae-Monument-Bow-2014.png
*(saved again with Gimp as progressive JPG with Quality 100%)
I cut it to 50 KiB (the desired size for a thumbnail of around 100px).
$ dd if=$if of=$of bs=1024 count=50
$ du -h Mansudae-Monument-Bow-2014.50KiB.* | sort -n 52K Mansudae-Monument-Bow-2014.50KiB.flif 52K Mansudae-Monument-Bow-2014.50KiB.jpg 52K Mansudae-Monument-Bow-2014.50KiB.png
The original image[1] in contrast to the 50 KiB FLIF[2] which outperforms progressive JPG[3] as well as interlaced PNG[4].
And I did another test with 250 KB to simulate a further download of the file:
Those tests show clearly an advantage of FLIF at this time since only JPG progressive has any chance of competing fair... which means we still have to rely on generation loss and lossy compression.
FLIF, on the other hand, does not have any (heavy) generation loss, as WebP, JPG or JPG2000 do have on lossy compression.
FLIF is also able to provide a 3.77% Download a 200px width thumbnail with very low distortion and with just 0.75% a 100 px width thumbnail with very low distortion. While the main competitor (PNG) still shows a black screen.
FLIF in comparison also provides the ability to store motion-pictures like GIF or APNG which is still not widely supported for PNG.
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Hope this helps a bit without extensive own testing to get an impression of the capabilities of this file format.
Best regards
Ruben