I notice that WPT reports some images as requiring further compression.
Using Imagick I re-compress at “70” and for most images I achieve the WPT compression target.
However, a few images still fail or get warnings that the target has not been achieved, for example:
FAILED - (62.2 KB, compressed = 28.2 KB - savings of 34.0 KB)
WARNING - (15.7 KB, compressed = 11.4 KB - savings of 4.3 KB)
If I then re-compress the same images at “20”, a very low compression, the images still don’t hit the target, because the target size has moved!
FAILED - (47.3 KB, compressed = 13.7 KB - savings of 33.6 KB)
WARNING - (10.1 KB, compressed = 6.2 KB - savings of 3.8 KB)
I see that whilst the image dimensions have not changed the expected target compression has moved from
_small.jpg 28.2KB > 13.7KB
_cover_thumb.jpg 11.4KB > 6.2KB
After processing over 2000 images cut to different sizes, the above 2 formats of 480x306 and 320x180 consistently result in “FAILED” or “WARNING” respectively.
Whilst I understand that there are other factors to take into account during compression, I still expect the compression target to remain roughly the same based on image dimensions, and if not, then at least a compression of “20” should be enough to satisfy the target value.
Are there any other approaches I should take to achieve the compression targets for these image dimensions ?
Is there a bug in the given target values ?
p.s. each time an image is cut/compressed it is done automatically from the same original image during upload to the site.