I get results that I hardly understand about image compression :
But nor Jpegtran nor ImageMagick does allow better compression of this image:
Original : 50.497 bytes
jpegtran -copy none -optimize : 53.244 bytes
jpegtran -copy none -progressive : 50.497 bytes
convert -strip : 53.273 bytes
convert -strip -interlace Plane : 50.550 bytes
I made the same constation on a lot of others images…
Could somebody tell me what tool could be used to obtain the gain displayed par the result page (about 20Kb !!!) ?
Try http://jpegmini.com By far the best jpeg compression tool ever.
You’re right, it’s pretty good! But maybe I should rephrase my question:
How WebPageTest can make such an assessment (ie, state that a gain of 40% is achievable as well known tools as jpegtrans, imagemagick can not even win one byte)?
It should be based on some specific tool/library that would be very interesting to know in order to try to achieve the above result (unless WebPageTest use JPEGMini? :)).
jpegtrans and imagemagick were trying to do lossless optimizations (stripping exif data, etc). WebPagetest checks jpegs for lossy savings by recompressing them at a quality level of 85 using libjpeg which is actually pretty conservative. Hand-tuning you should actually be able to get significantly better than that.
All is clear now, thanks for your answer!