(Ted Harding) ted.harding@nessie.mcc.ac.uk wrote: [...]
Yet, for example, one 75% file has 171064 bytes, and its 100% counterpart has 694833 bytes, 4 times larger.
At that difference in byte size, I'm surprised that there's no perceptible difference!
Any ideas?
JPEG uses a discrete cosine transform so that the early waveforms account for as much of the picture data as possible. As you move closer and closer to 100%, each extra wave adds less and less visible.
Secondly, the quality scale is neither linear or standardised.
Thirdly, screens might not show all the detail in the picture. When I got a better screen a few years ago, I was amazed how many ripples I noticed in images. Also, decoder performance varies massively. Never use JPEG on the web if hard edges are important, like on most company logos. It's great for photos, though.
Finally, the JPEG FAQ is online: http://www.faqs.org/faqs/jpeg-faq/ or there's more detail in Part 2, question 75 of the comp.compression FAQ http://www.faqs.org/faqs/compression-faq/
Hope that helps,