Quote:
Originally Posted by MattUK
I'm intrigued. How would you accomplish this?
Any JPG outputting software I use asks me for a quality setting when I save as JPG. As such, it automatically re-calculates the JPG compression when you save, regardless of whether you changed anything or not.
|
I suppose one way to try to describe it is it's almost like the JPEG algorithm is slightly sentient (ooh err) but think complete idiot capable of one thing rather than SkyNet...
If you open a JPEG file and re-save it with the quality at 100 (or compression at it's lowerst, whichever way round you want to think of it) the JPEG algorithm reads the MCUs and sees an intact, functional, unaltered DCT. If it can read and use the original quantisation tables, which it should be able to do if the DCT is functional, it won't re-write them. If you don't re-write the table, you don't scan. If you don't scan, you don't chuck data.
There is a massive caveat to this though...
The quantisation tables used are different for just about every single camera and program you can think of. For the tables to be functional you have to have the RIGHT table.
The practical upshot of this is that if you open a JPEG from your camera in Photoshop and save it, you will get a tiny loss of data (the tables aren't functional as the camera table doesn't match that of Photoshop).
Open the same JPEG file again in Photoshop and just save it and you won't get a loss as you've got the correct format of table for that particular run of the JPEG algorithm...
Quote:
Originally Posted by MattUK
I'm not being argumentative, I'm just curious  |
Oh... and there was me looking for another good argument!
