I'm working with the ImageJpeg function in PHP.

This function is documented on the PHP website here: http://www.php.net/manual/en/function.imagejpeg.php

and says that the 'quality' setting can be set in a range of values from 0 (max compression=worst quality) to 100 (no compression, max quality).

However, I've seen the quality set to -1 (but I've lost where I saw this). I've experimented myself, and it seems to produce acceptable results. But can anyone tell me what -1 is actually doing?