howto exec('nice convert ...')?

[GGallery]

that's more of a general php question:
I'd like coppermine to 'nice' the exec('convert ..') part in include/picmgmt.inc.php.

Right now it's being executed with

                $cmd = "{$CONFIG['impath']}convert -quality ...
                exec ($cmd, $output, $retval);

how can I modify the code so that the cpu consuming convert is done with a nice value=10?

there is proc_nice which can only increase the nice value but not decrease (unless you're root)