color model conversion question

hi, is there any easy way to convert from HSV->HLS and vice-verse without going through HSV->RGB->HLS. i read the book "computer graphics-Foley, VanDam". they gave an algorithm to convert between RGB and HLS/HSV (and also the other way round). but going to HLS->HSV i need an intermediate RGB representation. may be i'm wrong. so plz help.