transforms image from RGB to CMY

def rgb2cmy(self, img, whitebg=False):
        """transforms image from RGB to CMY"""
        tmp = img*1.0
        if whitebg:
            tmp = (1.0 - (img - img.min())/(img.max() - img.min()))
        out = tmp*0.0
        out[:,:,0] = (tmp[:,:,1] + tmp[:,:,2])/2.0
        out[:,:,1] = (tmp[:,:,0] + tmp[:,:,2])/2.0
        out[:,:,2] = (tmp[:,:,0] + tmp[:,:,1])/2.0
        return out