def _control_sample(self, sample):
        ''' Control the asked sample is ok '''
        if sample > float(self.SAMPLE_LAST_PIXEL):
            return int(self.SAMPLE_LAST_PIXEL)
        elif sample < float(self.SAMPLE_FIRST_PIXEL):
            return int(self.SAMPLE_FIRST_PIXEL)
        else:
            return sample
Control the asked sample is ok