corrected bug relating to size of _alpha
corrected bug relating to size of _alpha

sk1712/IRTK,ghisvail/irtk-legacy,BioMedIA/IRTK,ghisvail/irtk-legacy,BioMedIA/IRTK,sk1712/IRTK,BioMedIA/IRTK,ghisvail/irtk-legacy,BioMedIA/irtk-legacy,BioMedIA/irtk-legacy,sk1712/IRTK,BioMedIA/irtk-legacy,ghisvail/irtk-legacy,BioMedIA/irtk-legacy,sk1712/IRTK,BioMedIA/IRTK