detect face and mouth using viola jones technique in computer vision tool box

a=imread('img.jpg');
MouthDetect = vision.CascadeObjectDetector('Mouth','MergeThreshold',16);
BB=step(MouthDetect,a);
figure,
imshow(I); hold on
for i = 1:size(BB,1)
rectangle('Position',BB(i,:),'LineWidth',4,'LineStyle','-','EdgeColor','r');
end
title('Mouth Detection');
hold off;