在Matlab中将边界框映射到原始图像

i have a real time skin detection algorithm where it gives me a bounding box around the skin region with rectangle('Position',bb,'EdgeColor','r','LineWidth',2) from an original image. I wish to use the code to first detect the skin region from the original image before i use Viola Jones to detect the face region from the cropped skin region. I wish to know after i crop the skin region then use the face detection algorithm to detect the face, how can i map the bounding box of the face to the original image.i have a real time skin detection algorithm whe