如何在用户向下移动iphone / ipad相机时自动捕捉图像，以制作像照片一样的全景图？

I have an app where i scan the forms means i take its pictures and upload them . Now if the form is long so that the iphone /ipad camera could not take picture of entire form clearly. In that case i want the user to slowly move the camera down , so that continous images are taken until the whole form is looked or scanned . After that create a picture stitching all the frames together. I know there can be various ways of doing this manually like i can let user chose when to take pictures ad after merge them together.I have an app where i scan the forms means i ta