1. An image processing apparatus, capable of stitching two overlapped input images having an overlapped area into an output image, comprising: a first registration device configured to perform, by taking a first input image of the two overlapped input images as a reference image, a first registration on a second input image of the two overlapped input images, to find, in the second input image, a second pixel which is matched with each first pixel located in the overlapped area of the reference image; an output pixel location determination device configured to calculate, based on the locations of the first and second pixels, a location of an output pixel which is located in the overlapped area of the output image and corresponds to the first pixel, wherein the locations of the first and second pixels are respectively weighted, and the shorter the distance from the first pixel to a non-overlapped area of the reference image is, the greater a weight of the location of the first pixel is, and the smaller a weight of the location of the second pixel is; and an output pixel value determination device configured to calculate a pixel value of the output pixel according to a pixel value of at least one of the first and second pixels.