1. An edge-oriented interpolation method for a digital image, comprising the steps of: identifying each destination pixel, with a region determination circuit, as being located in a smooth region, a text region or an edge region of the digital image; performing Sobel edge detection, with a Sobel edge detector, for two neighboring pixels around the destination pixel to obtain two Sobel gradients; performing cubic convolution interpolation, with an interpolator, for the destination pixel with a first slope when the destination pixel is identified to be located in a smooth region; performing cubic convolution interpolation, with the interpolator, for the destination pixel with a second slope when the destination pixel is identified to be located in a text region, wherein the second slope is different from the first slope; and performing Sobel-based interpolation, with the interpolator, for the destination pixel with reference to the two Sobel gradients when the destination pixel is identified to be located in an edge region.