1. An image processing apparatus, comprising: a first memory, for storing an original image having a first width; a second memory; a buffer, having a second width smaller than the first width; a fetching module, for fetching a sub-image of the original image from the first memory and storing the sub-image into the buffer; wherein a width of the sub-image is smaller than or equal to the second width; and a processing module, for performing an image processing process on the sub-image image to generate a processed sub-image, and storing the processed sub-image into the second memory.