1. An image processing device for processing an image comprising a plurality of pixels, each pixel having a pixel value, the image processing device comprising: a memory for storing a part of the pixel values; a pixel enhancing circuit, coupled to the memory, for performing an edge enhancement operation on the pixel values to generate a result; a first filtering circuit, coupled to the memory, for performing a first filtering process on the pixel values according to a first filter coefficient to generate a first filtered result; a second filtering circuit, coupled to the pixel enhancing circuit and the first filtering circuit, for performing a second filtering process according to a second filter coefficient, the result and the first filtered result and outputting a first intermediate value; and a contrast adjusting circuit, coupled to the second filtering circuit, for adjusting a contrast of the image according to a contrast adjusting parameter to generate a contrast adjusted result and generating an output value according to the first intermediate value and the contrast adjusted result; wherein, the first filter coefficient, the second filter coefficient and the contrast adjusting parameter can be adjusted independently; wherein the contrast adjusting circuit further determines whether the differences among a part of the first intermediate values meet an edge condition, wherein the part of the first intermediate values corresponds to pixels arranged along the same direction perpendicular to an edge direction of the image.