1. An image processing method applied in an image output device for reducing jaggy effect, comprising the steps of: generating a continuous tone image having a plurality of continuous tone pixels, the co-ordinates of the continuous tone pixels being defined according to a first addressing space with each of the continuous tone pixels having a continuous image density and one of the continuous tone pixels being I(i, j); halftone-processing the continuous tone image to generate a halftone image, which has a plurality of halftone pixels corresponding to the continuous tone pixels, the halftone pixels being at an ON state or an OFF state, while the halftone pixel HT(i, j) corresponding to continuous tone pixel J(i, j); low-pass filtering the continuous tone image to generate a low-pass filtered image having a plurality of low-pass filtered pixels corresponding to the continuous tone pixels, a low-pass filtered pixel B(i, j) corresponding to the continuous tone pixel (i, j), and each of the low-pass filtered pixels having a low-pass filtered image density; determining whether a position of the low-pass filtered image whose density is approximate to a pre-determined density can be obtained in the low-pass filtered pixel B(i,j): if so, define the position according to a second addressing space to obtain a set of second addressing space co-ordinates (Vr, Vc) of a key point KP(i, j) corresponding to the positions, wherein the step of determining whether a position of the low-pass filtered image whose density is approximate to a pre-determined density can be obtained in the low-pass filtered pixel B(i, j) further comprises: (a1) calculating plural row averages of the low-pass filtered image densities of plural rows of the low-pass filtered pixels disposed at a corresponding local region of the low-pass filtered pixel B(i, j) as well as plural column averages of the low-pass filtered image densities of plural columns of the low-pass filtered pixels disposed at a corresponding local region of the low-pass filtered pixel B(i, j); (a2) interpolating a second addressing space row co-ordinate Vr 0 whose low-pass filtered image density is approximate to the pre-determined value according to the row averages, and if the second addressing space row co-ordinate Vr 0 is within the corresponding row region of the low-pass filtered pixel B(i, j), then proceed to step (a3), otherwise, proceed to (a4); (a3) determining whether the corresponding low-pass filtered image density column average of the low-pass filtered pixel B(i, j) is the median of the column averages: if yes, set MarkR(i, j), a row mark value of the low-pass filtered pixel B(i, j), to be a first value; otherwise, set MarkR(i, j) to be a second value; (a4) interpolation a second addressing space column co-ordinate Vc 0 whose low-pass filtered image density is approximate to the pre-determined value according to the column averages, and if the second addressing space column co-ordinate Vc 0 is within the corresponding column region of the low-pass filtered pixel B(i, j), then proceed to step (a5), otherwise, proceed to (a6); (a5) determining whether the corresponding low-pass filtered image density row average of the low-pass filtered pixel B(i, j) is the median of the row averages: if yes, set MarkC(i, j), a column mark value of the low-pass filtered pixel B(i, j), to be the first value; otherwise, set MarkC(i, j) to be the second value (a6) recording the second addressing space row co-ordinate Vr 0 and the second addressing space column co-ordinate Vc 0 , and setting the second addressing space co-ordinates (Vr 0 , Vc 0 ) to be the key point KP(i, j); determining whether the key point KP(i, j) corresponds to a convex image region or a concave image region according to the relationship between the key point KP(i, j) and at least two of its adjacent key points, obtain a moving vector corresponding to the key point, and regulate the position of a display point of the halftone pixel HT(i, j) according to the moving vector; and outputting the display point according to the position after regulation.