1. A method for rasterizing a primitive of an image, comprising: dividing the primitive at a position where a color discontinuously changes into a plurality of sub-primitives with continuously changing colors; and rasterizing each of the sub-primitives, wherein the step of rasterizing further comprises: setting respective resolution reduction coefficient nBlockSize for each sub-primitive in accordance with a ratio of the color change to the spatial size of each sub-primitive; reducing the resolution of each sub-primitive based on the respective resolution reduction coefficient; rasterizing each sub-primitive with the reduced resolution; and increasing the resolution of the rasterized sub-primitive to the original resolution, and wherein the step of setting respective resolution reduction coefficient further comprises: taking the maximum value max (d(s i )/d(l)) as a quantized value for the color change, wherein (d(s i )/d(l)) is a derivative with resect to a spatial length l for the spatial color function of the sub-primitive in each color field, where s i is a spatial color function in i th color field; and setting the resolution reduction coefficient of the sub-primitive nBlockSize based on the quantized value max (d(s i )/d(l)) as follows: 
 n BlockSize= L d /( S max /max( d ( s i )/ d ( l ))), where L d is a spatial length in the radial direction of the sub-primitive, and S max is the maximum color difference in the range of the sub-primitive.