1. An image recording system comprising: a vector data processor; a RIP part that converts data output from said vector data processor into variable raster data; a raster data composition part that combines fixed raster data that is raster data prepared in advance and said variable raster data to generate composite raster data; and an image recording apparatus that records an image to a recording medium in accordance with said composite raster data, wherein said vector data processor comprises; a thinning part that processes target vector data indicating an outline of a target figure that includes a linear figure having a thickness, to acquire thinned data that is vector data indicating a thinned figure obtained by thinning said target figure by an amount of thinning; a central line acquiring part that processes said target vector data to acquire central line data that is vector data indicating at least part of a central line of said target figure; and a superimposing part that generates data to be input to said RIP part, said data including an instruction to record a combined figure obtained by superimposing said thinned figure and said central line one on the other.