This invention discloses a transforming apparatus, according to a transforming curve, for transforming a first image to a second image. The first image includes a plurality of pixels. The transforming apparatus includes a differentiating module, a first transforming module, an adjusting module and a second transforming module. According a predetermined value, the differentiating module transforms the transforming curve into a plurality of sub-curves. The first transforming module generates a plurality of straight lines corresponding to the sub-curves. According to the sub-curves, the straight lines and a predetermined threshold, the adjusting module adjusts the straight lines to generate a plurality of approximate lines. According to the approximate lines and the pixels of the first image, the second transforming module generates the second image.