1. An image processing device comprising: a processor; and a memory storing computer-readable instructions therein, the computer-readable instructions, when executed by the processor, causing the image processing device to perform: acquiring target image data including each pixel value of a plurality of pixels, the target image data representing a target image by using the plurality of the pixels; extracting a plurality of edge pixels and a plurality of non-edge pixels from the plurality of the pixels in the target image; determining a first reference value by using the plurality of edge pixels without using the plurality of non-edge pixels and a second reference value by using the plurality of non-edge pixels without using the plurality of edge pixels, the second reference value being greater than the first reference value; generating correspondence data by using the first reference value and the second reference value; and converting, using the correspondence data, a plurality of original pixel values of the plurality of pixels into respective converted pixel values, the correspondence data including a first feature used to convert each of the original pixel values smaller than or equal to the first reference value, a second feature used to convert each of the original pixel values greater than the first reference value and smaller than the second reference value, and a third feature used to convert each of the original pixel values greater than or equal to the second reference value, the first feature, the second feature, and the third feature being different from each other; wherein the first feature is used to convert each of the original pixel values into the respective converted pixel values having the same values as each of the original pixel values, wherein the second feature is used to convert each of the original pixel values into each of the converted pixel values such that the converted pixel values increase monotonically, and wherein the third feature is used to convert each of the original pixel values into the respective converted pixel values all having a same value.