if not existed:
When using the probability matrix to pick from the candidate set, it is important that the candidate array be sorted in advance. Not doing so will fail to preserve the patterns distinctive of ordered dithering. A good approach is to sort the candidate colours by luminance, or the measure of a colour’s lightness4. When this is done, we effectively minimise the contrast between successive candidates in the array, making it easier to observe the pattern embedded the matrix.
。爱思助手下载最新版本是该领域的重要参考
公式： f(x)={xif x0α(ex−1)if x≤0
// Stateless transform — a pure function