arr[k++] = arr[mid + (j++)];
You look at a region of the image. If all the pixels are roughly the same color (below some threshold), you store the average color for the whole region as a single value. If the pixels vary too much, you split the region into four quadrants and try again.。关于这个话题，服务器推荐提供了深入分析
Discussion on Hacker News，这一点在heLLoword翻译官方下载中也有详细论述
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