Third, the gray-level histogram of image is equalized in order to reduce the influence of uneven light to the gray-level image.

第三步，为了减轻光线不均匀对灰度图像造成的影响，对产品图像进行灰度直方图均衡；

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At first, the image background is suppressed by setting a threshold. Then, both gray density and gray interval of the histogram are equalized.

该算法首先通过设置平台阈值对背景的增强进行抑制，然后将直方图在灰度密度和灰度间距两个方向进行均衡处理。

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