Noise filtering is a necessary step on image pre-processing stage in digital image processing.

在图像处理领域中噪声滤除是图像预处理阶段一项必不可少的工作。

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In order to improve the accuracy rate of recognition of handwritten number, the researchers have done a lot of work at the pre-processing stage, feature extraction and classification stage.

为了提高手写数字的识别率，研究者在预处理阶段、特征提取和分类识别阶段都提出了很多方法。

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