svg – Blur Filters: Box Blur

<svg width="900px" height="400px" viewBox="900 400">
      <filter id="GaussianHardEdge" >
         <feConvolveMatrix order="3" kernelMatrix=" 1 1 1
                                                    1 1 1
                                                    1 1 1"/>
   <image xlink:href="" x="20px" y="20px" width="300px" height="200px" preserveAspectRatio="xMinYMin meet" />
   <image filter="url(#GaussianHardEdge)" xlink:href="" x="340px" y="20px" width="300px" height="200px" preserveAspectRatio="xMinYMin meet"/>

enter image description here

(Source image by Daderot at Wikimedia Commons)

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svg – Blur Filters: Box Blur - CodeDay