Example
<svg width="800px" height="600px" >
<defs>
<filter id="posterize" color-interpolation-filters="sRGB">
<feComponentTransfer>
<feFuncR type="discrete" tableValues="0 0.25 0.75 1.0"/>
<feFuncG type="discrete" tableValues="0 0.25 0.75 1.0"/>
<feFuncB type="discrete" tableValues="0 0.25 0.75 1.0"/>
</feComponentTransfer>
</filter>
</defs>
<image xlink:href="https://upload.wikimedia.org/wikipedia/commons/4/42/Andy_Warhol_1975.jpg" x="20px" y="20px" width="300px" height="600px" preserveAspectRatio="xMinYMin meet" />
<image filter="url(#posterize)" xlink:href="https://upload.wikimedia.org/wikipedia/commons/4/42/Andy_Warhol_1975.jpg" x="340px" y="20px" width="300px" height="600px" preserveAspectRatio="xMinYMin meet"/>
</svg>
