bias1にしてコンボルーションフィルタの中心を-1
<filter id="filter" x="-20%" y="-20%" width="140%" height="140%" filterUnits="objectBoundingBox" primitiveUnits="userSpaceOnUse" color-interpolation-filters="linearRGB"> <feConvolveMatrix order="1 1" kernelMatrix="-1" divisor="1" bias="1" targetX="0" targetY="0" edgeMode="duplicate" preserveAlpha="true" x="0%" y="0%" width="100%" height="100%" in="SourceGraphic" result="convolveMatrix"/> </filter>
