Shadow map PCF vs. Variance filter

PCF(Percentage closer filer): Because texture precision is not enough to confirm lossless. Therefore, we can reference the neighbor pattern. I use four corner texel( left-top, right-top, left-bottom, right-bottom) and the current texel.
                           float2 texCoord;
                           float4 p;
                           texCoord.x = shadowPos.x + 1/shadowMapSize;
                           texCoord.y = shadowPos.y - 1/shadowMapSize;
                           p.x = tex( sm, texCoord );  // left-top

                           texCoord.xy = shadowPos.xy + 1/shadowMapSize;
                           p.y = tex( sm, texCoord );  // right-top

                           texCoord.xy = shadowPos.xy - 1/shadowMapSize;
                           p.z = tex( sm, texCoord );  // left-bottom

                           texCoord.x = shadowPos.x + 1/shadowMapSize;
                           texCoord.y = shadowPos.y - 1/shadowMapSize;
                           p.w = tex( sm, texCoord );  // right-bottom

                           float4 resultCorn = ( shadow.z > p ) ? 1 : 0;
                           float2 result;
                           result.x = dotProduct4( result, 0.25 );

                           texCoord.xy = shadowPos.xy;
                           p.x = tex( sm, texCoord );  // center

                           result.y = ( shadow.z > p.x ) ? 1 : 0;
                           float finalResult = dotProduct( result.xxyy, 0.25 );

Variance: We need two shadow map, individually they are record clip-depth and square of clip-depth.