pixel shader fade towards edges

gistfile1.txt

float2 texelsize; // 1/width


bool isalpha(uv,offset)
{
// if bilinear sampling, then you might want to increase this rather than checking for pure 0
 return sample( uv + offset*texelsize ).w < 0.1;
}

float getdistancetoedge(uv,float radius)
{
// can increase samples to improve quality but bileanear filtering might do this for you
 float2 offsets[8] = 
 {
  float2(-1,-1), float2(0,-1), float2(1,-1),
  float2(-1,0), /*float2(0,0),*/ float2(1,0),
  float2(-1,1), float2(0,1), float2(1,1)
  }

float mindistance = 999;
for ( int i=0;i<8;i++ )
{
 float distance = radius * length(offsets[i]);
 bool alpha = isalpha( uv, offsets[i]*radius );
 if ( alpha )
  mindistance = min( distance, mindistance );
}
return mindistance;
}


frag()
{
 float distancetoedge = getdistancetoedge( uv, blurrange );
 distancetoedge = min( distancetoedge, 1 );
 rgba = sample( uv );
 rgba.w *= min( 1, range( distancetoedge, 0, 0.3 ) );
 return rgba;
 }