Bryan Bentley

int prim;
vector uv;
vector hitPos;
vector dispDir;
float dispSwtich;
float dispMag;

// origP is the rasterized low poly world positions
//dispMag is the length to the hit
dispMag = xyzdist(1, v@origP, prim, uv);

//get position on high poly mesh
hitPos = primuv(1, "P", prim, uv);

//get the normalised direction to the high polygon mesh
dispDir = normalize(hitPos-v@origP);

// check the direction against the rasterized low polygon normal
dispSwtich = dot(v@N, dispDir);

//if the direction is opposite the normal direction reverse the displacement value
if(dispSwtich<0){
    dispMag *=-1;

}

@disp = dispMag;

float incrementX = (1.0/$XRES);
float incrementY = (1.0/$YRES);

float offsetX = incrementX * 0.5;
float offsetY = incrementY * 0.5;

vector udimoffset = detail($LOW, "__udimoffset", 0);

vector WorldN = $vec;
vector MyN = uvsample($LOW, "N", "uv", set((($IX+1)*incrementX - offsetX) + floor(udimoffset.x), (($IY+1)*incrementY - offsetY)+ floor(udimoffset.y), 0));
vector lowpos = uvsample($LOW, "P", "uv", set((($IX+1)*incrementX - offsetX) + floor(udimoffset.x), (($IY+1)*incrementY - offsetY)+ floor(udimoffset.y), 0));

$DISP = distance($pos, lowpos) * $Alpha;
$DISP = ($Alpha == 0.0) ? 0 : $DISP;

if (dot(normalize(lowpos-$pos), MyN) >= 0) {
    $DISP *= -1;
}

if ($RemapHeight == 1)
    $DISP = fit($DISP, $HeightRange.x, $HeightRange.y, 0, 1);