int dataSize = 11;
int imgSize = 200;
vec3 orig = [0.0, 0.0, 5.0];
vec3 cVec = [0.0, 1.0, 0.0];
vec3 rVec = [1.0, 0.0, 0.0];
image(3)[] img = load("../data/zimg11.nrrd");
field#2(3)[] F = img ⊛ bspln3;

strand RayCast (int row, int col)
{
    real rr = lerp(-0.5, real(dataSize)-0.5, -0.5, real(row), real(imgSize)-0.5);
    real cc = lerp(-0.5, real(dataSize)-0.5, -0.5, real(col), real(imgSize)-0.5);
    vec3 pos = orig + rr*rVec + cc*cVec;
    output real val = 0.0; 

    update
    {
        if (inside (pos,F)) {
	    tensor[3,3] H = ∇(∇F)@pos;
//	    val = trace(∇(∇F)@pos);
	    val = H[0,0] + H[1,1] + H[2,2];  // test subscripting
	}
        stabilize;
    }
}

initially [ RayCast(r, c) | r in 0..(imgSize-1), c in 0..(imgSize-1) ];