int outSize = 201;

field#1(2)[] F = ctmr ⊛ load("../../data/ddro-115.nrrd");

strand sample (int ui, int vi) {
   real uu = lerp(0.0, 1.0, -0.5, real(ui), real(outSize)-0.5);
   real vv = lerp(0.0, 1.0, -0.5, real(vi), real(outSize)-0.5);
   vec2 pos = [uu, vv];
   output vec3 out = [nan,nan,nan];

   update {
      vec2 grad = ∇F(pos) if (inside(pos,F)) 
                          else [2000.0,0.0];
      out = [grad[0], grad[1], |grad|];
      stabilize;
   }
}

initially [ sample(ui, vi) | vi in 0..(outSize-1), ui in 0..(outSize-1) ];