 const x = c * r;
      const y = s * r + sy;
      const a = -Math.atan2(lx - x, ly - y);

      if (data.eyeToSurface) {
        ctx.fillStyle = colors.eyeToSurface;
        ctx.strokeStyle = colors.eyeToSurface;
        diagram.arrow(ctx, lx, ly, x, y, false, true, 0.5);

        ctx.save();
        ctx.translate(x, y);
        ctx.rotate(a);
        ctx.fillStyle = '#000';
        ctx.strokeStyle = '#000';
        diagram.arrow(ctx, 0, 5, 0, 40, true, false, 0.5);
        ctx.restore();
      }

      c