shadow on transparent plane ThreeJS r76

frag.vert

uniform vec3 diffuse;
uniform vec3 emissive;
uniform float opacity;

varying vec3 vLightFront;

#ifdef DOUBLE_SIDED

  varying vec3 vLightBack;

#endif

#include <common>
#include <packing>
#include <lights_pars>
#include <shadowmap_pars_fragment>
#include <shadowmask_pars_fragment>

void main() {
  gl_FragColor = vec4(vec3(0.0), 1.0 - getShadowMask());
}