ShaderMaterial that only renders shadow

shadowMaterial.js

// Minimal version of MeshBasicMaterial
// But removed everything except shadow
// then only render the shadow

var shadowMaterial = {
    transparent: true,
    uniforms: THREE.UniformsUtils.merge([
        THREE.UniformsLib['common'],
        THREE.UniformsLib['shadowmap']
    ]),

    vertexShader: [
        THREE.ShaderChunk['shadowmap_pars_vertex'],
        'void main() {',
            THREE.ShaderChunk['default_vertex'],
            THREE.ShaderChunk['worldpos_vertex'],
            THREE.ShaderChunk['shadowmap_vertex'],
        '}'
    ].join('\n'),

    fragmentShader: [
        'uniform vec3 diffuse;',
        'uniform float opacity;',
        THREE.ShaderChunk['shadowmap_pars_fragment'],

        'void main() {',
        '   gl_FragColor = vec4( diffuse, opacity );',
            THREE.ShaderChunk['shadowmap_fragment'],
        '   if (gl_FragColor.r == diffuse.r) discard;',
        '   gl_FragColor = vec4(vec3(0.0), opacity);',
        '}'
   ].join('\n')
};

// And the mesh

var mesh = new THREE.Mesh(
    new THREE.PlaneBufferGeometry(128, 128),
    new THREE.ShaderMaterial(shadowShader)
);

// Changing color and opacity...
mesh.material.uniforms.diffuse.value = new THREE.Color(0xff00ff);
mesh.material.uniforms.opacity.value = 0.05;
mesh.material.needsUpdate = true;

// Don't forget this one
mesh.receiveShadow = true;