screenToWorldAtZ.js

function screenToWorldAtZ(positionX, positionY, z, camera){
    var vector = new THREE.Vector3();

    var dX, dY, dZ;
    if(this.curObject && this.curObject.parent ){
        dX = this.curObject.parent.position.x;
        dY = this.curObject.parent.position.y;
        dZ = this.curObject.parent.position.z;
    }else{
        dX = 0; dY = 0, dZ = 0;
    }

    vector.set(
        positionX,
        positionY,
        0.5 );

    
    camera.updateProjectionMatrix();
    camera.updateMatrixWorld();

    vector.unproject( camera );
    var dir = vector.sub( camera.position ).normalize();
    var distance = (z- camera.position.z) / dir.z;
    var pos = camera.position.clone().add( dir.multiplyScalar( distance ) );

    pos.x -= dX;
    pos.y -= dY;
    pos.z -= dZ;

    return pos;
}