text: function capture(source_index)
  {
      source_index=Math.floor(source_index);    
      var v=this.video_source(source_index);
      
      // make sure the video has adapted to the capture source
      if(!v || v.currentTime==0 || !v.videoWidth) return this; 
      
      if(!this._.videoTexture) this._.videoTexture=this.texture(v);    
      this._.videoTexture.loadContentsOf(v);
      this.draw(this._.videoTexture);
          
      return this;
  }
  