Centering tilingSprite whilst also scaling

Hi, I've experimented, googled and read the API and I still can't figure this out even though it is extremely simple

I want my image to remain in the center of my canvas as it becomes larger.

      function init() {            stage = new PIXI.Stage(0x66FF99);            renderer = new PIXI.autoDetectRenderer(512, 384, document.getElementById("game-canvas"));            var midTexture = PIXI.Texture.fromImage("resources/mid2.png");            mid = new PIXI.TilingSprite(midTexture, 256, 384);            stage.addChild(mid);            requestAnimFrame(update);      }      function update() {            mid.scale.x *= 1.001;             mid.tilePosition.y += 1.21;            // I want it to remain centered as it gets larger, this attempt does not work:            mid.position.x = (512 - mid.width) / 2 ;            renderer.render(stage);            requestAnimFrame(update);         }