Method clear

src/common/overlay/levelRenderer/Painter.js:1087–1138

* @function Painter.Layer.prototype.clear * @description 清空该层画布。

()

     * @description 清空该层画布。
     */
    clear() {
        var dom = this.dom;
        var ctx = this.ctx;
        var width = dom.width;
        var height = dom.height;

        var haveClearColor = this.clearColor;
        var haveMotionBLur = this.motionBlur;
        var lastFrameAlpha = this.lastFrameAlpha;

        if (haveMotionBLur) {
            if (!this.domBack) {
                this.createBackBuffer();
            }

            this.ctxBack.globalCompositeOperation = 'copy';
            this.ctxBack.drawImage(
                dom, 0, 0,
                width / Painter.devicePixelRatio,
                height / Painter.devicePixelRatio
            );
        }

        if (haveClearColor) {
            ctx.save();
            ctx.fillStyle = this.config.clearColor;
            ctx.fillRect(
                0, 0,
                width / Painter.devicePixelRatio,
                height / Painter.devicePixelRatio
            );
            ctx.restore();
        } else {
            ctx.clearRect(
                0, 0,
                width / Painter.devicePixelRatio,
                height / Painter.devicePixelRatio
            );
        }

        if (haveMotionBLur) {
            var domBack = this.domBack;
            ctx.save();
            ctx.globalAlpha = lastFrameAlpha;
            ctx.drawImage(
                domBack, 0, 0,
                width / Painter.devicePixelRatio,
                height / Painter.devicePixelRatio
            );
            ctx.restore();
        }
    }

}