updateParallax() {

		let indices = this.Reveal.getIndices();

		if( this.Reveal.getConfig().parallaxBackgroundImage ) {

			let horizontalSlides = this.Reveal.getHorizontalSlides(),
				verticalSlides = this.Reveal.getVerticalSlides();

			let backgroundSize = this.element.style.backgroundSize.split( ' ' ),
				backgroundWidth, backgroundHeight;

			if( backgroundSize.length === 1 ) {
				backgroundWidth = backgroundHeight = parseInt( backgroundSize[0], 10 );
			}
			else {
				backgroundWidth = parseInt( backgroundSize[0], 10 );
				backgroundHeight = parseInt( backgroundSize[1], 10 );
			}

			let slideWidth = this.element.offsetWidth,
				horizontalSlideCount = horizontalSlides.length,
				horizontalOffsetMultiplier,
				horizontalOffset;

			if( typeof this.Reveal.getConfig().parallaxBackgroundHorizontal === 'number' ) {
				horizontalOffsetMultiplier = this.Reveal.getConfig().parallaxBackgroundHorizontal;
			}
			else {
				horizontalOffsetMultiplier = horizontalSlideCount > 1 ? ( backgroundWidth - slideWidth ) / ( horizontalSlideCount-1 ) : 0;
			}

			horizontalOffset = horizontalOffsetMultiplier * indices.h * -1;

			let slideHeight = this.element.offsetHeight,
				verticalSlideCount = verticalSlides.length,
				verticalOffsetMultiplier,
				verticalOffset;

			if( typeof this.Reveal.getConfig().parallaxBackgroundVertical === 'number' ) {
				verticalOffsetMultiplier = this.Reveal.getConfig().parallaxBackgroundVertical;
			}
			else {
				verticalOffsetMultiplier = ( backgroundHeight - slideHeight ) / ( verticalSlideCount-1 );
			}

			verticalOffset = verticalSlideCount > 0 ?  verticalOffsetMultiplier * indices.v : 0;

			this.element.style.backgroundPosition = horizontalOffset + 'px ' + -verticalOffset + 'px';

		}

	}
Updates the position of the parallax background based
on the current slide index.