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+import { queryAll } from '../utils/util.js'
+import { colorToRgb, colorBrightness } from '../utils/color.js'
+
+/**
+ * Creates and updates slide backgrounds.
+ */
+export default class Backgrounds {
+
+	constructor( Reveal ) {
+
+		this.Reveal = Reveal;
+
+	}
+
+	render() {
+
+		this.element = document.createElement( 'div' );
+		this.element.className = 'backgrounds';
+		this.Reveal.getRevealElement().appendChild( this.element );
+
+	}
+
+	/**
+	 * Creates the slide background elements and appends them
+	 * to the background container. One element is created per
+	 * slide no matter if the given slide has visible background.
+	 */
+	create() {
+
+		// Clear prior backgrounds
+		this.element.innerHTML = '';
+		this.element.classList.add( 'no-transition' );
+
+		// Iterate over all horizontal slides
+		this.Reveal.getHorizontalSlides().forEach( slideh => {
+
+			let backgroundStack = this.createBackground( slideh, this.element );
+
+			// Iterate over all vertical slides
+			queryAll( slideh, 'section' ).forEach( slidev => {
+
+				this.createBackground( slidev, backgroundStack );
+
+				backgroundStack.classList.add( 'stack' );
+
+			} );
+
+		} );
+
+		// Add parallax background if specified
+		if( this.Reveal.getConfig().parallaxBackgroundImage ) {
+
+			this.element.style.backgroundImage = 'url("' + this.Reveal.getConfig().parallaxBackgroundImage + '")';
+			this.element.style.backgroundSize = this.Reveal.getConfig().parallaxBackgroundSize;
+			this.element.style.backgroundRepeat = this.Reveal.getConfig().parallaxBackgroundRepeat;
+			this.element.style.backgroundPosition = this.Reveal.getConfig().parallaxBackgroundPosition;
+
+			// Make sure the below properties are set on the element - these properties are
+			// needed for proper transitions to be set on the element via CSS. To remove
+			// annoying background slide-in effect when the presentation starts, apply
+			// these properties after short time delay
+			setTimeout( () => {
+				this.Reveal.getRevealElement().classList.add( 'has-parallax-background' );
+			}, 1 );
+
+		}
+		else {
+
+			this.element.style.backgroundImage = '';
+			this.Reveal.getRevealElement().classList.remove( 'has-parallax-background' );
+
+		}
+
+	}
+
+	/**
+	 * Creates a background for the given slide.
+	 *
+	 * @param {HTMLElement} slide
+	 * @param {HTMLElement} container The element that the background
+	 * should be appended to
+	 * @return {HTMLElement} New background div
+	 */
+	createBackground( slide, container ) {
+
+		// Main slide background element
+		let element = document.createElement( 'div' );
+		element.className = 'slide-background ' + slide.className.replace( /present|past|future/, '' );
+
+		// Inner background element that wraps images/videos/iframes
+		let contentElement = document.createElement( 'div' );
+		contentElement.className = 'slide-background-content';
+
+		element.appendChild( contentElement );
+		container.appendChild( element );
+
+		slide.slideBackgroundElement = element;
+		slide.slideBackgroundContentElement = contentElement;
+
+		// Syncs the background to reflect all current background settings
+		this.sync( slide );
+
+		return element;
+
+	}
+
+	/**
+	 * Renders all of the visual properties of a slide background
+	 * based on the various background attributes.
+	 *
+	 * @param {HTMLElement} slide
+	 */
+	sync( slide ) {
+
+		const element = slide.slideBackgroundElement,
+			contentElement = slide.slideBackgroundContentElement;
+
+		const data = {
+			background: slide.getAttribute( 'data-background' ),
+			backgroundSize: slide.getAttribute( 'data-background-size' ),
+			backgroundImage: slide.getAttribute( 'data-background-image' ),
+			backgroundVideo: slide.getAttribute( 'data-background-video' ),
+			backgroundIframe: slide.getAttribute( 'data-background-iframe' ),
+			backgroundColor: slide.getAttribute( 'data-background-color' ),
+			backgroundGradient: slide.getAttribute( 'data-background-gradient' ),
+			backgroundRepeat: slide.getAttribute( 'data-background-repeat' ),
+			backgroundPosition: slide.getAttribute( 'data-background-position' ),
+			backgroundTransition: slide.getAttribute( 'data-background-transition' ),
+			backgroundOpacity: slide.getAttribute( 'data-background-opacity' ),
+		};
+
+		const dataPreload = slide.hasAttribute( 'data-preload' );
+
+		// Reset the prior background state in case this is not the
+		// initial sync
+		slide.classList.remove( 'has-dark-background' );
+		slide.classList.remove( 'has-light-background' );
+
+		element.removeAttribute( 'data-loaded' );
+		element.removeAttribute( 'data-background-hash' );
+		element.removeAttribute( 'data-background-size' );
+		element.removeAttribute( 'data-background-transition' );
+		element.style.backgroundColor = '';
+
+		contentElement.style.backgroundSize = '';
+		contentElement.style.backgroundRepeat = '';
+		contentElement.style.backgroundPosition = '';
+		contentElement.style.backgroundImage = '';
+		contentElement.style.opacity = '';
+		contentElement.innerHTML = '';
+
+		if( data.background ) {
+			// Auto-wrap image urls in url(...)
+			if( /^(http|file|\/\/)/gi.test( data.background ) || /\.(svg|png|jpg|jpeg|gif|bmp|webp)([?#\s]|$)/gi.test( data.background ) ) {
+				slide.setAttribute( 'data-background-image', data.background );
+			}
+			else {
+				element.style.background = data.background;
+			}
+		}
+
+		// Create a hash for this combination of background settings.
+		// This is used to determine when two slide backgrounds are
+		// the same.
+		if( data.background || data.backgroundColor || data.backgroundGradient || data.backgroundImage || data.backgroundVideo || data.backgroundIframe ) {
+			element.setAttribute( 'data-background-hash', data.background +
+															data.backgroundSize +
+															data.backgroundImage +
+															data.backgroundVideo +
+															data.backgroundIframe +
+															data.backgroundColor +
+															data.backgroundGradient +
+															data.backgroundRepeat +
+															data.backgroundPosition +
+															data.backgroundTransition +
+															data.backgroundOpacity );
+		}
+
+		// Additional and optional background properties
+		if( data.backgroundSize ) element.setAttribute( 'data-background-size', data.backgroundSize );
+		if( data.backgroundColor ) element.style.backgroundColor = data.backgroundColor;
+		if( data.backgroundGradient ) element.style.backgroundImage = data.backgroundGradient;
+		if( data.backgroundTransition ) element.setAttribute( 'data-background-transition', data.backgroundTransition );
+
+		if( dataPreload ) element.setAttribute( 'data-preload', '' );
+
+		// Background image options are set on the content wrapper
+		if( data.backgroundSize ) contentElement.style.backgroundSize = data.backgroundSize;
+		if( data.backgroundRepeat ) contentElement.style.backgroundRepeat = data.backgroundRepeat;
+		if( data.backgroundPosition ) contentElement.style.backgroundPosition = data.backgroundPosition;
+		if( data.backgroundOpacity ) contentElement.style.opacity = data.backgroundOpacity;
+
+		const contrastClass = this.getContrastClass( slide );
+
+		if( typeof contrastClass === 'string' ) {
+			slide.classList.add( contrastClass );
+		}
+
+	}
+
+	/**
+	 * Returns a class name that can be applied to a slide to indicate
+	 * if it has a light or dark background.
+	 *
+	 * @param {*} slide
+	 *
+	 * @returns {string|null}
+	 */
+	getContrastClass( slide ) {
+
+		const element = slide.slideBackgroundElement;
+
+		// If this slide has a background color, we add a class that
+		// signals if it is light or dark. If the slide has no background
+		// color, no class will be added
+		let contrastColor = slide.getAttribute( 'data-background-color' );
+
+		// If no bg color was found, or it cannot be converted by colorToRgb, check the computed background
+		if( !contrastColor || !colorToRgb( contrastColor ) ) {
+			let computedBackgroundStyle = window.getComputedStyle( element );
+			if( computedBackgroundStyle && computedBackgroundStyle.backgroundColor ) {
+				contrastColor = computedBackgroundStyle.backgroundColor;
+			}
+		}
+
+		if( contrastColor ) {
+			const rgb = colorToRgb( contrastColor );
+
+			// Ignore fully transparent backgrounds. Some browsers return
+			// rgba(0,0,0,0) when reading the computed background color of
+			// an element with no background
+			if( rgb && rgb.a !== 0 ) {
+				if( colorBrightness( contrastColor ) < 128 ) {
+					return 'has-dark-background';
+				}
+				else {
+					return 'has-light-background';
+				}
+			}
+		}
+
+		return null;
+
+	}
+
+	/**
+	 * Bubble the 'has-light-background'/'has-dark-background' classes.
+	 */
+	bubbleSlideContrastClassToElement( slide, target ) {
+
+		[ 'has-light-background', 'has-dark-background' ].forEach( classToBubble => {
+			if( slide.classList.contains( classToBubble ) ) {
+				target.classList.add( classToBubble );
+			}
+			else {
+				target.classList.remove( classToBubble );
+			}
+		}, this );
+
+	}
+
+	/**
+	 * Updates the background elements to reflect the current
+	 * slide.
+	 *
+	 * @param {boolean} includeAll If true, the backgrounds of
+	 * all vertical slides (not just the present) will be updated.
+	 */
+	update( includeAll = false ) {
+
+		let config = this.Reveal.getConfig();
+		let currentSlide = this.Reveal.getCurrentSlide();
+		let indices = this.Reveal.getIndices();
+
+		let currentBackground = null;
+
+		// Reverse past/future classes when in RTL mode
+		let horizontalPast = config.rtl ? 'future' : 'past',
+			horizontalFuture = config.rtl ? 'past' : 'future';
+
+		// Update the classes of all backgrounds to match the
+		// states of their slides (past/present/future)
+		Array.from( this.element.childNodes ).forEach( ( backgroundh, h ) => {
+
+			backgroundh.classList.remove( 'past', 'present', 'future' );
+
+			if( h < indices.h ) {
+				backgroundh.classList.add( horizontalPast );
+			}
+			else if ( h > indices.h ) {
+				backgroundh.classList.add( horizontalFuture );
+			}
+			else {
+				backgroundh.classList.add( 'present' );
+
+				// Store a reference to the current background element
+				currentBackground = backgroundh;
+			}
+
+			if( includeAll || h === indices.h ) {
+				queryAll( backgroundh, '.slide-background' ).forEach( ( backgroundv, v ) => {
+
+					backgroundv.classList.remove( 'past', 'present', 'future' );
+
+					const indexv = typeof indices.v === 'number' ? indices.v : 0;
+
+					if( v < indexv ) {
+						backgroundv.classList.add( 'past' );
+					}
+					else if ( v > indexv ) {
+						backgroundv.classList.add( 'future' );
+					}
+					else {
+						backgroundv.classList.add( 'present' );
+
+						// Only if this is the present horizontal and vertical slide
+						if( h === indices.h ) currentBackground = backgroundv;
+					}
+
+				} );
+			}
+
+		} );
+
+		// The previous background may refer to a DOM element that has
+		// been removed after a presentation is synced & bgs are recreated
+		if( this.previousBackground && !this.previousBackground.closest( 'body' ) ) {
+			this.previousBackground = null;
+		}
+
+		if( currentBackground && this.previousBackground ) {
+
+			// Don't transition between identical backgrounds. This
+			// prevents unwanted flicker.
+			let previousBackgroundHash = this.previousBackground.getAttribute( 'data-background-hash' );
+			let currentBackgroundHash = currentBackground.getAttribute( 'data-background-hash' );
+
+			if( currentBackgroundHash && currentBackgroundHash === previousBackgroundHash && currentBackground !== this.previousBackground ) {
+				this.element.classList.add( 'no-transition' );
+
+				// If multiple slides have the same background video, carry
+				// the <video> element forward so that it plays continuously
+				// across multiple slides
+				const currentVideo = currentBackground.querySelector( 'video' );
+				const previousVideo = this.previousBackground.querySelector( 'video' );
+
+				if( currentVideo && previousVideo ) {
+
+					const currentVideoParent = currentVideo.parentNode;
+					const previousVideoParent = previousVideo.parentNode;
+
+					// Swap the two videos
+					previousVideoParent.appendChild( currentVideo );
+					currentVideoParent.appendChild( previousVideo );
+
+				}
+			}
+
+		}
+
+		// Stop content inside of previous backgrounds
+		if( this.previousBackground ) {
+
+			this.Reveal.slideContent.stopEmbeddedContent( this.previousBackground, { unloadIframes: !this.Reveal.slideContent.shouldPreload( this.previousBackground ) } );
+
+		}
+
+		// Start content in the current background
+		if( currentBackground ) {
+
+			this.Reveal.slideContent.startEmbeddedContent( currentBackground );
+
+			let currentBackgroundContent = currentBackground.querySelector( '.slide-background-content' );
+			if( currentBackgroundContent ) {
+
+				let backgroundImageURL = currentBackgroundContent.style.backgroundImage || '';
+
+				// Restart GIFs (doesn't work in Firefox)
+				if( /\.gif/i.test( backgroundImageURL ) ) {
+					currentBackgroundContent.style.backgroundImage = '';
+					window.getComputedStyle( currentBackgroundContent ).opacity;
+					currentBackgroundContent.style.backgroundImage = backgroundImageURL;
+				}
+
+			}
+
+			this.previousBackground = currentBackground;
+
+		}
+
+		// If there's a background brightness flag for this slide,
+		// bubble it to the .reveal container
+		if( currentSlide ) {
+			this.bubbleSlideContrastClassToElement( currentSlide, this.Reveal.getRevealElement() );
+		}
+
+		// Allow the first background to apply without transition
+		setTimeout( () => {
+			this.element.classList.remove( 'no-transition' );
+		}, 10 );
+
+	}
+
+	/**
+	 * Updates the position of the parallax background based
+	 * on the current slide index.
+	 */
+	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';
+
+		}
+
+	}
+
+	destroy() {
+
+		this.element.remove();
+
+	}
+
+}