function updateBackground()

in js/reveal.js [3545:3655]
16 lines of code
10 McCabe index (conditional complexity)

	function updateBackground( includeAll ) {

		var currentBackground = null;

		// Reverse past/future classes when in RTL mode
		var 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)
		toArray( dom.background.childNodes ).forEach( function( backgroundh, h ) {

			backgroundh.classList.remove( 'past' );
			backgroundh.classList.remove( 'present' );
			backgroundh.classList.remove( 'future' );

			if( h < indexh ) {
				backgroundh.classList.add( horizontalPast );
			}
			else if ( h > indexh ) {
				backgroundh.classList.add( horizontalFuture );
			}
			else {
				backgroundh.classList.add( 'present' );

				// Store a reference to the current background element
				currentBackground = backgroundh;
			}

			if( includeAll || h === indexh ) {
				toArray( backgroundh.querySelectorAll( '.slide-background' ) ).forEach( function( backgroundv, v ) {

					backgroundv.classList.remove( 'past' );
					backgroundv.classList.remove( 'present' );
					backgroundv.classList.remove( 'future' );

					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 === indexh ) currentBackground = backgroundv;
					}

				} );
			}

		} );

		// Stop content inside of previous backgrounds
		if( previousBackground ) {

			stopEmbeddedContent( previousBackground );

		}

		// Start content in the current background
		if( currentBackground ) {

			startEmbeddedContent( currentBackground );

			var currentBackgroundContent = currentBackground.querySelector( '.slide-background-content' );
			if( currentBackgroundContent ) {

				var 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;
				}

			}

			// Don't transition between identical backgrounds. This
			// prevents unwanted flicker.
			var previousBackgroundHash = previousBackground ? previousBackground.getAttribute( 'data-background-hash' ) : null;
			var currentBackgroundHash = currentBackground.getAttribute( 'data-background-hash' );
			if( currentBackgroundHash && currentBackgroundHash === previousBackgroundHash && currentBackground !== previousBackground ) {
				dom.background.classList.add( 'no-transition' );
			}

			previousBackground = currentBackground;

		}

		// If there's a background brightness flag for this slide,
		// bubble it to the .reveal container
		if( currentSlide ) {
			[ 'has-light-background', 'has-dark-background' ].forEach( function( classToBubble ) {
				if( currentSlide.classList.contains( classToBubble ) ) {
					dom.wrapper.classList.add( classToBubble );
				}
				else {
					dom.wrapper.classList.remove( classToBubble );
				}
			} );
		}

		// Allow the first background to apply without transition
		setTimeout( function() {
			dom.background.classList.remove( 'no-transition' );
		}, 1 );

	}