1 files changed, 31 insertions, 7 deletions

diff --git a/js/reveal.js b/js/reveal.js
index 3c06f8d..0e60745 100644
--- a/js/reveal.js
+++ b/js/reveal.js
@@ -136,6 +136,10 @@
 			// Parallax background size
 			parallaxBackgroundSize: '', // CSS syntax, e.g. "3000px 2000px"
 
+			// Amount of pixels to move the parallax background per slide step
+			parallaxBackgroundHorizontal: null,
+			parallaxBackgroundVertical: null,
+
 			// Number of slides away from the current that are visible
 			viewDistance: 3,
 
@@ -2672,15 +2676,35 @@
 				backgroundHeight = parseInt( backgroundSize[1], 10 );
 			}
 
-			var slideWidth = dom.background.offsetWidth;
-			var horizontalSlideCount = horizontalSlides.length;
-			var horizontalOffset = -( backgroundWidth - slideWidth ) / ( horizontalSlideCount-1 ) * indexh;
+			var slideWidth = dom.background.offsetWidth,
+				horizontalSlideCount = horizontalSlides.length,
+				horizontalOffsetMultiplier,
+				horizontalOffset;
+
+			if( typeof config.parallaxBackgroundHorizontal === 'number' ) {
+				horizontalOffsetMultiplier = config.parallaxBackgroundHorizontal;
+			}
+			else {
+				horizontalOffsetMultiplier = ( backgroundWidth - slideWidth ) / ( horizontalSlideCount-1 );
+			}
+
+			horizontalOffset = horizontalOffsetMultiplier * indexh * -1;
+
+			var slideHeight = dom.background.offsetHeight,
+				verticalSlideCount = verticalSlides.length,
+				verticalOffsetMultiplier,
+				verticalOffset;
+
+			if( typeof config.parallaxBackgroundVertical === 'number' ) {
+				verticalOffsetMultiplier = config.parallaxBackgroundVertical;
+			}
+			else {
+				verticalOffsetMultiplier = ( backgroundHeight - slideHeight ) / ( verticalSlideCount-1 );
+			}
 
-			var slideHeight = dom.background.offsetHeight;
-			var verticalSlideCount = verticalSlides.length;
-			var verticalOffset = verticalSlideCount > 1 ? -( backgroundHeight - slideHeight ) / ( verticalSlideCount-1 ) * indexv : 0;
+			verticalOffset = verticalSlideCount > 0 ?  verticalOffsetMultiplier * indexv * 1 : 0;
 
-			dom.background.style.backgroundPosition = horizontalOffset + 'px ' + verticalOffset + 'px';
+			dom.background.style.backgroundPosition = horizontalOffset + 'px ' + -verticalOffset + 'px';
 
 		}