I solved it by myself. So here for the rest of the world or maybe for myself in a couple of months.
The following config is working:
{
module: 'MMM-BackgroundSlideshow',
position: 'fullscreen_below',
config: {
imagePaths: ['modules/MMM-BackgroundSlideshow/BackgroundImages'],
transitionImages: true,
transitions: ['opacity'],
randomizeImageOrder: true,
slideshowSpeed: 500000,
gradientDirection: 'both',
gradient: [ "rgba(0, 0, 0, 0.00) 0%", "rgba(0, 0, 0, 0.00) 0%", "rgba(0, 0, 0, 0.00) 90%", "rgba(0, 0, 0, 0.75) 100%" ], //von oben nach unten, % = wo im Bildschirm startet es
horizontalGradient: [ "rgba(0, 0, 0, 0.75) 0%", "rgba(0, 0, 0, 0) 15%", "rgba(0, 0, 0, 0.00) 85%", "rgba(0, 0, 0, 0.75) 100%" ] //von links nach rechts, % = wo im Bildschirm startet es
}
},
In addition, you have to change a line in the MMM-BackgroundSlideshow.js file in the getDom function (line 316)
OLD: this.createGradientDiv('right', this.config.gradient, wrapper);
NEW: this.createGradientDiv('right', this.config.horizontalGradient, wrapper);
I will put this on GitHub as well.