Parallax Background Image
The is the background of a div rather than in the browser background so that parallax scrolling can be applied to it.
The image height can be set in pixels if required but the example on this page uses the option to set the height in viewport units. The height is set to suit the page content and, in this case, is set to 200vh.
Images
The background image are inserted in three sizes. The large image is 1200 x 720px and the medium image is 800 x 480px. The small image for mobile phones in portrait mode was cropped to an aspect ratio of 9:16 with an actual size of 470 × 836px.
To the inexperienced user, these sizes may seem small. Background images do not need the same quality as those in the forground and the smaller the size the better in terms of page download effiency.
Overlay
The background image has an overlay with controls for background color and its opacity. This can be used to tone down the background images to make the overlying more visible and the text easier to read.
Parallax Speed
The speed control is a slider with a default setting of 25. If set to zero the image will be fixed in position.
At maximum value there will be no parallax effect and the image will scroll with the rest of the page content.