Well I said probability because depending on how complex the background is such as photographs, video, or some weird scrolling behavior, there could be some hindrances to having a computer accurately or computationally-efficiently determine this.
Google uses filters, which run in different intervals apart from the main algorithm and have different effects for a page or the whole website. This is why spammy SEO tactics often "work for a while" - until said computationally intensive filter catches them and often punishes the whole website. Prominent examples for these filters are Panda [0] and Penguin [1].
