# A Sign of Things to Come: Predicting the Perception of Above-the-Fold Time in Web Browsing
## 2 Background  
In Figure 1, the loading process of a web page is illustrated. When a user first navigates to a website, a TCP or a UDP connection is made to the web server. The connection will be used for the data transport between the application server and the user's browser. The web application server processes the request and sends a response back to the browser. The browser then constructs a Document Object Model (DOM) and starts rendering the web page on the user's screen. Depending on how the web application is designed, the browser may paint the visual elements all at once, or at different points in time. At the time the browser completes painting the current viewport, the user perceives the website as visually complete (ATF time) and may already start interacting with the HTML elements [11]. The browser, however, may still load more visual and non-visual objects until the page gets fully loaded. ATF time can be used as an independent KPI, or as an input to time integral metrics for QoE estimation [6; 11]. The time integral metrics (e.g., SI) quantify how fast a web page is visually loaded by tracking the visual progress of a web page over time (see the gray area in Figure 2). SI uses the following equation to estimate the visual loading speed of a website:  
\[X=\int_{0}^{t_{\text{end}}}(1-\frac{vc}{100}), \tag{1}\]  
where \(X\) is the estimated speed, \(t_{end}\) is the time the last event occurs, and \(vc\) is the time evolution of the Visual Completion (VC) to reach \(t_{end}\) (as shown in Figure 2, it calculatesarea above-the-curve). For example, _PLT_ is generally considered as the \(t_{end}\) time, while _vc_ is the visual completion progress ratio of the web page over time. The VC progress ratio of SI is calculated based on a comparison of Mean Pixel Histogram Difference (MPHD) between the current state of the web page at time \(t\) and the state of the page at the PLT. SI uses a series of snapshots (at a rate of 10 frames per second) from the time that the URL is requested until PLT. The frames are analyzed in the same order to determine the VC progress ratio over time. The time that VC progress reaches 100% is referred to as Objective ATF time [12]. VC time and progress are related factors influencing the ATF time estimation and the result of SI. For instance, if the time increases and the VC progress has not reached 100%, the area above-the-curve also increases (impacting SI estimation). For example, from Figure 3, it can be seen that the existence of animated shopping items increases the above-the-curve area. The ATF time estimation is also prolonged until the animation stops or the page load event occurs (PLT). Figure 1: The loading process of a website from time that the URL is requested until the time that the website is completely loaded. Figure 2: Illustration of the Visually Complete (VC) time (\(x\)-axis) and progress (\(y\)-axis) with example page rendering. SpeedIndex (SI) calculates the area above-the-curve from page request time, \(t=0\), until the time at which the Above-the-fold (ATF) is 100% VC (i.e., ATF time). VC progress usually occurs before the Page Load Time (PLT) event. However, depending on the content, network, and computational conditions, ATF and PLT may occur simultaneously. Researchers have proposed various heuristic based metrics to estimate ATF time. In Reference [13], the authors have proposed TFVR (Time for Full Visual Rendering) which TFVR estimates ATF time by extracting loading events and rendering timing from the browser HTTP Archive (HAR) file. Similarly, [PERSON] et al. [14] proposed Approximated Above-The-Fold (AATF) that estimates ATF time from the browser's heuristics without requiring image processing. However, to the best of our knowledge, no previous studies have considered the impact of various content types while estimating ATF time. In Reference [10], we established a range of perceived ATF times and demonstrated that objective ATF is generally overestimating the perceived ATF time.