# Survey on Quality of Experience Evaluation for Cloud-Based Interactive Applications
## 5 Strategies for QoE Measurement in CIAs
### Strategies Based on Screen Updates
CloudReady allows the user to specify a text string that the tool locates in a program of the user's choice after opening. The common procedure of OCR systems is to binarize, segment, improve visual quality, and compare with known character patterns. However, as it is a commercial tool, there is not enough information regarding the exact procedure it uses. Another group of authors has tried to evaluate the elapsed time between when a user interacts with a CIA and the moment they perceive a response to their interactions (this metric is called the interactivity time [9] or responsiveness [10]). In [9], [PERSON] et al. proposed the thin client latency analysis (TeCLA) methodology. The aim of TeCLA is to quantify QoE on remote desktops, independent of the protocol used. The authors measured the interactivity time. To do this, TeCLA periodically (as fast as possible) performs a checksum of the user's screen. When TeCLA detects a keyboard or mouse interaction, it stores the most recent checksum, and it computes the client's screen checksum constantly until it is different from the stored one. At that point, TeCLA interprets that the screen has changed and regards that change as a response to the client's interaction. Then, it calculates the interactivity time as the elapsed time. A change in the screen may not be the actual response to a user's interaction and may generate shorter interactivity times compared to the actual ones. For example, there may be autonomous changes to desktop elements (a clock flashing every second), the current window (a banner containing an animation), or a different window (a video playing on the desktop). The authors call these phenomena instabilities. TeCLA identifies instabilities on the screen when a user is not performing any action. If no instabilities occur for a sufficiently long period, the statistical model of TeCLA interprets the subsequent samples of interactivity as reliable. TeCLA allows for the determination of the minimum interval between the detection of instabilities to reduce the probability of measurement error below a user-specified threshold based on studies with real users. Instead of obtaining bitmaps from the user's screen, other proposals opted to use external devices. These devices relate variations in light intensity in a particular area of the monitor to the user's keyboard or mouse interactions. When the tool detects a user interaction, it waits to detect a slight variation on the monitor. The tools interpret the variation as the representation of the response to the CIA user's request. These external devices have a photodetector to measure variations in light intensity. These proposals place the photodetector on the user's monitor to make the measurements. Leo Bodnar Electronics offers devices that allow one to obtain interactivity time through emulated interactions, which are used by researchers to evaluate the QoE in cloud gaming. The device in [55] supports video signals up to full-HD quality, whereas the model in [56] allows for signal measurements at 4K resolution. [PERSON] [57] (UXMeter) and the NVIDIA Latency Display Analysis Tool (LDAT) [58] offer similar external devices based on light-intensity variations. They allow for the detection of user-mouse interactions. Their external devices connect to the user's PC via USB and receive the user's interactions by directly connecting the mouse to them via a USB port. The NVIDIA solution not only detects the interactivity of the display device but also provides a mini-jack audio input to evaluate the audio delay. Ideally, in remote desktop or cloud gaming CIAs, the user's screen should be identical at the same instant of time as that of the cloud server. However, in practice, this is impossible due to the delay introduced by the network. Some authors evaluated the time difference between the screen representations of the cloud server and the client device. These QoE quantification proposals base their approaches on the fact that the shorter the time difference, the better the user experience.