## 4 Challenges in Glottal Gap Segmentation and Facilitative Playbacks  
Automatic methods for glottal gap segmentation and for the representation of FP are still being investigated. From a pure image processing perspective, the task of tracking vocal folds edges during an entire video sequence appears to be a standard tracking task. Thus, naively, it may seem that once the glottal area in a frame is delineated, it can easily be identified in successive frames, or that the glottal \begin{table}
\begin{tabular}{c c c} \hline \hline
**Author** & **Year** & **Playback** & **Dynamics** \\ \hline [PERSON] et al. & 1958 & Glottal Area Waveform (GAW) & Global \\ \hline [PERSON] and [PERSON] & 1983 & Glottal shape Data Representation (GSD) & Global \\ \hline [PERSON] and [PERSON] & 1996 & Videokymography (VKG) & Local \\ \hline [PERSON] et al. & 2001 & Vibration Profiles (VP) & Global \\ \hline [PERSON] et al. & 2001 & Empirical Orthogonal Eigenfunctions Analysis (EOF) & Global \\ \hline [PERSON] et al. & 2002 & Eigenfolds Analysis (EFA) & Global \\ \hline [PERSON] et al. & 2005 & Hilbert Transform Analysis (HTA) & Global \\ \hline [PERSON] et al. & 2007 & Two-Dimension Spatiotemporal Series Analysis (2D-STSA) & Global \\ \hline [PERSON] et al. & 2007 & Vocal Folds Trajectories (VFT) & Local \\ \hline [PERSON] et al. & 2008 & Mucosal Wave Kymography (MKG) & Local \\ \hline [PERSON] and [PERSON] & 2008 & Phonovibrogram (PVG) & Global \\ \hline [PERSON] et al. & 2010 & Laryngotopography (LGT) & Global \\ \hline [PERSON] et al. & 2012 & Glottovibrogram (GVG) & Global \\ \hline [PERSON] et al. & 2013 & Phonovibrographic Wavegram (PVG-wavegram) & Global \\ \hline [PERSON] et al. & 2013 & Waveform Decomposition Analysis (WDA) & Global \\ \hline [PERSON] et al. & 2014 & Glottaltopogram (GTG) & Global \\ \hline [PERSON] et al. & 2016 & Phasegram Analysis (PGAW) & Global \\ \hline [PERSON] et al. & 2017 & Glottal Optical Flow Waveform (GOFW) & Global \\ \hline [PERSON] et al. & 2017 & Optical Flow Kymogram (OFKG) & Local \\ \hline [PERSON] et al. & 2017 & Optical Flow Glottovibrogram (OFGVG) & Global \\ \hline [PERSON] et al. & 2018 & Glottocorrelogram (GCG) & Global \\ \hline [PERSON] & 2018 & Optical Flow Based Waveform (OFW) & Local \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the main studies that synthesise the vocal folds dynamics in FP. gap can be obtained intuitively by subtracting successive frames. However, glottal-gap segmentation is not easy due to many different factors: limited spatial resolution, contrast inadequacies during acquisition (see Figure 15a), camera rotation (see Figure 15b), side movement of the laryngoscope, movements of the patient, brightness changes (see Figure 15c), depth differences between videos and demanding cases as hourglass closure, irregular closure, vocal hemorrhage (see Figure 15d), nodules (see Figure 15e), polyps, cysts, scars, mucus, specular reflection, and vocal folds occlusion (see Figure 15f) among others. In addition, there are no official guidelines for laryngeal imaging footage analysis; the literature is limited by the relatively low or nonexistent correlations among measures of irregularity in vocal folds vibration and acoustic parameters; and the lack of normative values of the parameters and intervals, which are needed to determine the severity of pathological voice production. Since the pattern of vocal folds vibration is difficult to evaluate by simply observing the successive video frames, the researchers have introduced the concept of FP to easily and better visualize the features of vocal folds dynamics. The purpose of FP is to synthesize the time-varying data in a few static images, or in an unidimensional temporal which permits to visualize hidden features. Despite the great advances that have been reached condensing the data coming from laryngeal imaging, many of the FP in the literature present some drawbacks that restrict their applicability. For instance, the motion analysis is focused only on those points belonging to the glottal contours.