# 3D Approaches and Challenges in Facial Expression Recognition Algorithms--A Literature Review
## 5 Databases  
In the literature, there are numerous databases related to FER for comparative and extensive experiments; some of them are used in particular for conventional FER approaches with decision methods, others for FER systems based on deep learning with recognition algorithms. Traditionally, human facial emotions have been studied using 2D data, static or dynamic, with many difficulties attached. A 3D-based analysis of facial emotions will facilitate handling significant pose variations and subtle facial behaviors but has specific problems such as a high computational cost. The databases mainly used in this research field are described below and summarized in Table 4.  
To date, various databases for 3D facial expression recognition exist and have been employed by the research community for the evaluations of the developed algorithms. Of these, only three publicly available have been designed specifically for emotion analysis, having datasets displaying the six basic emotions or different AUs of the FACS, and are BU-3 DFE, BU-4 DFE, and Bosphorus. The databases listed above are not the only ones to contain facial expressions; other public databases, for example, FRGC v2.0 and GavabDB, present a set of expressions variations, but incomplete or with an irregular distribution. The main databases with 3D images and video sequences used for the study of facial expressions are described below. \begin{table} \begin{tabular}{c c c c c c c c} \hline \hline
**Name** & **Year** & **Data 1** & **Datasets 2** & **Expressions** & **Occlusions 3** & **Head Poses 4** & **\# of Landmarks 5** \\ \hline JAFFE [135] & 1999 & 2D images & 213 (10) & 6 basic emotions + neutral & No & No & - \\ FRGC ver2.0 [136] & 2005 & 3D scans & 4950 (577) & 6 basic emotions & No & No & - \\ MMI [137] & 2005 & 2D RGB images + 2D videos & 740 + 2900 (75) & 6 basic emotions + neutral + AUs & No & No & - \\ BU-3 DPE [138] & 2006 & 3D facial models & 2500 (100) & & + neutral & No & No & 83 \\ Bosphorus [139] & 2008 & 2D RGB images + 3D data & 4666 (105) & 6 basic emotions + neutral + 28 AUs & Yes (4) & Yes (13) & 24 \\ BU-4 DPE [140] & 2008 & 3D video sequences & 606 (101) & 6 basic emotions + neutral & No & No & 83 \\ CK+ [141] & 2010 & 2D video sequences & 593 (123) & & neutral + AUs & No & No & - \\ D3 DFACS [142] & 2011 & 2D+3D static + dynamic & 519 (10) & & 19-97 AUs individually + & & & \\  & & & combination & & & & 47 \\ RGB-D [143] & 2012 & 2D RGB images + depth & 1581 (31) & Neutral, smile, sad, yawn and angry & No & Yes (17) & - \\ UPM-3 DFE [144] & 2012 & 3D images & 350 (50) & 6 basic emotions + neutral & No & No & 32 \\ BP4D [41, 42] & 2013 & 3D video sequences & 328 (41) & 8 facial expressions + 27 AUs & No & No & 49 \\  & & & 2D RGB images + 2.5D & & & & \\ KinectFaceDB [145] & 2014 & depth maps + 3D point & 936 (52) & Neutral, smile and yawn & Yes (3) & Yes (2) & 6 \\  & & & sequences & & & & \\ FERG-DB [146] & 2017 & 2D images & 55767 (6) & 6 basic emotions + neutral & No & No & - \\ \hline \hline \end{tabular}   
The most common for 3D FER systems is the Binghamton University 3D Facial Expression (BU-3 DFE) database [138], the first one to become publicly available. It consists of raw 3D facial scans of 100 subjects, 56 females and 44 males, with different ethnic and racial ancestries, and ranging age from 18 to 70 years old. Each person performed the expressions corresponding to the six basic emotions, acquired at four levels of intensity (low, middle, high, and higher) using 3D sensors, along with a neutral expression. In total, therefore, the database contains 2500 3D facial expression models, 25 for each subject, plus the associated texture images captured at two views (about +45\({}^{\circ}\) and -45\({}^{\circ}\)). The manually annotated dense landmark set is provided with the release, contributing to its diffusion and use. The BU-4 DFE [140] is a 3D dynamic facial expression database, built to analyze the facial behavior in a dynamic 3D space.