AirContour:Building Contour-based Model for In-Air Writing Gesture Recognition 44:7 B Plane Human B -0.1 frame D C -0.2 0.1 0.1 y8m)0.10.1 m (a)Hand movements (b)3D contour across different planes Fig.5.In-air gesture across different planes. (a)Writing with left hand (b)Writing with right hand Fig.6.Viewing angles for writing with different hands. contour,i.e.,the projected contour in the principal plane keeps the essential feature of the 3D contour.Therefore,we are expected to adaptively project the 3D contour into the principal plane and obtain the essential contour feature of the handwritten character,as the contour"k"shown in the red circle in Figure 5(b). 3.3 Some Definitions about In-air Gestures According to Section 3.2,the improper viewing angle will lead to the distortion of the observed gesture contour.To mitigate the confusion or misrecognition of gesture contours caused by view- ing angles,we first define the appropriate range of viewing angles based on people's writing habits, i.e.,when the user writes in the air,her/his eyes track the movement of the hand naturally. As shown in Figure 6(a),when the user writes with the left hand,she/he tends to write in front,left side,or below;the corresponding viewing angle comes from behind,right side,or up side.Accordingly,we select a reference coordinate plane for each viewing angle,i.e.,xh-zh plane, yh-Zh plane,and xh-yh plane,respectively.Similarly,as shown in Figure 6(b),when the user writes with the right hand in front,right side,or below,the corresponding viewing angle comes from behind,left side,or up side.The selected reference coordinate plane under the viewing angles are xh-zh plane,(-yh)-zh plane,and xh-yh plane,respectively.Therefore,there is a mapping relationship between a reference coordinate plane and a viewing angle.With the selected reference coordinate plane,the user will not view a character contour in the right orientation as a reversed contour(referring to Figure 3(a)and Figure 3(c)).It is worth mentioning that the selected reference coordinate plane is used to indicate the possible orientation of the projected contour in principal plane,as described in Section 4.2.It does not mean that the user can only write on xh-zh,yh -Zh, or xy-yh planes;in fact,the user can write towards arbitrary directions in 3D space. ACM Transactions on Sensor Networks,Vol.15,No.4.Article 44.Publication date:October 2019.AirContour: Building Contour-based Model for In-Air Writing Gesture Recognition 44:7 Fig. 5. In-air gesture across different planes. Fig. 6. Viewing angles for writing with different hands. contour, i.e., the projected contour in the principal plane keeps the essential feature of the 3D contour. Therefore, we are expected to adaptively project the 3D contour into the principal plane and obtain the essential contour feature of the handwritten character, as the contour “k” shown in the red circle in Figure 5(b). 3.3 Some Definitions about In-air Gestures According to Section 3.2, the improper viewing angle will lead to the distortion of the observed gesture contour. To mitigate the confusion or misrecognition of gesture contours caused by view￾ing angles, we first define the appropriate range of viewing angles based on people’s writing habits, i.e., when the user writes in the air, her/his eyes track the movement of the hand naturally. As shown in Figure 6(a), when the user writes with the left hand, she/he tends to write in front, left side, or below; the corresponding viewing angle comes from behind, right side, or up side. Accordingly, we select a reference coordinate plane for each viewing angle, i.e., xh − zh plane, yh − zh plane, and xh − yh plane, respectively. Similarly, as shown in Figure 6(b), when the user writes with the right hand in front, right side, or below, the corresponding viewing angle comes from behind, left side, or up side. The selected reference coordinate plane under the viewing angles are xh − zh plane, (−yh ) − zh plane, and xh − yh plane, respectively. Therefore, there is a mapping relationship between a reference coordinate plane and a viewing angle. With the selected reference coordinate plane, the user will not view a character contour in the right orientation as a reversed contour (referring to Figure 3(a) and Figure 3(c)). It is worth mentioning that the selected reference coordinate plane is used to indicate the possible orientation of the projected contour in principal plane, as described in Section 4.2. It does not mean that the user can only write on xh − zh, yh − zh, or xy − yh planes; in fact, the user can write towards arbitrary directions in 3D space. ACM Transactions on Sensor Networks, Vol. 15, No. 4, Article 44. Publication date: October 2019