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# A new numerical method for level set motion in normal direction used in optical flow estimation

Work supported by grants VEGA 1/0728/15, APVV-15-0522 and APVV-16-0431. The authors are grateful for a support of company Tatramed in Bratislava, Slovakia

• We present a new numerical method for the solution of level set advection equation describing a motion in normal direction for which the speed is given by the sign function of the difference of two given functions. Taking one function as the initial condition, the solution evolves towards the second given function. One of possible applications is an optical flow estimation to find a deformation between two images in a video sequence. The new numerical method is based on a bilinear interpolation of discrete values as used for the representation of images. Under natural assumptions, it ensures a monotone decrease of the absolute difference between the numerical solution and the target function, and it handles properly the discontinuity in the speed due to the dependence on the sign function. To find the deformation between two functions (or images), the backward tracking of characteristics is used. Two numerical experiments are presented, one with an exact solution to show an experimental order of convergence and one based on two images of lungs to illustrate a possible application of the method for the optical flow estimation.

Mathematics Subject Classification: Primary: 65D18, 65M25; Secondary: 35L60.

 Citation: • • Figure 1.  The example with exact solution: the function $F$ (left), the function $G$ (middle), and the deformation $-\vec{D}^{ex}$ (right)

Figure 2.  Comparison of the exact deformation $\vec{D}^{ex}$ (the blue arrows) with the numerical one $\vec{D}$ (the red arrows) for the discretization steps $h = 0.1$ (top left), $h = 0.05$ (top right), $h = 0.025$ (bottom left), and $h = 0.0125$ (bottom right)

Figure 3.  The images of lungs scan: the source image $F$ (left), the target image $G$ (middle), the difference image $|G-F|$ (right)

Figure 4.  The plot and the table of the normalized norm $e^n$ in (36)

Figure 5.  The image given by the values $F(x_{ij}-\vec{D}_{ij})$ (left) and the difference image given by the values $|G_{ij}-F(x_{ij}-\vec{D}_{ij})|$ (right)

Figure 6.  The plot of deformation $\vec{D}$ for the example with the images of lungs

Figure 7.  The plot of deformation $\vec{D}$ for the example with the images of lungs. Only arrows in the points where $|G_{ij}-F_{ij}| > E_{crit}$ are plotted

Table 1.  The error norm (35) and the corresponding $EOC$ for the example with exact solution

 $I$ $N$ $E_{RT}$ EOC $E_{CTU}$ EOC 10 1 0.00703 - 0.00312 - 20 2 0.00401 0.81 0.00171 0.87 40 4 0.00235 0.77 0.000893 0.94 80 8 0.00160 0.55 0.000458 0.96 160 16 0.00281 -0.81 0.000232 0.98

Table 2.  The $\text{L}_1$-norms and the experimental rates of convergence for the example with exact solution

 $I$ $N$ $E_L$ EOC $E_D$ EOC 10 1 0.003120 - 0.004433 - 20 2 0.001307 1.2556 0.002379 0.8985 40 4 0.000528 1.3075 0.001259 0.9175 80 8 0.000220 1.2667 0.000659 0.9339 160 16 0.000096 1.1947 0.000339 0.9590
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