**2.1. Local graph structure (LGS)**

LGS algorithm [8] applied the graph structure in the calculation of the feature values, the main idea is as follows: take the center pixel as the target pixel, and take the two pixels of the left side and three pixels of the right side in the neighborhood of 3 × 4 to constitute the graph structure. Then compare the pixel value of the target pixel and its two pixels on the left by the counterclockwise. Set to 1 when the pixel value becomes larger, to 0 when it becomes smaller. Three pixels on the right side of the target pixel are compared in the clockwise direction. The same as above, set to 1 when the pixel value becomes larger, to 0 when it becomes smaller. Finally, the target pixel value is compared with the adjacent pixel value on the right. The whole procedure is shown in **Figure 1**, where X<sup>0</sup> is the target pixel. The comparison order is conducted as X<sup>0</sup> → X<sup>1</sup> → X<sup>2</sup> → X<sup>0</sup> → X3 → X<sup>4</sup> → X5 → X3 → X<sup>0</sup> . The final result is composed by these eight binary numbers. Finally, we convert the 8-bit binary numbers to its decimal format.

A specific example of LGS operator is shown in **Figure 1**. By comparing the value of the pixel with its adjacent pixel along the direction of arrows, the binary value of X<sup>0</sup> is 10,001,011. As a result, the decimal value of the target pixel is obtained as 139.
