3.5. Using TSV Pictures and the GVI Formula to Calculate GVI Values

The GVI formula for every sampling site is as follows:

Formula (1) uses Area_{g_i} as the total number of pixels of green vegetation extracted by the image segmentation algorithm in direction i. Area_{t_i} is the total pixel number in the whole picture in direction i. The parameter m is the number of pictures facing different directions in the horizontal plane; in this study, m = 6. The GVI calculation was adjusted based on the specific configuration of TSV pictures. Vertical-view GVI was calculated by adding SVPs in vertical or sub-vertical directions. A formula incorporating the upward direction would be more appropriate in a study of the relationship between tree crown density and GVI, for example. Therefore, based on the work of previous studies, the authors provide the general GVI Formula (2):

In Formula (2), Area_{g_ij} represents the number of pixels of green vegetation extracted by the segmentation method in the horizontal direction i and the vertical direction j.

Similarly, Area_{t_ij} represents the total area of the SVP in the horizontal direction i and the vertical direction j. Regarding Formula (2), the symbol “g” means “general”. Parameters m and n represent the numbers of horizontal and vertical directions, respectively. Considering Li’s GVI calculation formula, m = 6 and n = 3; hence, the number of SVPs at a single sampling site was 18 [20]. When n = 1 and the heading angle is 0 degrees, Formula (2) is equal to Formula (1).

Additionally, when m = 1, n = 1 and a single image can represent the whole visual environment, the authors defined the GVI as the panoramic green view index (PGVI), as described by Formula (3):

In Formula (3), Area_g is the area of green vegetation in the panorama, and Area_t is the total area of the panorama.

Selection of the most appropriate GVI calculation formulae depended on the configuration (horizontal and vertical directions and number of pictures) of the SVPs employed by the researchers. Section 5.3 will explore the differences among the calculation results of various GVI formulas.