## 3 Results
### Image Processing of CT Images under Confining Stress
The aperture boundary is a linear curved object, and some image misidentifications may appear due to the image noise generated around the aperture boundary or on the rough profile. The aperture value difference and the histogram at 3.0 MPa are illustrated in Figure 19. The aperture difference of Slice A generates a pixel number of around 300 to 400 at 3 MPa, and a big aperture difference of Slice E occurs in the pixel size of around 200 to 300, as shown in Figure 19a. A membrane region located around the rock may also result in image segmentation noise, which will also produce an aperture value difference. Moreover, the aperture becomes narrower, and this will result in a more rough boundary  
Figure 17: CT image comparison under RG and ED methods at 3.0 MPa (Slice A to E). profile. Therefore, the boundary profile of Slice A is influenced by a relatively low-density value around the rock. From observing the histogram results of 3 MPa Slice A and Slice E, it can be seen that the aperture value is completely different under RG and ED segmentation. A large aperture value difference is observed through a comparison with Slice A and E, as shown in Figure 19b,c. Several rough boundary profiles and image noise are admitted under different image processing methods. This great difference also corresponds to the image calculation results in Figure 18. It is also illustrated that the low-density area exists within the aperture, and it becomes much harder to distinguish. Moreover, at 3.0 MPa, a much narrower aperture will result in a change of grayscales. Therefore, the image processing becomes much more difficult when analyzing all of these characteristics that occurred on the fractured rock CT image. Several aperture positions of Slice A and Slice E at 3.0 MPa are picked up and employed to make a comparison in Figure 20. After RG segmentation, results show that some void positions are difficult to be distinguished. Some void regions are misrecognized and assimilated into the rock region, and this results in image misidentification, as presented in Figure 20(a1,a2). Moreover, this will also result in the inaccurate calculation of the contact area ratio. Figure 19: Aperture value difference of Slice A and Slice E at 3.0 MPa. (**a**) Aperture difference under RG and ED methods. (**b**) Aperture value histogram (RG). (**c**) Aperture value histogram (ED). Figure 18: Comparative result of the aperture boundary profiles under RG and ED methods (3 MPa, Slice A–E). Image misidentification positions are also selected and presented as follows: In the \(x\)-direction, the aperture position of Slice A with the pixel number of 100-335 in Figure 20(a1) and the aperture position of Slice E with the pixel number of 80-220 in Figure 20(a2) are presented with several misidentifications. These positions on the original CT images and the CT segmentation images under RG and ED are compared. Results shows that the Slice A has a large difference in RG and ED at 3.0 MPa. The RG segmentation misrecognizes some different regions around the void region as the rock voxel continuity, and the aperture boundary is neglected. In contrast, ED only concentrates on the boundary, and the segmentation result is quite similar to that of the original CT image. Moreover, the yellow profile line is picked up and compared, i.e., profile line 275 of Slice A in the \(x\)-direction, and profile line 142 of Slice E in the \(x\)-direction, as depicted in Figure 20(b1,b2). From observing the 3 MPa Slice A, it is presented that the ED profile line is well reflected from the original CT image. However, the RG profile line value is equal to 0, as shown in Figure 20(c1), which is corresponds to the RG segmentation results. The relative aperture narrow position is misrecognized as the connectivity of the rock region. However, the 3 MPa Slice E also shows a significant difference in the profile line comparison. The profile line under the RG method shows an oscillation variation in Figure 20(c2).