This enumeration specifies types of edge map operation.
Values:
| IG_EDGE_OP_PREWITT | Prewitt. |
| IG_EDGE_OP_ROBERTS | Roberts. |
| IG_EDGE_OP_SOBEL | Sobel. |
| IG_EDGE_OP_LAPLACIAN | Laplacian. |
| IG_EDGE_OP_LOG | Laplacian of Gaussian. |
| IG_EDGE_OP_HORIZONTAL | Horizontal. |
| IG_EDGE_OP_VERTICAL | Vertical. |
| IG_EDGE_OP_DIAG_POS_45 | Diagonal positive. |
| IG_EDGE_OP_DIAG_NEG_45 | Diagonal negative. |