Detection of lines can be done using the masks shown below.

In an image, lines can be in any direction and detecting these lines would need different masks.

All these masks have a sum equal to zero, and hence all of them are high pass masks.

The first mask would respond strongly to lines that are oriented horizontally.

The second mask would respond to lines at an angle of +45°, the third mask would respond strongly to vertical lines and the last mask would respond strongly to lines at an angle of - 45°.

Figure 18