This slide shows the local flow defined by a slant tile.
Each "slant tile" defines ...
And here is the definition.
Local flow defined by ... is the three consecutive flat tiles of { ... }.
Shown below are the local flows around a flat tile (colored white).
For example, if the gradient of the flat tile is x1x2 as shown in the left figure, then the "blue" slant tile on this side of the "white" slant tile specifies the downward flat tile and the "bule" slant tile on the other side specifies the upward flat tile.
Similarly adjacent blue slant tiles specify the upward and downward flat tiles in the other cases.