t;o&lt;-|-----------o   |     |
|     |               |`````|               |     |
|     |   o&lt;----------|--o--|----------&gt;o   |     |
|     |      dy (dx)  |``|``|  dx (dy)      |     |
|     o               o``|``o               o     |
|      \               \`|`/               /      |
|       \               \|/               /       |
|        \               |               /        |
|         \             /|\             /         |
|          o-----------o | o-----------o          |
|     