                |
|                      dx|dy                      |
|                        |                        |
|          o-----------o | o-----------o          |
|         /             \|/             \         |
|        /       x       |       y       \        |
|       /               /|\               \       |
|      /               /`|`\               \      |
|     o               o``|``o               o     |
|     |      dy (dx)  |``v``|  dx (dy)      |     |
|     |   o-----------|-&g