I want to do a very simple animation effect. I have a texture which is split into two horizontal bars, one black and one white, thus:

```
+----+
|####|
|####|
| |
| |
+----+
```

I have applied it to a square mesh of two triangles:

```
v00 v01
+----+
| /|
| / |
| / |
|/ |
+----+
v10 v11
```

by varying the V coordinates of the corners, I can make the black/white border march up and down the square, as I desire. The problem I have is making it do so with uniform speed, for which I need to know the formula that gives the UV coordinate for a given XY position on the square. Can anyone tell me what this is?