As opposed to rectangles?
That's a bit more complicated...

What do you already have?

yep, instead of rectangles.

Well... triangles are a bit more complicated than rectangles because you have to account for different positions.
This post might be of use...

Basically (depending on the way you are drawing), you need to alternate between two different types of triangles; for example, up and down, or right and left.

Just use triangle() instead of rect()...
But you have to compute the coordinates, at regular intervals.
And indeed, you first have to make your mind about the kind of triangles you want: half rectangles, equilateral triangles, etc.