Rolling a folded surface up and down

[Myle_McKay]

how to roll a zebra roller blind up and down using grasshopper?

Here is a reference image of the zebra blind

It is a single long sheet of fabric folded at the bottom so they become two layers. One side of the fabric is fixed, while the other side is attached to the roller tube. And here is an illustration of the movement I wish to achieve:

Zebra blind phone number 7670966070

Thank you