Occlude Peripheral Visual Field

[Qiliang He]

Hello,

I'm wondering how to occlude the peripheral visual field, as indicated in the attached figure (From a to b)?

https://drive.google.com/open?id=0Bx...nFsLUtQRzRndms

I tried the following codes, but the problem is I don't know how to make the texture transparent at the center but opaque in the peripheral.

	quad = viz.addTexQuad(parent=viz.HEAD, size=5)
	quad.color(1,0,0)
	quad.setPosition([0,0,0.1])
        quad.alpha(0.5)

Any help is appreciated, thank you!