Making the background see-through, with a recticle staying visible I'm modelling a rifle, and am up to the scope. I now know how to use cycles with its nodes, but I don't get how to the scope crosshairs. The image is a crosshair on a white background. However when I put it into blender, even with the transparency, translucent, or glass BSDF nodes, the white background is still there.
You can use this kind of node setting:



The principle is the following:


Your image is used as diffuse
But it is also used as input to drive the mix shader
This is driven by the white parts here: the more white the more the mix shader will use its second input which is here a glass


Using a glass shader can help to have this kind of zoom like effect, just changing the IOR value of the node: