Why can I see curves that should be behind meshes?

There are three known problems with the Grasshopper preview geometry. Two of these are there because we use short-cuts to make the preview as fast as possible, sacrificing a little bit of quality in the process. The last one is there because fixing it would cause another problem, which we feel is more serious. Solutions may well be forthcoming in the future though.

1. Grasshopper curves and points are drawn more towards the camera than they really are. This is a conscious decision. Often Rhino geometry and Grasshopper geometry exist in the same place. If we would draw the Grasshopper preview in place, then there's no telling whether you'd see the Rhino curve or the Grasshopper curve. We feel it's important that you always see the Grasshopper curve on top. This is why we draw all curves and points slightly towards the camera. However we don't do this for meshes. This results in something akin to the image below. The eye represents the location of the viewport camera, the shaded box represents the actual location of the geometry and all the thick black lines represent the edges of the geometry moved towards the camera. As you can see, the red lines will be visible, even though they should be behind the shaded box. This effect can get very strong when the camera is close to some geometry relative to the size of the boundingbox of all geometry.

2. Wires behind the camera are sometimes visible. This is a bug I don't know how to solve. We'll get around to it eventually. When an object is behind the camera the display transform sometimes makes it visible in front of the camera in some weird inverted perspective mode.

3. Meshes are not z-sorted prior to display. This means that the order in which they are drawn is not back-to-front, but fairly arbitrary. This means that a transparent mesh may appear to punch a hole in the mesh behind it. If this is annoying you to no end, you can use Ctrl+F on the Grasshopper components that contain the meshes that are punching holes and then press F5 to recompute. The draw order should now be different. Of course sometimes it will only 'fix' it for a specific camera angle.