Tumbling does not work after setting camera to straight down and setting orientation.

[chamric]

I used a combination of the camera's setEyePoint(), setOrientation(), and setPivotDistance() methods to initialize an overhead view of my scene with a desired orientation about the global Y. When I tried to tumble off of the initial point, there was an issue on line 223 of CameraUI.cpp.

There is a cross product between the view direction point and world up. The vectors are parallel, so the cross product is undefined, giving a NaN in the mU variable. The view was lost from this point forward.

[gaborpapp]

Does setting worldUp help as described in #1965?

[shehabattia96]

I also ran into this bug.

Steps to reproduce:

1. Set mCam->lookAt(vec3(0,200,0), vec3(0,0,0)); so that the eyePoint is only y=200.
2. Try to drag with left button to trigger tumble.

Expected Result: tumbling happens normally Actual Result: camera is rotated to NaN. In other words, eyePoint becomes vec3(-nan,-nan,-nan) at https://github.com/cinder/Cinder/blob/master/src/cinder/CameraUi.cpp#L223