@ferdinand

Thanks, that is exactly correct - in fact I did not need to make the orthonormal vector, all one needs do is set the objects Y-axis equal to the camera, and get the x-axis by cross product .

I think I correctly handle the pathological situation after looking at the limiting cases.

The modified function is below.

Randy

EPSILON = 1E-5 # The floating point precision we are going to assume, i.e., 0.00001 # This is based on the python Look at Camera example # https://github.com/PluginCafe/cinema4d_py_sdk_extended/tree/master/plugins/py-look_at_camera_r13 def GetLookAtTransform(host: c4d.Matrix, target: c4d.Matrix, reverseZ=True) -> c4d.Matrix: """Returns a transform which orients its z/k/v3 component from #host to #target. """ # Get the position of both transforms. p: c4d.Vector = host.off q: c4d.Vector = target.off # The normalized offset vector between 'host' (object to be reoriented) and 'target' (the camera) # will become the z-axis of the modified frame for the object . # # If reverseZ = True, the new z-axis is points from camera toward object, if False the reverse # I turn reverseZ on by default, as my initial application is to text splines, which are meant to be # viewed looking down the object z-axis. # In the original implementation # (https://github.com/PluginCafe/cinema4d_py_sdk_extended/tree/master/plugins/py-look_at_camera_r13 ) # the modified y-axisis computed using the global y-axis, and this does not consistently # keep the text upright in the view of the camera. Instead, simply take the object y-axis same as camera y. # # In the pathological case of new object z-axis parallel to camera y : # If reverseZ : set object z = camera y , object y = -camera Z # else : set object z = -camera y, object y = -camera z # if reverseZ : z: c4d.Vector = ~(p - q) if 1. - abs(z * target.v2) > EPSILON : y = target.v2 else : z = target.v2 y = -target.v3 else : z: c4d.Vector = ~(q - p) if 1. - abs(z * target.v2) > EPSILON : y = target.v2 else : z = -target.v2 y = -target.v3 # get x using cross product x: c4d.Vector = ~(y % z) # Return the frame (x, y, z) plus the offset of #host as the look-at transform. return c4d.Matrix(off=p, v1=x, v2=y, v3=z)

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