How to connect a 3-D line to the edge of a sprite

I have a 3-D scene with a Sprite that displays some text. Currently I have a 3-D Line that connects the sprite to a point on the 3-D model. The coordinates of the end of the line that touches the sprite are the same as the coordinates of the sprite itself, and the line touches the centre of the sprite. What I’d really like is to have the line touch one of the edges of the sprite. When I try it, the line touches the right point sometimes but drifts around as I rotate the scene, presumably because the scene and the sprite are not rotating in the same way.
Can anyone suggest a way of getting the effect I want?
Thank you.

Hello robertf,

Do you have an image of the effect you are trying to achieve? If you could create a JSfiddle with sample code, that would great too…

++

Thanks for your interest. I apologize - I should have posted a message here saying that I’d concluded that stackoverflow seems to be the preferred place for this sort of thing. I posted a question at https://stackoverflow.com/q/44488145 and had some comments from WestLangley, and then I posted an ‘answer’ saying that I’d found a related question from 2014 that included two possible solutions. I’ve implemented the less elegant of the two and it solves my immediate problem. You can see the effect at http://audilab.bme.mcgill.ca/~funnell/davis3d/thrwpny.html (the colours and the trackball control are currently broken).

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