After vtk.js' ImageCPRMapper class example is run, how do I get the image coordinates of the center line on the cpr plane in stretch and straighten mode

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Does the actor have any transform? If that’s the case, you should apply its transform to your line.
The transform of an actor is a mat4 that you can get using actor.getUserMatrix().
You are very close to something that works :+1:
Your Y axis seems flipped, and you have an offset on the X axis, but I don’t recommend changing the computations of newX and newY because this is how they are computed internally. If you are using the example for the ImageResliceMapper, the actor has a transform that is set at lines 210 and 310: actor.setUserMatrix(...).