I have a line which is bevelled with a circle to make a cylinder. I want to taper it with the curve on the right side. When I choose the taper object to be the curve, the cylinder dissapears. How do I use the curve to taper the cylinder ?
The file is available here.
It's a problem with the taper object and the curve itself.
First of all, like @Chris says, your curves have non applied rotations and those always tend to mess things up.
Another problem is that your modeled your taper object in the Z direction but Blender takes the taper value from the coordinates along the Y axis :
This is a screenshot from the official documentation about the taper object :
Look at what happens when I move one of the taper object's control points along the x axis :
With that said, your curves still responds to the taper in a funny way and I had real trouble trying to figure out how it was supposed to work so the best solution I found is to redo everything from the beginning without rotating the curves or transforming them in Object Mode. The steps are simple :
Add a Bezier Curve
Scaling everything by 0 on the Y axis, to get a cylinder
And add another Bezier for the taper object
You can now model your taper object but just remember that only the positions of the curve's vertices on the local X and Y axis will be taken into account :
To add to Mqbaka and Chris answers, once you've applied the taper rotation and aligned it with the X axis, it looks like you need to switch your curve handle type from Vector to Automatic or Aligned (select its vertices in Edit mode and press V)
