There is not much documentation on Bullet constraints. Even the Bullet User Manual is very sparse. Here is what I know or understand:
One of the two transform defines the location of the slider pivot axis. Which of the two transforms is determined by the last (boolean) parameter. The transform has both an offset and an orientation. The offset defined a point on the pivot. The orientation is to be considered as a “direction” here. So a line is defined by the first transform. Mind that the transform is relative to the related body object.
The second transform defines how the other body can be located in relation to the pivot.
Hope this helps. The best source of information is to try out different settings, and see how the objects behave. Oh, and don’t forget to turn verbose mode on for the debug renderer - this way you will be able to see the pivot and the limits.
This explanation and your constraint sample helped a lot, thank you! The slider looks like it is working now.
One thing I don’t understand: I’ve seen in your sample that you set Vec3(0,0,45) to set the slider axis 45° downward from the static box. This also worked for me (using 90° instead for vertical sliding), however, the behaviour seems strange to me. Shouldn’t rotation values on the z axis rotate around it, in the same way as setting the heading of a node works? This way, it rotates around the y axis as far as I can see in the sample.
Yes, I noticed this also. Not very intuitiv. I’m currently just passing the values to Bullet. Might be that I don’t understand Bullet here good enough, might be that Bullet just works this way. Maybe we will add a mapping or a more Panda3D-like API (HPR?) in the future. But this has not a high priority currently.