KinematicBody3D Move_and_slide vs. Move_and_collide ... Any logic in different input and output ???

Question

According to documentation:

Move_and_collide
... takes a Vector3 "relative velocity"
... returns a KinematicCollision-object (possibly empty if no collision occured)

Move_and_slide
... takes a Vector3 "linear velocity"
... returns a Vector3 "linear velocity" (rotated/scaled if collision occured)

Questions:

1. What is the difference between "Linear velocity" and
   "Relative velocity"?

2. What is the logic in returning different things, for functions that
   essentially does the same thing, with a slight twist?

3. I would prefer if both functions returned same-type velocities, as
   they require for input, and then for me to query possible collisions
   manually ... What am I missing?

4. Why are Vector-maths melting my brain?

Answer 1

move_and_collide() is the basic movement method for kinematic bodies. It attempts to move the body along the given vector, stopping if it collides with another body. If a collision happens, it returns a collision object containing the details.

move_and_slide() was designed as a special case, because a common use case was to calculate a slide vector along the collision. However, because it slides, it's possible for there to be more than one collision in a frame (think when you hit a corner between the floor and the wall. For this reason, it can't return a collision, so you need to use the get_slide_collision() method to retrieve the details. Fun fact: move_and_slide() calls move_and_collide() internally to process the movement.

Both take a vector for the movement, but move_and_collide(), being the more basic method, is just the frame-based (ie multiplied by delta) movement vector. move_and_slide(), however, takes the pre-delta velocity (yes, this can be confusing).

Hopefully that clarifies it at least somewhat questions 1-3.

As for #4, what is melting your brain about vectors? Have you read this:
https://docs.godotengine.org/en/3.0/tutorials/math/vector_math.html ? I think it's a pretty good overview of the topic specifically applied to its use in games.

Comments

Thanks for your answer

So, as to 1) ... Relative vs. Linear is "Vector3*delta" vs. just "Vector3" ... Both are "Relative to Frame-time"? ... If so, I think the chosen nomenclature is somewhat confusing :)

Just to clarify:
move_and_collide() should be fed a pre-multiplied velocity Vector3 (Vector3*delta).
move_and_slide() should be fed a raw velocity Vector3, and does the delta internally.

And yes, I have read the tutorial on vector-math, along with several other resources on the subject ... Still I'm having troubles :/

PS: the most helpful info I got from you answer, is the fact that move_and_slide() calls move_and_collide() internally ... that helps my understanding of what really is going on.