XNA move from start position to target position exactly in 3D

Question

If I have a list of positions that map out a path a character should follow. What would be the best way to move at a constant speed to each position making sure the character lands exactly at each position before moving onto the next?

For example the character is at position A, we then queue up position B and position C. The character cannot move towards position C until it reaches position B exactly.

It would be great if the solution worked at slower frame rates/update speeds as well.

Answer 1

Why do you necessarily want the character to get to a point when a frame is drawn? You want your logic to be separated from view, that's why you use a timer and not onEnterFrame event, in first place. at smooth 30 FPS or more, a player won't see a difference.

What you want to do is not actually drawing the character on desired position, but behaving like if he was there. What you do now is probably calculating a delta_time everyframe by substracting old timestamp from an actual time: delta_time = Now() - old_timestamp; old_timestamp = Now();. And then using this data to move objects (e.g. x += vx * delta_time) etc. and then update them on screen. What you want to do in this case is to calculate objects (at least these that can be altered by the character's position) twice - at the moment the character reached a point, and at a moment right after, that is, delta_time after previous frame:

1. Calculate when the character will come to point B (from A): journey_time = distance / speed.

2. On next frame, get the delta_time from previous frame.

3. If delta_time >= journey_time go to next point (4). Else journey_time -= delta_time and go to point 2.

4. Calculate everything for the moment the character reaches destination point, e.g. simulatePhysics ( journey_time );

5. Add actions special to the event (because you want to detect reaching the point for some particular reason I suppose), e.g. make the character ghostlike: physics.removeChild ( character );

6. Calculate second part of the 'frame': simulatePhysics ( delta_time - journey_time );.

7. Redraw the frame.

Answer 2

There is no solution that satisfies this demand precisely.

1. You would need to cap the framerate to a framefrate the player's machine can handle constantly. (the lowest framerate the game reaches when computation is heavy)
2. Calculate the distance from the current position to the first destination.
3. Divide that distance by the speed you wish the character to move at divided by the current framerate to get the amount of frames it would take to reach the destination. distance / (speedPerSecond * (1.0/frameRate)) = framesLeft
4. This could be a none integer number such as framesLeft = 54.76. We round this number to the nearest integer number roundFrameCount = 55 and calculate a fake speed of fakeSpeed = (distance / roundFrameCount).
5. We now move the object at this rate each frame.

Otherwise, when the object is too close to the first destination, you have to deduct the distance from that destination from the stepSize = speed * delta and use the rest of the stepSize - distance to move towards the next destination.