Help with a maths problem « Sparrow Forum

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Help with a maths problem

(12 posts) (2 voices)

Started 10 months ago by Oscar
Latest reply from Oscar

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Oscar
Member

Hi folks,

I'm implementing a feature in my platform game that allows the player to "climb" up edges. It should work as if the character grabs the edge of a higher wall/floor and then pulls himself up, but I'm having trouble making it work.

Have a look at this picture:
http://admin.deletionquality.net/dump/climb.jpg

Depending on when the player grabs the edge the distance he should move upwards is different, but I can always find out the value.

Because I can't use sparrow to tween a box2D body (as I originally tried), I have to apply a force to push him up instead. The thing I need to know is some sort of formula that always applies the right amount of force to push the player upwards the total distance. It should also work no matter what the framerate is.

On top of this, the whole movement should always happen in an exact period of time (like a second). This is so that I can match it with a sprite sheet animation of the player pulling up.

If anyone has ideas on how I can work this out please share, thanks.

Posted 10 months ago #
baike
Moderator

I think you should just use b2Body->SetLinearVelocity. You say you know the distance the player has to move, and you know the time (a second), so you know what value to set the velocity to. When the tween completes, change the linear velocity back to whatever is normal for your game (probably zero) and let forces take over again.

Posted 10 months ago #
Oscar
Member

Yes, I've been playing around with that a lot. I do know both the time and distance but how will that lead me to what velocity I should specify?

I currently have an on enter frame event in which I can set the velocity for every frame update.

Posted 10 months ago #
baike
Moderator

You can just use Velocity = Distance / Time. Just make sure your distance is in Box2D units. Your velocity vector will only have a vertical component, so you can leave the x component as 0.

Posted 10 months ago #
Oscar
Member

This sounds too simple to be true, haha! I'll give it a try first thing in the morning.

Just to clearify, if time is one second - the distance is actually what I would specify as the velocity? Shouldn't I take passed time or something similar into consideration?

Posted 10 months ago #
baike
Moderator

Yes, in the case of 1 second, that is right. You shouldn't keep changing the velocity as the player climbs the ladder; it should just remain whatever you calculate at the bottom of the ladder.

However, if your game has gravity, you should also apply a force equivalent to negative gravity to the player while they're climbing the ladder so that gravity doesn't mess with your velocity calculation.

Posted 10 months ago #
Oscar
Member

Ah, things got a bit more complicated there. So you mean I only set the velocity once before the "tween" kicks in?

To counter act the gravity, I should apply a negative gravity force (9,81) on thw player body. I assume this is for each step of the box2d world or would each frame update work just as well?

Posted 10 months ago #
baike
Moderator

There might be easier ways to do this, but I am not familiar with them. For example, you might be able to call b2Body->SetActive(false) and then just tween its position in your onEnterFrame routine. That would make it much simpler.

However, if you leave forces active, you would set the LinearVelocity once at the bottom of the ladder, and then turn it off at the top. But you would apply the negative gravity force in onEnterFrame, because the b2World will apply a positive gravity on every world step, so you would need to cancel it out.

Posted 10 months ago #
Oscar
Member

I've been thinking of other ways to do it but I haven't come up with something easier either. I will definitely try the set active to false method, that didn't even cross my mind. That way I would also be able to work with easings of the tween.

The rest makes perfect sense. It should work in theory. I'll give it a try, thanks!

Posted 10 months ago #
Oscar
Member

Ok, I thought I'd share this if anyone runs into some similar error.

First off, I wasn't able to tween the body while it was set as inactive. Nothing happened at all. It makes sense because if the body is inactive you can't mess with it.

The problem with applying a force to counter gravity is that the gravity accelerates the object. This makes the body you are trying to "nullify", to slowly descend. I couldn't figure out a way to work around this. I do know a few sites on the web that show how to work around this by manipulating the b2Island class to make a custom gravity, but I think this was overkill for my project. Also this method seems to rely on my frame rate as well.

So what I did was to simply make my character keep being pushed up until he was above the edge. The time needed for this was different though, so I'm going to have problems matching this with a sprite sheet animation. I might be able to set the frame rate of the MovieClip or something similar.

Posted 10 months ago #
baike
Moderator

Can you leave the body active and just move it after the world step? When the ladder climb starts you would record the body's position in an ivar. Then tween that ivar as you would any other numeric value in Sparrow. After the world step, just update the body's position to that of the tweened ivar. This is effectively ignoring Box2D simulation until the ladder climb is complete (although any collisions would still register, which is good).

Posted 10 months ago #
Oscar
Member

I actually tried that exact method as well. I might have moved the body in the wrong time of the time step (should probably be just after the step like you said). I should also probably have applied a force like do now that tries to counter act the gravity. My tests resulted in a whole lots of shaky movement on the character which was just terrible, some tweaking and it may work though.

I'm happy with my current result though It looks pretty good and it's matching the sprite sheet animation. The time wasn't as important as I first thought it would be.

EDIT:
I managed to get a perfect tween by setting the position of the body just after the time step. This removed any shakes like I had in my earlier attempts. Because a tween is way more accurate I think I'll go ahead and use this method!

Posted 10 months ago #

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