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Physics engine for SHMUP?


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#1 derpness   Members   -  Reputation: 109

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Posted 07 January 2014 - 08:04 PM

I am currently working on SHMUP with a new twist.

 

Players can link together and create a barrier between them. It's essentially a line that connects the two players and destroys any bullets that pass through it.  However I'm having a hard time with the tunneling effect. I've essentially got to detect collisions between two moving line segments.  

 

I've tried a lot of things but at this point I'm wondering if I should do a full physics engine like box2d or I'm hoping that there might be something simpler out there for CCD in a shump environment.

 

The full engine seems like overkill since I won't really need any other features from it.

 

You can see an example of this in the screenshot. I activated the shield. moved one player up and missed a ton of bullets. 

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  • badcollision.png


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#2 jHaskell   Members   -  Reputation: 1053

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Posted 08 January 2014 - 09:12 AM

How are you testing for collision detection?

 

Testing for collision between small and fast moving objects can be problematic with a simple detection scheme.  One method to try is to generate line segments between each bullets previous position and their current position and perform line segment intersection tests between them and your 'barrier'.

 

If the bullets are moving fast enough, and/or your framerate is low enough, then bullet positions can change substantially from frame to frame and potentially jump from one side of the barrier to the other without actually colliding with it.  Generating a line segment and testing for intersection eliminates this possibility.

 

If the player ships can also move pretty fast (and thus the barrier itself can move fast), you may even need to generate a polygon encompassing the 'swept' area of the barrier and test for intersection with this polygon to assure 100% detection.



#3 derpness   Members   -  Reputation: 109

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Posted 08 January 2014 - 11:04 AM

I'm actually testing with line segments already. I think the polygon test is probably what i really need. because the players can move fast enough that even the line segments between bullet positions will pass the barrier.  I'll see what I can do to check if the line segment of the bullets movement intersects the polygon created by the player's movement.

 

Thanks!



#4 BeerNutts   Crossbones+   -  Reputation: 2978

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Posted 09 January 2014 - 02:17 PM

I wouldn't dismiss using a physics engine even for a shump.  You can do some cool things with those engines, and it really costs you nothing.  I personally use chipmunk-physics, but Box2d would be fine too.

 

For example, you might want to create a weapon that it's bullet, instead of killing (or damaging) a enemy, it draws close enemies towards it (think a large electromagnetic bullet), and if they hit the bullet, it either explodes, or maybe the enemy sticks to it and it explodes when it reaches the edge of the screen.

 

Anyway, just thinking out loud.


My Gamedev Journal: 2D Game Making, the Easy Way

---(Old Blog, still has good info): 2dGameMaking
-----
"No one ever posts on that message board; it's too crowded." - Yoga Berra (sorta)

#5 derpness   Members   -  Reputation: 109

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Posted 09 January 2014 - 03:07 PM

Good point. However this is my first real game so I'm trying to keep things as simple as possible. I'm actually afraid that I'm already being too ambitious for a first project.

 

FWIW I also found out that one of my major issues wasn't with collision but rather with the way I was cleaning objects out of a vector. I was using ".erase" to remove but I didn't realize that erase invalidates the current iterator and that I have to use it's return value instead.

 

so

for (auto itr=bullets.start; itr != bullets.end(); ++itr) {
    if ((*itr)->wasDestroyed()) bullets.erase(itr);
}

Needed to be:

for (auto itr=bullets.start; itr != bullets.end()) {
    if ((*itr)->wasDestroyed()) itr =bullets.erase(itr);
    else ++itr;
}

So that my bullets could actually get cleaned up in the correct frame.



#6 phantom   Moderators   -  Reputation: 7400

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Posted 09 January 2014 - 04:03 PM

This is where the standard STL algorithms come in handy, certainly when coupled with C++11 lambdas;

bullets.erase(std::remove_if(std::begin(bullets), std::end(bullets), [](BulletVector::Type &itr) {return (*itr)->wasDestroyed();}), std::end(bullets));


Where;
BulletVector::Type is whatever type you put in your vector.

This is also known as the Erase-remove idiom

Edited by phantom, 09 January 2014 - 04:03 PM.


#7 ferrous   Members   -  Reputation: 2075

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Posted 09 January 2014 - 06:42 PM


I wouldn't dismiss using a physics engine even for a shump.  You can do some cool things with those engines, and it really costs you nothing.  I personally use chipmunk-physics, but Box2d would be fine too.

 

While the OP has already stated he wants to keep things simple.  (Which is good!)  I just wanted to chime in and say that you can get a lot of bang for your buck by including a free physics engine in your game.  It can be pretty amazing how much emergent gameplay can come from just having actual physics at your fingertips.



#8 derpness   Members   -  Reputation: 109

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Posted 09 January 2014 - 09:03 PM

To that end which engine would you advice a c++/gamedev novice to look into first? Or which makes the most sense in a 2D SHMUP situation? (i.e. No gravity/mostly unrestricted movement.)



#9 BeerNutts   Crossbones+   -  Reputation: 2978

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Posted 10 January 2014 - 08:12 AM

To that end which engine would you advice a c++/gamedev novice to look into first? Or which makes the most sense in a 2D SHMUP situation? (i.e. No gravity/mostly unrestricted movement.)

 

Both chipmunk-physics and box2d are good options.  Take a look at them, and you can decide which you'd rather use.

 

There's a fairly small learning curve, but, moving all your objects and detecting collision as well as automatic collision responses are done for you automatically.

 

Also, my old blog (in my sig) details me making a top-down game using chipmunk-physics;  you're welcome to take a look at it to see how I use the physics engine.

 

If you'd rather do it on your next project, that's fine too, but I do recommend trying it at some point.


My Gamedev Journal: 2D Game Making, the Easy Way

---(Old Blog, still has good info): 2dGameMaking
-----
"No one ever posts on that message board; it's too crowded." - Yoga Berra (sorta)




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