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2D Platformer - Shooting in the direction of the mouse cursor


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#1 Mekuri   Members   -  Reputation: 299

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Posted 09 December 2012 - 04:02 PM

I've been working quite a while on my 2D tile based side scroller, and it's been going smooth for quite a while, but now I've run into a problem that I'm hoping someone could help me with.

I've just implemented my first throwing weapon- a snowball. Now I want the player to be able to throw in the direction of the mouse curser, instead of just throwing to either sides. The problem is I can't seem to grasp how to do this the right way.
When I create a snowball I give it a starting position and a velocity (a Vector2). I add the velocity to the position, and update the velocity with gravity and such accordingly.
My current solutions, that I haven't had good results with are the following:
- Convert the mouse position into tile coordinates, and use the distance in tiles on the y axis to determine the up- or downwards speed in worldspace. No changes to the X velocity,
- Do some math with radians or degrees, to get the angle between the player and the cursor. This solution is one I haven't tested, since I have a hard time figuring out the math

My last suggestion is just trial and error and simply hardcode some values that I find works out, based on the mouse position, but this is really not the way to do it (unless there really aren't any other way, which I doubt).

I am going to sneak in another quick question about weather.
I've recently added rain and snow to my game. Snow will slowly gather on the ground and be "harvestable". Now I want the snow to smelt again, when it gets warmer, but I'm looking for a smart way of doing this. Currently I just randomly pick a spot on the x-axis in tile terms within a set distance of the player, and then I iterate through the Y axis to check for snow, and remove it. I do this to add a sense of smelting over time, instead of the snow just dissapering in a predictable manner. Is my approach good enough, or are there better ideas?

If you have a hard time picturing what I mean, I have a video you can look at to see the snow in effect. If you don't want to see the whole video go to 1.28 to see where the snow starts, or go to 2:05 for the harvest part.



So to sum it up:
- How do I shoot/throw an object in the direction of the mouse in a 2D tile based platformer?
- (Not so important) Are there a smart way to make the snow smelt over time, or is my method good enough?

Any help is greatly appreciated. - Thanks in advance :-)

Check out the game I am making here - http://www.youtube.com/user/NasarethMekuri


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#2 Sik_the_hedgehog   Crossbones+   -  Reputation: 1492

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Posted 09 December 2012 - 04:28 PM

To get the angle between two points: atan2(y2 - y1, x2 - x1). Yeah, there isn't much else to it. Just beware that atan2 returns the value in radians, so you'll need to convert it to degrees (or whatever your game uses).

EDIT: atan2 assumes Y goes upwards and angles are counterclockwise. If either (not both) of these is true, you should use y1 - y2 instead to invert the angle.

Edited by Sik_the_hedgehog, 09 December 2012 - 04:30 PM.

Don't pay much attention to "the hedgehog" in my nick, it's just because "Sik" was already taken =/ By the way, Sik is pronounced like seek, not like sick.

#3 Trienco   Crossbones+   -  Reputation: 2054

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Posted 09 December 2012 - 11:08 PM

Not sure why anyone would want to deal with angles, when your problem sounds like all you want is "mousePosition - shooterPosition". If all throws are supposed to be equally strong, normalize and multiply with appropriate value. Otherwise just multiply with appropriate factor (and maybe clamp). However if distance to player decides initial speed, it makes aiming weak throws more difficult.
f@dzhttp://festini.device-zero.de

#4 Mekuri   Members   -  Reputation: 299

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Posted 10 December 2012 - 01:36 AM

To get the angle between two points: atan2(y2 - y1, x2 - x1). Yeah, there isn't much else to it. Just beware that atan2 returns the value in radians, so you'll need to convert it to degrees (or whatever your game uses).

EDIT: atan2 assumes Y goes upwards and angles are counterclockwise. If either (not both) of these is true, you should use y1 - y2 instead to invert the angle.

Radians will work fine for me. I use radians for swinging weapons and tools around, so I've already read up on those, so might as well use them. Thanks for the info, this does seem quite simple :-)

Not sure why anyone would want to deal with angles, when your problem sounds like all you want is "mousePosition - shooterPosition". If all throws are supposed to be equally strong, normalize and multiply with appropriate value. Otherwise just multiply with appropriate factor (and maybe clamp). However if distance to player decides initial speed, it makes aiming weak throws more difficult.


You might be right. I just woke up, and after reading your post I suddenly remembered that a long time ago I played around with one of those XNA tutorials.. and if I remember correctly you actually create a very basic game where you shoot in the direction of the mouse. Then I just apply the appropriate physics, and I should be set to go. I'll go dig around for it.

Thank you both for your replies. I'll post back here with whatever solution I come up with.

I am still open for comments regarding my smelting snow :-)

Check out the game I am making here - http://www.youtube.com/user/NasarethMekuri


#5 Mekuri   Members   -  Reputation: 299

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Posted 10 December 2012 - 03:52 AM

I'm back, and with success.
So to get the snowball to "shoot" towards the mouse pointer, I do the following:

First I convert the mouse position into worldspace position. I then create a Vector2 for the direction, and give it the value of the convertedMousePosition minus the spawn point for the snowball.
Vector2 direction = mousepos - snowballSpawnPos;

After that I normalize the direction vector, which gives a vector with a length of one, which is then used to determine the direction to move. Then simply multiply direction with a float with the speed of the snowball. If you multiplay with gametime.ElapsedGameTime.TotalSeconds the speed will be the number of units it moves pr second.

So basically it would be something like this:
Vector2 direction = mousePos - snowballSpawnPos;
float speed = 200.0f;
if(direction != Vector2.Zero) //If direction is 0,0 normalize would throw a divide by zero exception.
	 direction.Normalize();

//Update code
public void Update(GameTime gameTime)
{
	 Position += direction * speed * (float)gameTime.ElapsedGameTime.TotalSeconds;
}

I found this solution on StackExchange - here's the link: http://gamedev.stack...e-mouse-pointer

Check out the link if my explanation is to vague Posted Image

Thanks for your help guys!

Check out the game I am making here - http://www.youtube.com/user/NasarethMekuri





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