Back to For Beginners

sfml distance between player and enemy

For Beginners

Started by Elit3d August 22, 2015 04:43 AM

9 comments, last by Elit3d 8 years, 8 months ago

Elit3d

359

Author

August 22, 2015 04:43 AM


void Enemy::EnemyDetectPlayer(sf::Vector2f playerPos, sf::Vector2f enemyPos){
	float distance = (abs(sqrt(((playerPos.x - enemyPos.x) * (playerPos.x - enemyPos.x)) + ((playerPos.y - enemyPos.y) * (playerPos.y - enemyPos.y)))));

	if (distance < 200 && m_gunIsOut == true){
		//do something here
	}
}

So I currently have the distance between enemy and player, but the problem is, wherever the enemy is eg (500,50) this distance < 200 wont work correctly because the enemies x+y isn't 0, it would be 500,50.

So my question is, how would I get the distance all around the enemy correctly so when the player is in distance, something will happen?

Lactose

11,696

August 22, 2015 09:24 AM

wherever the enemy is eg (500,50) this distance < 200 wont work correctly because the enemies x+y isn't 0, it would be 500,50.

The distance formula will work regardless of where the entities are.

Neither of the entities need to be at (0, 0).

Are you actually experiencing a problem with this code? If so, for which specific values (both player & enemy)?

Also, you can remove the abs() call completely. It's never ever going to do anything.

Later on, you can also remove the sqrt() call. You can use squared distances instead, and it will still work (but I would suggest you make sure your current code works correctly before doing this kind of change).

Hello to all my stalkers.

Istarnion

692

August 22, 2015 11:40 AM

Just skimmed through the code, but it looks like you're risking to take the square root of a negative number there. I suppose that's why you got that abs() call in there, bur you need to movre it inside the sqrt() call.
I would recommend something like this:
float xDist = abs(playerPos.x - enemyPos.x);
float yDist = abs(playerPos.y - enemyPos.y);
float distance = sqrt(xDist*xDist + yDist*yDist);

But if playerPos and enemyPos is vectors, you probably have a dist() function or something similar? Or (playerPos - enemyPos).length()?
Idk, I haven't used SMFL. The top solution should work just fine in any case.

EDIT:
I had forgotten basic math, so much of what I wrote is wrong. See below.

Programmer of HolyPoly games.

Alberth

10,209

August 22, 2015 11:54 AM

Just skimmed through the code, but it looks like you're risking to take the square root of a negative number there.

That is false. Taking the square of any real number is a non-negative value.

float distx = player.x - enemt.y; // distx can be negative.
float disty = player.y - enemy.y; // disty can be negative.

float distx2 = distx * distx; // distx2 is ALWAYS non-negative (+2 * +2 gives +4, 0 * 0 gives 0, -3 * -3 gives +9)
float disty2 = disty * disty; // disty2 is ALWAYS non-negative

float dxy2 = distx2 + disty2; // dxy2 is ALWAYS non-negative
float dxy = sqrt(dxy2);

Istarnion

692

August 22, 2015 12:24 PM

Woah you learn something new every day c:
I thought it was like normal math, and square root of negative numbers resulted in imaginary numbers.. I believe Math.sqrt() in Java returns NaN on a negative number.http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#sqrt(double)
Guess it was stupid to expect the same behaviour in C++

Programmer of HolyPoly games.

Alberth

10,209

August 22, 2015 12:38 PM

It is normal math that the square of a real or integer number gives a non-negative value

The square root of negative numbers (but that's not happening in the problem of distance calculations!) is normally either a floating point exception or a Nan, I think you can set that behavior (but never tried it).
With a complex math library I'd indeed hope you get an imaginary number, but that is not 'standard' math (which assumes the floating point number domain).

Elit3d

359

Author

August 22, 2015 08:03 PM

Just skimmed through the code, but it looks like you're risking to take the square root of a negative number there.

That is false. Taking the square of any real number is a non-negative value.
float distx = player.x - enemt.y; // distx can be negative.
float disty = player.y - enemy.y; // disty can be negative.

float distx2 = distx * distx; // distx2 is ALWAYS non-negative (+2 * +2 gives +4, 0 * 0 gives 0, -3 * -3 gives +9)
float disty2 = disty * disty; // disty2 is ALWAYS non-negative

float dxy2 = distx2 + disty2; // dxy2 is ALWAYS non-negative
float dxy = sqrt(dxy2);

This is really nicely written thank you, but my same issue still occurs. if i set my enemy to (200,50) and I go over the top of him, my distance will be 200, if I set him to (500,500) im getting in the 700's so how am I going to get the distance around the enemy if the values are totally different depending on where they are?

eg. If I am on the left of the enemy and I am < 200 away from enemy { do something }

If I am on the right of the enemy and I am < 200 away from enemy { do something }

If I am on the top of the enemy and I am < 200 away from enemy { do something }

If I am on the bottom of the enemy and I am < 200 away from enemy { do something }

Lactose

11,696

August 22, 2015 08:21 PM

if i set my enemy to (200,50) and I go over the top of him, my distance will be 200

Then your bug isn't in the code you posted.

EIther your values going into the function are wrong (check in a debugger), or you're doing something wrong somewhere else.

It sounds like maybe you're always comparing to something that's at (0, 0). If the values change when you move the enemy, verify that your player position is correct (again, use a debugger).

Hello to all my stalkers.

Elit3d

359

Author

August 22, 2015 09:38 PM

if i set my enemy to (200,50) and I go over the top of him, my distance will be 200

Then your bug isn't in the code you posted.

EIther your values going into the function are wrong (check in a debugger), or you're doing something wrong somewhere else.

It sounds like maybe you're always comparing to something that's at (0, 0). If the values change when you move the enemy, verify that your player position is correct (again, use a debugger).

I don't really know where to start looking. I am calling the function in my main update function like so:

enemy.EnemyDetectPlayer(player.playerPosition, enemy.enemyPosition);

and the position of each are vector2f

Lactose

11,696

August 22, 2015 09:53 PM

if i set my enemy to (200,50) and I go over the top of him, my distance will be 200

Then your bug isn't in the code you posted.

EIther your values going into the function are wrong (check in a debugger), or you're doing something wrong somewhere else.

It sounds like maybe you're always comparing to something that's at (0, 0). If the values change when you move the enemy, verify that your player position is correct (again, use a debugger).

I don't really know where to start looking. I am calling the function in my main update function like so:

enemy.EnemyDetectPlayer(player.playerPosition, enemy.enemyPosition);

and the position of each are vector2f

That's why I said to use a debugger.

Set a breakpoint inside the function, and step through line by line, inspecting each variable to see what it is.

Debugging is a fundamental skill -- the sooner you learn it, the better.

EDIT: A helpful link. Clicky.

Hello to all my stalkers.

sfml distance between player and enemy

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

sfml distance between player and enemy

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

Reticulating splines