CODEWITHSUNDEEP
c++algorithmsfmltrigonometry
user2663041
user2663041

Reputation: 229

How to rotate a sprite to a point?

I'm trying to to rotate a sprite, let's say it's a gun, to a point, by taking the smallest route possible on a 360° (right or left depending on what side the point is closer to) and I'm having a few problems.

On a circle, it jumps from 359° to 0° so I can't direcly use my target angle - current angle.

Leep in mind that I'm using SFML so it's executing the function everyframe while isRotating is true.

This is the information that is available to me :

//The angle in degrees of my sprite
float currentAngle = pSprite.getRotation();
//The angle is needs to be once the rotation is over
float targetAngle = float(atan2(deltaY,deltaX) * 180 / (atan(1)*4) + 180);

I'm using a speed variable to increment or decrement the value of the angle every frame.

distance = Speed*Time.asSeconds();
currentAngle += distance;

Upvotes: 2

Views: 481

Answers (1)

svinja
svinja

Reputation: 5576

First, get the difference:

diff = target - current

diff is either the "short" angle (the one resulting in a shorter rotation), or the "long" angle (rotation in the other direction which is longer). Notice that:

  • you never need to rotate for more than (as an absolute value) 180 degrees to get from one angle to another.

  • the "short" angle and the "long" angle have opposite signs (+/-) and their absolute values add to 360.

Example: 0 - 359 = -359. This is the "long" angle. We know this because its absolute value is > 180. The "short" angle will have the opposite sign and its absolute value will add up to 360 with 359, so it is 1.

An easy way to calculate this is:

while (diff < -180) 
    diff += 360;
while (diff > 180)
    diff -= 360;

diff is now the angle you are looking for. So if it is negative, multiply your Speed by -1.

The while (as opposed to if) is there in case the angles are not in [0, 360] - for example, you have current = 1440, target = 359.

Upvotes: 1

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