C++ Collision Detection causing objects to disappear - c++

Question

I am currently working on some basic 2D RigidBody Physics and have run into an issue. I have a function that checks for collision between a Circle and a AABB but sometimes the Circle (in this case the player) will collide then disappear and if I print out the position when this happens I just set "nan".
bool Game::Physics::RigidBody2D::CircleAABB(RigidBody2D& body)
{
sf::Vector2f diff = m_Position - body.m_Position;
sf::Vector2f halfExtents = sf::Vector2f(body.m_Size.x / 2.0f, body.m_Size.y / 2.0f);
sf::Vector2f diffContrained = diff;
if (diff.x > halfExtents.x)
{
diffContrained.x = halfExtents.x;
}
else if (diff.x < -halfExtents.x)
{
diffContrained.x = -halfExtents.x;
}
if (diff.y > halfExtents.y)
{
diffContrained.y = halfExtents.y;
}
else if (diff.y < -halfExtents.y)
{
diffContrained.y = -halfExtents.y;
}
sf::Vector2f colCheck = diff - diffContrained;
sf::Vector2f VDirNorm = NormVector(colCheck);
sf::Vector2f colToPlayer = NormVector(m_Position - (diffContrained + body.m_Position));
float dist = getMagnitude(colCheck) - m_fRadius;
//std::cout << dist << std::endl;
if (dist < 0)
{
OnCollision((diffContrained + body.m_Position) - m_Position);
m_Position += (VDirNorm * abs(dist));
body.m_Position -= (VDirNorm * abs(dist))* (1.0f - body.m_fMass);
return true; //Collision has happened
}
return false;
}
This happens randomly and with almost no clear reason although it seems to happen more often when the circle is moving fast but can happen as well when it is moving slowly or one or two times when it is not moving at all.
An added note is that I apply gravity to the Y velocity and on collision set the velocity of the coordinating axis to 0.
So my question is, is something clearly wrong here to those with more physics experience than me?
Note: Using SFML for drawing and Vector2 class physics code is all mine.
EDIT: The OnCollision function checks the side the collision so that objects that inherit can use this (e.g. check if the collision was below to trigger a "isGrounded" boolean). In the this case the player checks the side and then sets the velocity on that axis to 0 and also trigger a isGrounded boolean when it is below.
void Game::GamePlay::PlayerController::OnCollision(sf::Vector2f vDir)
{
if (abs(vDir.x) > abs(vDir.y))
{
if (vDir.x > 0.0f)
{
//std::cout << "Right" << std::endl;
//Collision on the right
m_Velocity.x = 0.0f;
}
if (vDir.x < 0.0f)
{
//std::cout << "Left" << std::endl;
//Collision on the left
m_Velocity.x = 0.0f;
}
return;
}
else
{
if (vDir.y > 0.0f)
{
//std::cout << "Below" << std::endl;
//Collision below
m_Velocity.y = 0.0f;
if (!m_bCanJump && m_RecentlyCollidedNode != nullptr)
{
m_RecentlyCollidedNode->ys += 3.f;
}
m_bCanJump = true;
}
if (vDir.y < 0.0f)
{
//std::cout << "Above" << std::endl;
//Collision above
m_Velocity.y = 0.0f;
}
}
}
From debugging out velocity and position no real reason has come to the surface.
inline sf::Vector2f NormVector(sf::Vector2f vec)
{
float mag = getMagnitude(vec);
return vec / mag;
}
Solution:
if (colCheck.x == 0 && colCheck.y == 0)
{
std::cout << "Zero Vector" << std::endl;
float impulse = m_Velocity.x + m_Velocity.y;
m_Velocity.x = 0;
m_Velocity.y = 0;
m_Velocity += NormVector(diff)*impulse;
}
else
{
VDirNorm = NormVector(colCheck);
dist = getMagnitude(colCheck) - m_fRadius;
}

Answer 1

One issue I see is NormVector with a zero vector. You'll divide by zero, generating NaNs in your returned vector. This can happen in your existing code when diff and diffContrained are the same, so colCheck will be (0,0) causing VDirNorm to have NaNs in it, which will propagate into m_position.
Typically, a normalized zero length vector should stay a zero length vector (see this post), but in this case, since you're using the normalized vector to offset your bodies after the collision, you'll need to add code to handle it in a reasonable fashion.