2D C++ Collision detection almost perfect, but not quite?

Question

Just to preface this question please note I am not asking 'fix my code', rather what techniques would I employ to fix this problem. I also apologise if my spelling is not very good.

Okay so I have a 2D platformer game which compares the players position with all of the tiles (in a loop), the resolves the collision accordingly. This is pretty much the structure of the main game loop:

1. Check all collisions (And enable jumping if a collision bellow the player occurred)
2. Get input and change player velocity accordingly
3. Add gravity to the Y velocity
4. Apply velocity and friction to the players position
5. Draw the game
6. repeat

But despite this system working there are two minor, but noticeable problems with the collision system (I have provided images to make it easier). There are two problems, the first is not that bad, but the second renderers the game almost unplayable!

Problem 1. When just moving left and right across the floor in the game, occasionally the player looses all the velocity it has gained and then has to re-accumulate that velocity. I think this is because every now and then my collision detection function does not return properly. here is a image:

I hope that was clear, the problem only really becomes apparent when moving across lots of flat land.

Problem 2 (This one is way worse) The problem is that player can essentially jump up walls, because if you say for example hold down left arrow and hold jump, the player will jump up the wall. I am assuming this is because My collision detection function is returning true if the collision is coming from the side (although it should not). Here is another picture (the text is small, sorry):

So here is my collision detection function, which should take in two 'Objects' then return the direction from the first object at which the collision occurred, I think the problem arouses when It comes to determining the direction as this is causing problems, as shown above:

//Find the collision vectors
        float vectorX = (a.Position.x + (a.Scale.x / 2)) - (b.Position.x + (b.Scale.x / 2));
        float vectorY = (a.Position.y + (a.Scale.y / 2)) - (b.Position.y + (b.Scale.y / 2));

        //Find the distance between the two objects
        float deltaWidth = (a.Scale.x / 2) + (b.Scale.x / 2);
        float deltaHeight = (a.Scale.y / 2) + (b.Scale.y / 2);

        //Stores the direction of collision
        Direction collisionDir = Direction::None;

        //Check if the two objects are intersecting on the x and y axis
        if (fabs(vectorX) < deltaWidth && fabs(vectorY) < deltaHeight)
        {
            //The direction of collision
            float directionX = deltaWidth - fabs(vectorX);
            float directionY = deltaHeight - fabs(vectorY);
            
            //Check for vertical collision
            if (directionX >= directionY)
            {
                //Check for collisions from the top
                if (vectorY > 0) 
                {
                    a.Velocity.y = 0;
                    a.Position.y += directionY;
                    collisionDir = Direction::Up;
                }

                //Collisions form the botttom
                else
                {
                    a.Velocity.y = 0;
                    a.Position.y -= directionY;
                    collisionDir = Direction::Down;
                }
            }

            else if (directionX < directionY / 2)
            {
                //Check for collisions from the left
                if (vectorX > 0 )
                {
                    a.Velocity.x = 0;
                    a.Position.x += directionX;
                    collisionDir = Direction::Left;
                }

                //Collisions form the right side
                else
                {
                    a.Velocity.x = 0;
                    a.Position.x -= directionX;
                    collisionDir = Direction::Right;
                }
            }
        }

        //Return the direction.
        return collisionDir;

This will return a direction, My other code also checks if that direction == Bottom, then it will allow jumping.

Thank-you for any help. I am practising for Ludum Dare, because I plan on (probably) making a platformer and If I cant figure out collision detection I don't know how good my game will be.

Answer 1

First thing I would recommend is make yourself a Vector2D class which holds your x and y coordinates and a few overload some operators to allow for addition and subtraction of two Vector2Ds and multiplication and division by ints, floats and doubles. Trust me it will make your life a lot easier as they can hold all your forces and collision points.

Next when I have used the style of collision you are currently using I have always found that it's:

A)Harder to debug.

B)Harder for other people to follow your code.

So I would recommend creating a Rectangle2D class which handles collisions with other Rectangles and other needed functionality.

As a recommendation have the top left corner and the bottom right corner as a vector from the center of the rectangle which makes scaling and collision detection much easier this also means you can derive the other corners without directly needing to store them.

Here's a code example that will probably help what I'm trying to explain:

bool Intersects(Rectangle2D other)
{
    //Checks the right, left, bottom then top of the rectangle
    //against the other.
    if(other.topLeftCorner.x >= bottomRightCorner.x       //Checks the right
        || other.bottomRightCorner.x <= topLeftCorner.x   //Checks the left
        || other.topLeftCorner.y >= bottomRightCorner.y   //Checks the bottom
        || other.bottomRightCorner.y <= topLeftCorner.y)  //Checks the top
        return false;
    else
        return true;
}

You can easily manipulate this code to give you the direction of the collision. Hope this helps.