How i can do colisions with irregular hitboxes?

Question

I'm developing my new platform engine and i don't know how to program collisions with slopes - I'm using hitboxes. For the collisions of rectangles i use this:

if (player.rec(Position, level.scale).Intersects(t.rec(level.scale)))

but this doesn't work for sloped collisions or triangular hitboxes. How can I do this?

Answer 1

You want to read up on PIP algorithms. You'll be also able to find many implementations, even in C#. Here is my suggestion:

public static List<Vector3> intersectShape
    (Geometry polygon_to_cross_0, Geometry polygon_to_cross_1)
{
    List<Vector3> list_vertices = new List<Vector3>();

    for (int i = 0; i <= polygon_to_cross_1.Points_.Count - 1; i++)
    {
        if (Geometry.isInsideOf(polygon_to_cross_1.Points_[i], polygon_to_cross_0))
            list_vertices.Add(polygon_to_cross_1.Points_[i]);

        if (i < polygon_to_cross_1.Points_.Count - 1)
        {
            foreach (Vector3 p in Geometry.getIntersectionVertexGeometry(
                new Vector3[]{ polygon_to_cross_1.Points_[i], 
                polygon_to_cross_1.Points_[i + 1] }, polygon_to_cross_0))
            {
                list_vertices.Add(p);
            }
        }
    }

    for (int i = 0; i <= polygon_to_cross_0.Points_.Count - 1; i++)
    {
        if (Geometry.isInsideOf(polygon_to_cross_0.Points_[i], polygon_to_cross_1))
                list_vertices.Add(polygon_to_cross_0.Points_[i]);

        if (i < polygon_to_cross_0.Points_.Count - 1)
        {
            foreach (Vector3 p in Geometry.getIntersectionVertexGeometry(
                new Vector3[] { polygon_to_cross_0.Points_[i], 
                polygon_to_cross_0.Points_[i + 1] }, polygon_to_cross_1))
                {
                    list_vertices.Add(p);
                }
            }
        }

        return sortPolygonClockWise_XZ((list_vertices));
    }

Count(intersectShape(Geom1, Geom2)) > 0 == true stands for collision.

Answer 2

It is not just a rectangle/rectangle test. It is also a dot product test to determine which side of the slope line the other object is on. For this you will need to initialize the 2 end points of your slope. This becomes a line segment and the corners of your slope's collision rect. So your slope and rect both share point A & B.

The algorithm gos like this:

If other object is in the rect, check which side of the line it is on,

If above,

 No collision,

else

 Collision

In XNA, that would go something like this,

if(objectRect.Intersects(slopeRect)
{
  Vector2 objectRelativePosition = objectPosition - PointA;
  Vector2 slopeRelativePosition = PointB - PointA;
  Vector3 slopeNormal3 = Vector3.Cross(Vector3.UnitZ, Vector3(slopeRelativePosition,0));
  Vector2 slopeNormal = Vector2(slopeNormal3.X, slopeNormal3.Y);

  if(Vector2.Dot(slopeNormal, objectRelativePosition) > 0)
    {
      //object is above slope, no collision
    }
    else
    {
      // object is below slope, collision
    }
}

The slopeRelativePosition, slopeNormal3, & slopeNormal can all be done only once in the initialization of your map, not each frame. but objectRelativePosition must be recalculated each time you want to run a collision check with the slope.

You can also run this check against each point that makes up the object (say, each corner of a rectangle) if there is no single "position" for the object.

Answer 3

I'm think it can be done like this:

class Tile
{
    public virtual float GetTileY(float pX)
    {
        return 1; // Mean tile size is 1x1 units; I'm using 1 unit = 8 pixels, for example.
    }
}

class SlopeTile : Tile
{
    public bool IsFlipped; // If true look like |\, if false - like /|.
    public override GetTileY(float pX)
    {
        return this.IsFlipped ? 1 - pX : pX;
    }
}

You send to GetTileY() value from 0 to 1 that mean how far on this tile player standing and get height, also as value from 0 to 1.

Vector2 position = ...;
Vector2 offset = ...;
Tile tile = ...;
float new_x = position.X + offset_x;
float new_y = position.Y + tile.GetTileY(new_x % 1);

Note, I'm not a professional but I'm also trying to make platformer and this is way I'm want to make slopes.