//Checks to see if the pixels overlap in two graphicsObjects
private static bool PixelCollision(
GraphicsObject a, GraphicsObject b,
Matrix transformA, int pixelWidthA, int pixelHeightA,
Matrix transformB, int pixelWidthB, int pixelHeightB)
{
//set A transformation relative to B. B remains at (0,0)
Matrix atoB = transformA * Matrix.Invert(transformB);
//Generate normal vectors to each rectangle's side
Vector2 columnStep = Vector2.TransformNormal(Vector2.UnitX, atoB);
Vector2 rowStep = Vector2.TransformNormal(Vector2.UnitY, atoB);
//Calculate the top left corner of A
Vector2 rowStartPosition = Vector2.Transform(Vector2.Zero, atoB);
// Search each row of pixels in A. Start at the top and move down
for (int rowA = 0; rowA < pixelHeightA; rowA++)
{
//begin at the left
Vector2 pixelPositionA = rowStartPosition;
// for each column in the row (move left to right)
for (int colA = 0; colA < pixelWidthA; colA++)
{
// get the pixel position
var x = (int)Math.Round(pixelPositionA.X);
var y = (int)Math.Round(pixelPositionA.Y);
//if the pixel is within the bounds of B
if (x >= 0 && x < pixelWidthB && y >= 0 && y < pixelHeightB)
{
//Get colors of overlapping pixels
Color colorA = a.GetPixelColor(colA + rowA * pixelWidthA);
Color colorB = b.GetPixelColor(x + y * pixelWidthB);
//If both pixels are not completely transparent
if (colorA.A != 0 && colorB.A != 0)
return true; // Colision!
}
// move to the next pixel in the row of A
pixelPositionA += columnStep;
}
// move to the next row of A
rowStartPosition += rowStep;
}
return false;
}
//Main method to check for a collision between two graphics objects
public bool CheckCollisions(GraphicsObject a, GraphicsObject b)
{
//transforms the rectangles which surrounds each sprite
Matrix aTransform = Transform(a.Center, 0.0f, a.Position);
Rectangle aRectangle = TransformRectangle(aTransform, a.Width, a.Height);
Matrix bTransform = Transform(b.Center, 0.0f, b.Position);
Rectangle bRectangle = TransformRectangle(bTransform, b.Width, b.Height);
// collision checking
if (aRectangle.Intersects(bRectangle))
{// rough collision
if (PixelCollision(a, b, aTransform, a.Width, a.Height,
bTransform, b.Width, b.Height))
return true; //What to do if there is a collision - in this case, stop the movement
}
return false;
}
