Game Development Reference
In-Depth Information
Fig. 27.2
Calculating the
overlap rectangle using the
minimal and maximal
x
and
y
coordinates
The
Intersection
method takes two bounding boxes and returns another rectangle that
represents the overlap of the two boxes.
In order to calculate this overlap rectangle, we need to know what the minimal
and maximal
x
and
y
coordinates are of the rectangle (see Fig.
27.2
).Usingafew
useful properties from the
Rectangle
class in combination with the
Min
and
Max
meth-
ods of the
Math
class, we can calculate these values quite easily:
int
xmin = (
int
)MathHelper.Max(rect1.Left, rect2.Left);
int
xmax = (
int
)MathHelper.Min(rect1.Right, rect2.Right);
int
ymin = (
int
)MathHelper.Max(rect1.Top, rect2.Top);
int
ymax = (
int
)MathHelper.Min(rect1.Bottom, rect2.Bottom);
Now we can calculate the position and size of the overlap rectangle, and return it
from the method:
−
xmin, ymax
−
ymin);
return new
Rectangle(xmin, ymin, xmax
Inside the
CollidesWith
method in
SpriteGameObject
, we store the overlap rectangle by
calling the
Intersection
method from the
Collision
class:
Rectangle b = Collision.Intersection(BoundingBox, obj.BoundingBox);
In order to check for collision within the overlap rectangle, we use a nested
for
-
instruction to walk over all the pixels in the rectangle:
for
(
int
x = 0; x < b.Width; x++)
for
(
int
y = 0; y < b.Height; y++)
checkifthepixelsatposition(x,y)arebothnottransparent
Inside the nested loop, we have to calculate what the local pixel coordinates are in
the current sprite, as well as the sprite that was passed as a parameter. Again, we
need to calculate these local coordinates using global positions, and we need to take
the origin into account:
