Shifted grid (if I can remember :)) basically involves dividing the play area into a grid and then only checking for collisions between the objects that lie within the same grid. That is, only check for collisions against other objects that are near by - this avoids redundant checks such as checking if an object is colliding with another on the other side of the play area.
However, it's possible a collision can occur between two objects that lie on the edge of two adjacent grids so the grid has to be 'shifted' up, down, left, right and the collision tests performed again. There are other refinements that can be made to the basic algorythm but that's basically it. It's an especially effective technique when you has loads of objects in a large (scrollable) playing area.
However, it's possible a collision can occur between two objects that lie on the edge of two adjacent grids so the grid has to be 'shifted' up, down, left, right and the collision tests performed again. There are other refinements that can be made to the basic algorythm but that's basically it. It's an especially effective technique when you has loads of objects in a large (scrollable) playing area.