Grids (The Chickadee Game Toolkit)

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5.6.5 Grids

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The (chickadee data grid) module provides a simple spatial -partitioning system for axis-aligned bounding boxes -(see Rectangles) in 2D space. The grid divides the world into -tiles and keeps track of which rectangles occupy which tiles. When -there are lots of moving objects in the game world that need collision -detection, the grid greatly speeds up the process. Instead of -checking collisions of each object against every other object (an -O(n^2) operation), the grid quickly narrows down which objects could -possibly be colliding and only performs collision testing against a -small set of objects. -

In addition to checking for collisions, the grid also handles the -resolution of collisions. Exactly how each collision is resolved is -user-defined. A player bumping into a wall may slide against it. An -enemy colliding with a projectile shot by the player may get pushed -back in the opposite direction. Two players colliding may not need -resolution at all and will just pass through each other. The way this -works is that each time an object (A) is moved within the grid, the -grid looks for an object (B) that may possibly be colliding with A. A -user-defined procedure known as a “filter” is then called with both -A and B. If the filter returns #f, it means that even if A and -B are colliding, no collision resolution is needed. In this case the -grid won’t waste time checking if they really do collide because it -doesn’t matter. If A and B are collidable, then the filter returns a -procedure that implements the resolution technique. The grid will -then perform a collision test. If A and B are colliding, the resolver -procedure is called. It’s the resolvers job to adjust the objects -such that they are no longer colliding. The grid module comes with a -very simple resolution procedure, slide, that adjusts object A -by the smallest amount so that it no longer overlaps with B. By using -this filtering technique, a game can resolve collisions between -different objects in different ways. -

Procedure: make-grid [cell-size 64]: Return a new grid partitioned into cell-size tiles. -

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Procedure: grid? obj: Return #t if obj is a grid. -

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Procedure: cell? obj: Return #t if obj is a grid cell. -

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Procedure: cell-count cell: Return the number of items in cell. -

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Procedure: grid-cell-size grid: Return the cell size of grid. -

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Procedure: grid-cell-count grid: Return the number of cells currently in grid. -

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Procedure: grid-item-count grid: Return the number of items in grid. -

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Procedure: grid-add grid item x y width height: -
Add item to grid represented by the axis-aligned bounding -box whose lower-left corner is at (x, y) and is -width x height in size. -

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Procedure: grid-remove grid item: Return item from grid. -

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Procedure: grid-clear grid: Remove all items from grid. -

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Procedure: grid-move grid item position filter: Attempt to move item in grid to position (a 2D -vector) and check for collisions. For each collision, filter -will be called with two arguments: item and the item it collided -with. If a collision occurs, position may be modified to -resolve the colliding objects. -

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Procedure: for-each-cell proc grid [rect]: Call proc with each cell in grid that intersects -rect, or every cell if rect is #f. -

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Procedure: for-each-item proc grid: Call proc for each item in grid. -

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Procedure: slide item item-rect other other-rect goal: -
Resolve the collision that occurs between item and other -when moving item-rect to goal by sliding item-rect -the minimum amount needed to make it no longer overlap -other-rect. -

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