Grids (The Chickadee Game Toolkit)

From 0e699be281c8dea53e589e08a5831837e0eae7ea Mon Sep 17 00:00:00 2001 From: David Thompson Date: Thu, 7 Oct 2021 20:17:50 -0400 Subject: Updates for Chickadee 0.8.0 and Guile-SDL2 0.7.0. --- manuals/chickadee/Grids.html | 214 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 214 insertions(+) create mode 100644 manuals/chickadee/Grids.html (limited to 'manuals/chickadee/Grids.html') diff --git a/manuals/chickadee/Grids.html b/manuals/chickadee/Grids.html new file mode 100644 index 0000000..ef364be --- /dev/null +++ b/manuals/chickadee/Grids.html @@ -0,0 +1,214 @@ + + + + + + +Grids (The Chickadee Game Toolkit) + + + + + + + + + + + + + + + + + + + + +

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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