About CollisionManagerGrid

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Aug 28, 2018, 9:29:19 AM8/28/18
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when calculating cell_width in __init__() function of CollisionManagerGrid, it says: 

1. in objects with same size case:
    cell_width = 1.25 * object width

2. in mixed width case:
    cell_width = 1.25 * {width (object): all objects not exceptionally big}
    what does it mean??? 
    what is the value of cell_width on earth in such case?


claudio canepa

Aug 28, 2018, 10:42:40 AM8/28/18
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Suppose you have  100 collidables with dimensions between 16 and 32, and then four big collidables  400 x 400
A good cell_width would be 1.25 * 32

A grid collision maneager works by
  - breaks the world in cells
  - each object is added to all the cells its overlap
  - when asking for collision with object A, we need to look only in the cells overlapped by A

So, if the cell size is too big they are too many objects to look in each cell.
On the other side, if cell size is too small, object A will overlap too many cells.

 If all the collidables are same size,and small relative to world size, cell_width as 1.25 * cell_width works well.

What if all the collidables are small relative to world size but not same size?. Well, 1.25 * max_width may be good enough.

What if a few big objects are added to the previous scenary ? Discard the big objects to calculate cell_width

What if theres no small size bigger than most of collidables sizes? Then probably the grid manager will not behave well, a quadtree would be better suited

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Aug 29, 2018, 4:12:38 AM8/29/18
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Thanks Claudio so much for your clear explanation. 

so based on my understanding,  I  guess  the statement like this would be better:
cell_width = 1.25 * max {width (object): all objects not exceptionally big} 

claudio canepa

Aug 29, 2018, 4:52:34 AM8/29/18
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Yes, i will change the docstring, thanks.

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