cpHashset size does not decrease after removal of shape obje

[psudheer28]

Hello everyone,

i am using 25 objects ( shapes) in my game, cpHashSet size is 47, after collision object(shape) will remove from the space.

I think after removing the objects the cpHashSet size should be decrese, but its not.

I am unable to understand what exactly happening.

plz give explanation for this

thank you

sudheer

[Tam Toucan]

Code: Select all

typedef struct cpHashSet {
	// Number of elements stored in the table.
	int entries;
	// Number of cells in the table.
	int size;

Why would the numer of cells decrease?

[slembcke]

As Tam pointed out, hash tables have a separate table size and number of entries they store. Think of it as the amount of space allocated compared to the amount of space used.

[psudheer28]

Thanks Tam and slembcke,
Now I understand concept of Hashset,and also able to visualize it. Thank you once again.

Actually I am developing a game and have to do a performance tuning. I think game performance can be improve by applying collision filtering(previously I filtering collision in call back function). recently I implement group filtering and performance improve a little bit. I think, more performance improve if I implement Bounding Box but I don't understand it and unable to implement it.

can you please explain me.

Thanks

[maximile]

Chipmunk automatically rejects collisions if the bounding boxes don't intersect. There's no need to do anything extra there.

[psudheer28]

Thanks maxmile,
is chipmunk automatically define bounding boxes for shape we create ?
and also are we able to define layer and group to same shape?

can you please explain me,How we can use layer I think description in the document is insufficient.

[Tam Toucan]

For chipmunk to try the "expensive" collision code checking it first does the following fast checks to do early rejection.

Code: Select all

queryReject(cpShape *a, cpShape *b)
{
	return
		// BBoxes must overlap
		!cpBBintersects(a->bb, b->bb)
		// Don't collide shapes attached to the same body.
		|| a->body == b->body
		// Don't collide objects in the same non-zero group
		|| (a->group && b->group && a->group == b->group)
		// Don't collide objects that don't share at least on layer.
		|| !(a->layers & b->layers);
}

So Chipmunk automatically does BB checking, it then checks that the shapes aren't in the same group (so you generally give all shapes for the same object the same (non-zero) group value.

Finally it checks that the two shapes share the same layer. Think of the layer a 32 flat planes stacked parallel to each other. The bit pattern of the layer value determines which planes the shape is on. For two shapes to collide they must have a common plane.

So if shapeA has layer = 0x00000001 and shapeB has layer = 0x00000003 they can collide since they are both on the plane for bit0. If shapeB didn't have bit0 set then it could never collide with shapeA.

[psudheer28]

Hi Tam,
Thank you very much for such a good explanation about collision filtering.

I have implement group and layer, now game has much better performance.


Thanks