Box2d Filter and doc

Hi

Filter collision is probably one of the hardest part of Box2d.
In documentation the ShouldCollide example has syntax bug.
Should be :

Local Collide:Int = (filter1.GetMaskBits() & filter2.GetCategoryBits()) <> 0 And ..
(filter1.GetCategoryBits() & filter2.GetMaskBits()) <> 0
return collide

Isn't there any way to register a b2ContactFilter ?
Overrided ShouldCollide() is never called.

About Collision Bits now
Do you think it's better to do like this :

Const PLAYERONLY_COLLISION_BYTE:Short = $0001:Short
Const GOLFBALL_COLLISION_BYTE:Short = $0002:Short
Const GOLFBALL_SENSOR_COLLISION_BYTE:Short = $0004:Short
Const OBJECT_COLLISION_BYTE:Short = $0008:Short
Const PLAYER_COLLISION_BYTE:Short = $0016:Short
Const HOLE_COLLISION_BYTE:Short = $00032:Short
Const GROUND_COLLISION_BYTE:Short = $0064:Short

Or values should be rather like that :

Const PLAYERONLY_COLLISION_BYTE:Short = $0001:Short
Const GOLFBALL_COLLISION_BYTE:Short = $0002:Short
Const GOLFBALL_SENSOR_COLLISION_BYTE:Short = $0004:Short
Const OBJECT_COLLISION_BYTE:Short = $0008:Short
Const PLAYER_COLLISION_BYTE:Short = $0010:Short '$0010:Short = 16
Const HOLE_COLLISION_BYTE:Short = $00020:Short '$0020:Short = 32
Const GROUND_COLLISION_BYTE:Short = $0040:Short '$0040:Short = 64

The thing is I found it very hard to setup correctly my player collision mask.
Player should collide with everything at certain moment (see FULL_MASK_BITS)
Sometimes Golfball must pass through (see PLAYER_MASK_BITS) when refiltering.
And a certain kind of object (PLAYERONLY) shall never collide with anything except player.

Const FULL_MASK_BITS:Short = $FFFF:Short
Const PLAYER_MASK_BITS:Short = $FFFF:Short ~ GOLFBALL_COLLISION_BYTE
Const PLAYERONLY_MASK_BITS:Short = $FFFF:Short ~ OBJECT_COLLISION_BYTE ~ GOLFBALL_COLLISION_BYTE ~ GOLFBALL_SENSOR_COLLISION_BYTE ~ HOLE_COLLISION_BYTE ~ GROUND_COLLISION_BYTE

I know it sound confusing but I'm wondering if it's the good way to setup collision filters since everything work like that.

Thanks if you can help

I've fixed the docs, and tweaked the collision filter example to work properly. It seems that Short in BlitzMax is an unsigned 16 bit. Which confused things slightly, as the C++ example used uint16 and int16... so where -1 for int16 is still -1, for a Short, -1 = 65535.
So now, the collision filter example works as expected.

It also uses a contact filter object, of which the ShouldCollide method is called as expected, once the contact filter is added to the world.

Thanks for this fix.

So if I understand well the next Collision Bits values after $0001 $0002 $0004 $0008 are $0010 $0020 $0040 and not $0016 $0032 $0064 right ?

Also, if I'm not changing the world.SetFilter() is the default ShouldCollide() method (hard coded in C++ I suppose) is quicker than the blitzMax one ?

Bits are always like this in each column : $0, $1, $2, $4, $8..
so, after $08 it is $10... and so on. (since each hex column is a set of 4 bits)


The default is probably quicker, yes. The ability to define your own, is useful of course if you want to apply your own special collision handling.

I think the difficulty is understanding how the different flags and groups interact. Hopefully, now that the example works as it should, you can see what all the flags/groups do.

Perfect !
Now things are more clear to me.

Thanks :)