Box2d Filter and doc

BlitzMax Forums/Brucey's Modules/Box2d Filter and doc

Armitage 1982

(Posted 2008) [#1]

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

Brucey

(Posted 2008) [#2]

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.

Armitage 1982

(Posted 2008) [#3]

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 ?

Brucey

(Posted 2008) [#4]

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.

Armitage 1982

(Posted 2008) [#5]

Perfect !
Now things are more clear to me.

Thanks :)