Re: [pygame] extending pygame masks

[Michael George <mdgeorge@xxxxxxxxxxxxxx>]

Nirav Patel wrote:
> It's not necessarily a good idea to have stuff that specific to an app
> in a library.  I think it would be good to have the convolution
> function built into mask, and leave the rest of the implementation up
> to the user.  As for how to deal with the interface for convolving,
> perhaps it would be useful to have one of the masks specified as being
> the "world", and the other as the object being moved? Something like:
> Mask.convolve(world = Mask) returning a mask the size of the world mask.
>
>   
I think it makes more sense to do it the other way - world.convolve(obj) 
rather than obj.convolve(world).  The two are flips of each other 
anyway, so it's probably not a big deal.  In either case, the output 
needs to be bigger than both of the inputs (the size of the convolution 
is the sum of the sizes of the inputs, minus one).

I guess it's not too hard to compute the rects if you're careful. 
> It would be fairly easy to construct this world mask by using Mask.draw.
>
>   
Yeah, that's probably best.  In fact, I was thinking that I might need 
to do it that way anyway.  The only reason to do it for a collection in 
one go is that you can avoid allocating all of those intermediate masks, 
and shifting everything twice.  I guess masks are pretty small and 
shifts are pretty fast so it's not such a big deal.

So here's a revised proposal:

Mask.convolve(other = Mask): return Mask
   compute the convolution of self and other.  The (x,y) bit of the 
return value will be set if shifting the lower-right corner of other to 
position (x,y) would cause it to overlap with self.  The return value 
has size equal to self.getsize() + other.getsize() - (1,1).


(and possibly)
Mask.convolve(other = Mask, world = Mask, offset = (x,y)): return None
   Functionally the same as world.union(self.convolve(other), offset) 
but more efficient

Then the stuff that computes the rectangles and masks for a sprite group 
would be up to the application developer (i.e. me).

On a side note, does Mask.draw do an "or" or a copy of the two masks?  
They're probably both useful.  Assuming it's a copy, I've used 
world.union to denote orring the bitmasks.

--Mike