[racket] Doing collision detection in universe.ss

On Nov 27, 2012, at 11:04 PM, John Clements wrote:

> 
> On Nov 27, 2012, at 5:56 PM, Yaron Minsky wrote:
> 
>> I've been weaning my son off of Scratch in favor of Racket, and trying
>> to get him to write interactive games using universe.ss and image.ss.
>> I'm wondering if anyone has suggestions for how to do things like
>> collision detection.  image.ss has these nice first-class images, but
>> I don't see a good way of querying two images to see if they overlap.
>> 
>> Has anyone else had luck in doing this?  universe has a nice
>> programming model, but I've found it challenging to find simple ways
>> of doing the kinds of things that Scratch makes easy.
> 
> Sounds like you want a raster model for images.  2htdp/image *almost kinda sorta* provides it, in that it exposes a "freeze" primitive that renders an image as a bitmap.  For images that are frozen, it should be possible to determine collision in a straightforward way. In fact, I suppose you could just use memoization to keep around a frozen version of any image.
> 
> It seems to me like the simplest interface would be one that accepted the two images and their relative positions, and returned #t if they overlap.
> 
> I think that the hardest part will be writing the collision detector, and I think even that part won't be too horrible; first do a fast check to see if their bounding boxes overlap. If they do, then blit the alpha channels of the overlapping region into a small fresh buffer, and ... um... yeah, that part gets a bit interesting. It's probably faster to pack the bitmask into 64-bit ints, and then just check the matching ints for non-zero-ness. Lots of fiddly details.

Followup: I see that the "just-alpha" arg to "get-argb-pixels" is undocumented. Investigating....

John

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