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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