On Nov 27, 2012, at 11:21 PM, John Clements wrote: > > 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....
Okay, the totally-dumb collision detection implementation was shorter than expected; only 73 lines, including (light) testing. Here's a link to that code: https://gist.github.com/4159778 It's observed to be darn slow: it took 4ms for collision detection between the two 65x65 images. This could be acceptable for a game with one or two collision detections per frame. You could improve this *hugely* just by doing some bounding-box computation. Should I put this code in 2htdp/private/image-more.rkt somewhere? John
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