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. John
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