"Paul McGuire" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > "Tim Chase" <[EMAIL PROTECTED]> wrote in message > news:[EMAIL PROTECTED] >>>> I'm wondering if anyone knows of a way to extract data from a numeric >>>> array along a line. I have a gridded dataset which I would like to be >>>> able to choose two points and extract a 1-d array of the data values >>>> along the line between the two points. Any ideas? >>> >>> Are these all 45 degree lines or what? >>> >>> If not, you'll have to use trigonometry and approximate the closest >>> matching cell. (Don't worry, Python has maths functions!! :)) >> >> >> There are a couple of optimal/easy cases where the run is horizontal, >> vertical, or as you describe, 45-degrees (where x_step = y_step or x_step >> = -y_step) >> >> Any other set of arbitrary points, and you will have to specify the >> behavior a little more finely. >> >> You can use something like Bresenham's algorithm to march either "over" >> or "up and over" to just pick out the "one point at each step that falls >> closest to the actual line" along the run. There's also the Wu >> Anti-aliasing line algorithm which takes something akin to Bresenham's >> algorithm and then samples the potential points to yield an >> "anti-aliased" result which averages the two potential data-points >> (traditionally colors, but they could really be any average-able data >> values). >> >> I'm not sure I've seen either of them implemented in Python before >> (though actually *looking* for them might increase those odds ;-) >> >> http://en.wikipedia.org/wiki/Xiaolin_Wu's_line_algorithm >> >> has details and a pseudo-code implementation (which shouldn't be too far >> from the Python code). It's also got links to stuff on Bresenham's. >> >> -tkc >> >> > No need for Bresenham's algorithm here. The point behind BA is to draw a > connected line using best selection of points, going cell-by-adjacent-cell > through the pixel map, so that you actually get a solid line in the > result. The OP's problem is much simpler - he/she (is "Theresaak" a boy > name or a girl name?) just wants the array of grid coordinates between a > starting and ending cell, so "continuousness" of the line is not an issue. > > -- Paul >
Hmmm, maybe I was a little hasty. Re-reading the OP's post, he/she may in fact want the Bresenham-type traversal of the grid from point A to point B. Need a little more info on requirements, Theresaak. -- Paul -- http://mail.python.org/mailman/listinfo/python-list
