"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 -- http://mail.python.org/mailman/listinfo/python-list