Thread: [Matplotlib-users] draw to memory

Hi all,

i need to draw a polygon (non-convex) into memory, so that i can query which pixels are inside the polygon and which not. I came across matplotlib which has some Polygon class in it so i presume i could use this (although so far, i didn't understand the terminology behind the library and how to string things together... patches? artists? O_O) 

What is the easiest way to draw the filled polygon, defined by a sequence of points, into some 1bit raster, so that i can answer queries like "Is the pixel at [10, 12] inside the polygon?"

Cheers!

On Wed, Mar 18, 2009 at 2:48 PM, crwe crwe <cr...@po...> wrote:
> Hi all,
>
> i need to draw a polygon (non-convex) into memory, so that i can query which pixels are inside the polygon and which not. I came across matplotlib which has some Polygon class in it so i presume i could use this (although so far, i didn't understand the terminology behind the library and how to string things together... patches? artists? O_O)
>
> What is the easiest way to draw the filled polygon, defined by a sequence of points, into some 1bit raster, so that i can answer queries like "Is the pixel at [10, 12] inside the polygon?"
>
> Cheers!
>
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hi, did you find this?
http://matplotlib.sourceforge.net/faq/howto_faq.html?highlight=codex%20nxutils#test-whether-a-point-is-inside-a-polygon

> hi, did you find this?
> http://matplotlib.sourceforge.net/faq/howto_faq.html?highlight=codex%20nxutils#test-whether-a-point-is-inside-a-polygon

I used the search function but no, i didn't find that link :) Shame on me and thank you Brent.

Looking at the implementation, it's really simple and i will do without matplotlib altogether and roll my own. In case anyone else needs this, the code is:

def insidePoly(x, y, xs, ys):
    """Decide whether point (x, y) lies inside the 2d polygon defined by its vertices zip(xs, ys).
    """
    result = False
    for now in xrange(len(xs)):
        if (ys[now] < y and ys[now - 1] >= y) or (ys[now - 1] < y and ys[now] >= y):
            if xs[now] + 1.0 * (y - ys[now]) * (xs[now - 1] - xs[now]) / (ys[now - 1] - ys[now]) < x:
                result = not result
    return result

crwe crwe wrote:
> Looking at the implementation, it's really simple and i will do
> without matplotlib altogether and roll my own. 

It sounds like you have a solution, but if you need to test a LOT of 
points, it can be efficient to rasterize, and then get a lightning fast 
check.

It that case, though, I'd be inclined to use something like PIL's 
ImageDraw, rather than MPL -- all of MPL's scaling, anti-aliasing, etc. 
will just make it harder.

-Chris



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