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*Subject*: Re: Repost of Vertex question...*From*: "Richard G. French" <rfrench(at)wellesley.edu>*Date*: Fri, 29 Jun 2001 10:55:40 GMT*Newsgroups*: comp.lang.idl-pvwave*Organization*: Wellesley College*References*: <3B3BC443.34DA9BCC@rainbow.uchicago.edu>*Xref*: news.doit.wisc.edu comp.lang.idl-pvwave:25474

> I would love to be able to use IDL to find the vertices in this > image. > Essentially this will give me all the corners in the image. Is > there a > function in IDL that does such a thing (find intersections or > verticies)? Here's an idea that might get you started. 1) Find a way to sort the pixels that contain the 'line' information so that they are contiguous - i.e., so that they are ordered in such a way that if you used 'plots,/dev,xvals,yvals' on this set of xvals and yvals, they would follow the edges of a given polygon as if you were drawing it with a pen on the screen - there are several ways of doing this, but the details depend on the nature of your images, so I leave this part to you. If you can't do this on your image immediately, then you can set up a test image that has this information in it - that is, simplify your image so that it contains a single polygon with known pixel coordinates in order. 2) Given this list of pixels, compute the slope of the line containing each contiguous pair of pixels - something like theta_vals = atan(yvals-shift(yvals,1), xvals-shift(xvals,1)) (syntax is close but may not be perfect here) 3) Now you can search for vertices by defining them as successive values in the array theta_vals whose difference (in absolute value) exceeds some threshold level. You may have some wrap-around modulo two-pi problems when theta goes between 0 and 2*!pi, but this is the general idea. If you have a very low resolution image, this approach might not work very well, due to the granularity of the pixel locations, but it is a start. If you are only after vertices with very sharp angles, though, this might be close to what you want. Hope this sparks some ideas from others, Dick French Astronomy Dept, Wellesley College

**References**:**Repost of Vertex question...***From:*Brian Koss

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