polygon

I've been struggling with this problem for a few days. I have a shapefile that contains zip code polygons. I'm trying to plot a certain portion of it with a transparent color ( alpha=0.7 ), but some of the polygons at the edge of the plot end up with no color. If alpha=1.0 , all polygons are fine. The shapefile is stored as a Large SpatialPolygonsDataFrame . # Code that you can use (Thanks jbaums) library(rgdal); download.file('http://www.naturalearthdata.com/http//www.naturalearthdata.com/do‌​wnload/110m/cultural/ne_110m_admin_0_countries.zip', {f <- tempfile()}); unzip(f, exdir=tempdir());

Assume you are given the equation of a line (in 2d), and the equations of lines that form a convex polygon (the polygon could be unbounded). How do I determine if the line intersects the polygon? Furthermore, are there computational geometry libraries where such tasks are pre-defined? I ask because I'm interested not just in the 2D version but n-dimensional geometry. For the 2D case, I think the problem simplifies a bit. The line partitions the space into two regions. If the polygon is present in only one of those regions, then the line does not intersect it. If the polygon is present in both

I have a two coplanar polygons. I tried doing. glEnable(GL_POLYGON_OFFSET_FILL); glPolygonOffset(0,1); and expected one to be distinctly "on top of" the other. This is the case until about 70-75 units away (with a near clipping plane of 1, and a far clipping plane of 10,000). Then a region of about 50 units where there is z-fighting, and then the alternate polygon seems to appear on top. Does the Polygon Offset get added before or after the normal z-calculations? If it was after, I would have thought it would have "just worked" at all distances. Am I using the wrong values? Am I

Given is a 2D are with the polygons. I need to find out the polygons visible in a perpendicular line of sight from the a given line segment lying within that area. e.g. Further, What can be the optimizations when the polygons have only vertical and horizontal edges. I'd suggest the following ... Rotate the problem so your 'line of sight' segment is aligned to the x axis. Find the (axis aligned) bounding rectangle (BR) of each polygon. Sort the polygons using the Y coordinate of the bottom edge of each BR Create a clipping 'range buffer' to mark the portions of the viewing segment that will be

I have a code that lets the user draw a shape, I'm using UIBezierPath for this. But I need to see if the shape crosses itself, for example like this: http://upload.wikimedia.org/wikipedia/commons/0/0f/Complex_polygon.svg Then it's not a a valid shape. How can I find this? Edit: I still haven't solved this. I save all the points between the lines in the path in a array. And then I loop through the array and try to find if any lines intersects. But it does not work, sometimes it says that there is an intersection when it isn't. I think that the problem is somewhere in this method. -(BOOL

'lo all. I am a self-described admitted noob in iPhone programming (having a much longer perl & web background -- 30 years)...but took the plunge last week and bought a couple of good books. After cramming and reading well over 1000 pages -- and understanding it pretty well, I am well on my way to a good first Native iPhone app. My problem is this: I do not know how to do a simple Geographic (lat/long) point-in-polygon routine in Objective-C. I have 2 ways of doing this. One in C (the first code example) and one in JavaScript (the second code example): // this is the poly.h file int pnpoly(int