From: Eric Firing <efiring@ha...>  20061224 19:46:17

shuwj5460@... wrote: > Hi, > matplotlib can make a contour plot on a masked array z using > contourf(x,y,z).but it seems that it draws in a raster mode.there is > sawtooth at the edge of the picture when it is zoomed.can contour be > drawed in a vector mode? i means that > 1) give an array z which has not been masked. > 2) contourf > 3) using a ploygon array as a mask instead of using a masked grid array. > the part out of the polygon will be clipped.it seems that cairo and agg can > both work in this way. contourf is not drawing in raster modeit is generating filled polygons. What you are seeing is the fact that when it follows the edge of a masked region, it does so in a stairstep fashion. This is inherent in the algorithm being used. I have thought a little bit about ways of making it at least take a diagonal path when reasonable so as to reduce the jaggy effect. I am not at all sure it would be worth the trouble, thoughI expect it would be a lot of work for only a little gain. To get around this you may want to use some interpolation/extrapolation to extend your data one grid point into the masked region, then contour with the reduced masked region, then plot your own filled polygon on top of the contour plot to define the masked region more precisely to your taste. Yes, I agree that this is a pain, and that the present contour/contourf routine is not ideal (although it is very clever and I suspect very fast). If you can find a better filled contour routine with a BSDstyle license, or even a clear and complete description of a superior algorithm that could be implemented without running into copyright or patent problems, please point it out to us. There is one interesting difference between the algorithm we use (taken from the gist package) and the one used by Matlab: ours generates polygons that enclose only a given levelpolygons do not overlapwhereas Matlab's generates polygons that stack. As a result, ours is suitable for use with alpha blending, whereas Matlab's is not (last time I looked, anyway). Eric 