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From: David MacMahon <davidm@as...>  20100218 05:07:05

plshade has three "groups" of parameters that I am having a hard time understanding: 1) xmin,xmax,ymin,ymax  "Define the 'grid' coordinates. The data a [0][0] has a position of (xmin,ymin), a[nx1][0] has a position at (xmax,ymin) and so on." 2) defined  "User function specifying regions excluded from the shading plot. This function accepts x and y coordinates as input arguments and must return 0 if the point is in the excluded region or 1 otherwise. This argument can be NULL if all the values are valid." 3) pltr, pltr_data parameters which translate 2D data indices into world coordinates. I think I understand pltr,pltr_data since those are also used in the plcont functions, but I have questions about the other two groups. What is the purpose of explicitly specifying xmin,xmax,ymin,ymax? Based on the comments/documentation, it sounds like xmin,ymin could be determined by passing (0,0) to pltr and xmax,ymax could be determined by passing (nx1,ny1). At least that's the case for pltr0 and pltr1 (with monotonic x and y grid axes), but for pltr2 the mapping from 2D data indices to world coordinates seems arbitrary (e.g. polar to rectangular) so determining xmin etc from pltr is not really viable. While that sounds like a valid reason for specifying xmin etc explicitly, the code for plshade looks like it uses (xmax xmin)/nx to determine "dx" (and likewise for "dy"), which seems to assume a linear mapping from i and j indices to x and y world coordinates, so I'm back to being confused. Since "defined" does not have a corresponding "user data" pointer, it seems that whether a point is "defined" or not must be determinable solely from the x,y coordinates themselves and not from the 2D data values (or accompanying "data valid" flags array). Is the primary intent of this to limit the region that is shaded (kind if like kx,lx,ky,ly for plcont, but in world coordinates rather than 2D data indices)? FWIW, these questions arose because I'm trying to figure out the best way to deal with these parameters in my Ruby bindings. Thanks, Dave 