Questions:

1) How does ferret generate boundaries for the 3-argument shade command?

2) Can these boundaries instead be user specified?

Example:

There are two ways of specifying the coordinate arguments: Note the different cell boundaries in the x direction.

Explanation:

In the 3-argument syntax you specify

 yes? SHADE values, xcoords, ycoords

Say
 nVx - is the size of the "x" dimension of the values argument
 nCx - is the size of the "x" dimension of the coordinate arguments

If nCx = nVx then the xcoords argument is presumed to give the locations ofthe points in the values argument, and the boundaries betweenpoints are computed to be the midpoints.

However, if nCx = nVx + 1 then the xcoords argument is presumed to give thelocations of the cell boundaries. The identical discussion would apply to the cell boundaries in the y direction.

For both ways of specifying the coordinate arguments, make sure that the size of the xcoord argument always matches the size of the ycoord argument.

Method:

The Ferret commands for the above plot:

 yes? set view upper
 yes? shade i[i=1:5]+j[j=1:5], i[i=1:5]+0*j[j=1:5], 0*i[i=1:5]+j[j=1:5]

 yes? set view lower
 yes? shade i[i=1:5]+j[j=1:5], i[i=1:6]+0*j[j=1:5], 0*i[i=1:6]+j[j=1:5]

You'll note that in the upper panel that X cell boundaries are at 1.5, 2.5,... (the midpoints of the xcoord argument), whereas in the lower panel theyare at x=1, 2, 3, ... (the values of the xcoord argument.)