Short vectors are not plotted

[Dima K. <gn...@di...>]

Hi all, a non-version-control question for yall!

I'm trying to plot a vector field (attached). The vectors start at an
evenly-spaced 20x20 grid. On my machine when I run the script, plotting
into an x11 terminal, I don't see all 400 vectors, I see maybe 1/4 of
them; the rest don't show up at all. It turns out what happens is this:

  do_plot()
  {
    ...
    plot_vectors();
    ...
  }

  plot_vectors()
  {
    ...
    for(vectors)
    {
      double x1_data_coords, x2_data_coords, ...;
      ...
      int x1_terminal_coords = map_x(x1_data_coords);
      int x2_terminal_coords = map_x(x2_data_coords);
      ...
      (terminal->arrow) (x1_terminal_coords, y1_terminal_coords, ...);
    }
  }

  arrow(int x1, int y1, ...)
  {
     if( x1 == x2 && y1 == y2 )
       return;

     ...
  }


I.e. we convert the vector start/end to integer terminal coords; short
vectors become 0-vectors in this representation, so the
terminal-specific arrow() function plots nothing.

This is a bug: checking for a 0-vector should happen before we convert
to integer pixel coordinates.

I'm attaching a patch series to fix this. The fundamental difference is
a change to the arrow() terminal callback to take FLOATING-POINT
terminal coordinates, so the vector direction, length can still be
accurately computed. This feels incomplete, since I suspect there're
other terminal functions that can benefit from such a change, but this
works.

Note that these patches are made with 'git format-patch', so once the
conversion is complete, they should be applied with 'git am'. This
imports the patch series together with the commit metadata (author,
message, etc).