Re: Short vectors are not plotted

[sfeam <sf...@us...>]

On Sunday, 29 October 2017 13:26:39 Dima Kogan wrote:
> 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 am not following the logic.

If it is a 0-length vector then isn't it correct to not draw it?
Even if you keep the floating point representation for a bit longer,
at the time you convert to integer terminal coordinates it will
again become 0-length and thus not drawn.  What am I missing?

Is the idea that you want to draw an arrowhead even if the
shaft length is zero?
 
	Ethan


> 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).
> 
> 
>