Re: Short vectors are not plotted

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

sfeam <sf...@us...> writes:

>> 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?

Right. It's not actually a length-0 vector, but its length is < 1 pixel
long. We should still draw an arrowhead, and the arrowhead should point
in the correct direction. Once we convert to integers, then even if we
wanted to draw something, we can't do it, since the direction has been
lost.

Here's a (hopefully convincing) thought: let's say you're looking at a
plot of a vector field. Then you zoom out repeatedly. I would expect the
plotted vector lengths to get shorted as you zoom out. The arrowheads
maybe would get smaller too, depending on how they're plotted. But the
transitions should be smooth. What currently happens is that as you zoom
out, some subset of the vectors disappears. As you zoom out more, some
OTHER subset disappears. This subset could be entirely different, and
some vectors actually come back. Eventually they all disappear, but as
they do so, sometimes they're plotted in an intermediate state where the
arrowhead IS plotted, but its direction is completely wrong.

Try with the example attached in the last email; use the mouse wheel to
zoom out.