Summary: zero-length arcs (i.e. arc distance of zero) seem to be impossible 
to create, and this is what I would expect. Zero-radius arcs show some 
display problems. Zero-length track disappears without being deleted, and 
this is a bug.

At 10:38 AM 3/6/01 +1100, Ian Wilson wrote:

>It is a bug - 0 to 0 should either display as an invisible arc or be 
>identical in behavior to 0 to 360 (I prefer the latter).  It should not 
>display as  per 0 to 360 but behave differently.

Both a zero length track and a zero length arc should display as they would 
in gerber, that is, as a circle of the draw diameter. If zero length arcs 
did not so display it would be a bug. Here's why.

If I import a gerber file to a layer, the import layer should appear 
exactly as the film would have appeared. Gerber will plot a track with no 
head movement as a print, basically, of the aperture.

set aperture, locate start, turn light on, move straight to end, turn light 

For design convenience, Protel should delete zero length track and arc. 
Arcs are only tracks drawn about a finite radius instead of with infinite 
radius. Such objects are already deleted under some conditions; for 
example, if I am placing a track and I click twice on the same location, 
the "additional" segment is not created.

Polygon segments should behave like track segments; bringing the endpoints 
of a segment together should eliminate the segment. This is a much easier 
way to remove segments than the present cumbersome and non-inyuitive procedure.

I did some experimentation in PCB. I was unable to create an arc with range 
from 0 to 0. If I edited an arc to that value, it was immediately converted 
to 0 to 360. I do agree that 0 to 0 should be treated the same as 0 to 360, 
and, apparently, it is.

I also could not make any completely invisible arcs. However, arcs with 
zero *radius* are not visible at moderate zoom even though a track with the 
same width as the arc was visible. Zooming in, the arc displayed as a 
circle of the draw radius. Except for the minor display issue, this is 
correct behavior. I hope that the arcs are photoplotted, I haven't tested that.

However, zero length track, however it is created, does disappear, but 
remains in the database and active. This is definitely a bug. Do I recall 
that we tested this with regard to whether it photoplots or not?

It should not be easy to create zero length track; I would have all forms 
of graphical edit which create a zero length track result instead in the 
deletion of the track segment involved. I would, however, allow zero length 
track created by modification in the edit dialog or by gerber import. Such 
track should always display as if it were a pad. It can be useful to create 
such objects where one wants a filled circle without creating a pad. They 
could be used as fiducials, for example, without affecting the pad count on 
a board. Sometimes I want to put a circle by pin 1 of a part, and I have 
used very short tracks to do this. A zero length track should behave 
exactly like a very short length track.

Zero length arc is not a problem because it seems to be always converted 
into a 360 degree arc. It's not just start 0 and end 0, but also start 90 
and end 90, for example. If the endpoint equals the start point, and it 
were interpreted as 0 degrees of rotation rather than as 360, it would be 
quite analogous to a zero length track. However, I see no reason whatever 
to allow a true zero length arc, and apparently Protel has implemented this 
restriction. There would be no easy way to specify such an arc, anyway.

Zero radius arcs should display the same as zero length track.

It would also be useful if the conversion of arcs to track segments in the 
photoplot routines was according to a user-set parameter, and it should be 
possible to display the track as if it were such segments, and it should 
also be possible to convert the arc into segments. I won't go into the 
reasons at this point, however.

Meanwhile, we have a bug; zero-length track is not displayed, which can 
make it hard to find. I could not find a way to make the query manager 
distinguish zero-length track.
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433

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