Poly Arc segments with Width=2R

If you are having problems with QCAD, post here. Please report bugs through our Bug Tracker instead.

Always attach your original DXF or DWG file and mentions your QCAD version and the platform you are on.

Moderator: andrew

Forum rules

Always indicate your operating system and QCAD version.

Attach drawing files and screenshots.

Post one question per topic.

Post Reply
Premier Member
Posts: 1786
Joined: Wed Sep 27, 2017 4:17 pm

Poly Arc segments with Width=2R

Post by CVH » Thu Jan 13, 2022 6:53 am

Coming back on an issue with arc segments of a polyline with a width equal to 2R.
https://www.ribbonsoft.com/bugtracker/i ... sk_id=2155
A nice example of such can be viewed in the mentioned topic:
https://qcad.org/rsforum/viewtopic.php? ... 889#p30905

Managed to explain it myself.
The inner offset shape of an arc with distance R is a Null-lenght arc.
The outer contour gets crippled and the fill is faulty.
Also meaning that polylines with width are rendered as solid hatched contours.

In the past there was a disagreement on that Null-entities should not be cast to a document.
Then again, I fully agree with Andrew that they have a meaning mathematically.
There are two kinds:
- Zero length entities having a certain position, visible as a blob.
- Invisible Null-entities only selectable with TA and those will turn up at the origin when a file is re-opened.
They also occur as a Null-length segment of a polyline.

(125.5 KiB) Downloaded 64 times
Referring to the arc segment in red : (Select the poly-Arc to see its reference points :shock:)
The inner offset is disregarded and in the absence of its endpoints the outer offset arc is closed (Cyan) as an arc segment.
Not disregarded, that arc would have center=start=endpoint = (180;-30)
Even when avoiding to cast the Null-arc itself, the orange lines would be valid and it would display as an arc sector.

I used that as 2D centroid markers, knowing the issue and avoided W=2R.
But the error also turns up in any polyline with variable arc segments with a given width.
E.g.: The explosion of a spline or that of an ellipse.
Or on rounded polyline shapes where the width would match 2R.
W=2R issue.png
W=2R issue.png (4.28 KiB) Viewed 714 times

It seems that there is a mixed consensus on Null-entities.

I now also can explain the issues encountered when mixing widths and non-zero Z values.
A complete different topic and off topic for QCAD that is considered as 2D.


Post Reply

Return to “QCAD Troubleshooting and Problems”