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Wondering why it is so difficult to Autosnap to the center of a short arc ...

Posted: Sun Jun 07, 2020 9:06 pm
by Husky
Hi,
my Autosnap is active for "Center Points". Works great for arcs with a decent size but if I have a short arc it is next to impossible to snap to the center even if I have a good clue where to search ...

Why works that differently?

Husky-2020.06.07-04.png
Husky-2020.06.07-04.png (13 KiB) Viewed 2351 times
Autosnap center snap.dxf
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Re: Wondering why it is so difficult to Autosnap to the center of a short arc ...

Posted: Sun Jun 07, 2020 10:05 pm
by CVH
I have no problems with your examples.
But I know examples where it doesn't work out that well.
Select only snap to center (SC) and go near the Arc and not near the center itself.


Regards,
CVH

Re: Wondering why it is so difficult to Autosnap to the center of a short arc ...

Posted: Mon Jun 08, 2020 8:55 am
by andrew
Yes, the trick is to move the cursor to the visible part of the arc, not close to the assumed center.

It's possible to implement this differently. That would mean that the user would have to navigate the cursor close to the assumed center. This can be very tricky for short arcs with large radii. My assumption is that the user knows which arc should be used to snap to the center but doesn't necessarily know where the center might be (QCAD can do that part).

It all boils down to the question which point you want to snap to with "Snap to Center": 1 or 2?
Screenshot 2020-06-08 at 09.49.45.png
Screenshot 2020-06-08 at 09.49.45.png (43.96 KiB) Viewed 2328 times
QCAD snaps to 1, i.e. the center of the arc closest to the cursor.

Re: Wondering why it is so difficult to Autosnap to the center of a short arc ...

Posted: Tue Jun 09, 2020 7:56 am
by Husky
@Andrew

Thanks for explanation.
andrew wrote:
Mon Jun 08, 2020 8:55 am
Yes, the trick is to move the cursor to the visible part of the arc, not close to the assumed center.
Well, the whole trick is to get rid of the Autosnap function - which includes Snap to Center - and activate the single Snap to Center. It is not obviously why the same named snap works in one case and fails in an other. That is the reason for this topic.
andrew wrote:
Mon Jun 08, 2020 8:55 am
It's possible to implement this differently. That would mean that the user would have to navigate the cursor close to the assumed center. This can be very tricky for short arcs with large radii. My assumption is that the user knows which arc should be used to snap to the center but doesn't necessarily know where the center might be (QCAD can do that part).
I understand your point but in my cases I can't agree. If I design an (short) arc which is positioning regarding to the center of the arc then I remember why and where it was positioned. If I forgot that - what happens from time to time - then I just select the little arc and I see the blue center reference what makes it easy to hover over the center with the mouse pointer. Unfortunately I don't know the limits for a working Autosnap for an "short arc" what means always guessing: Will it work this time or not ... :wink:

Re: Wondering why it is so difficult to Autosnap to the center of a short arc ...

Posted: Tue Jun 09, 2020 8:03 am
by andrew
Unfortunately I don't know the limits for a working Autosnap for an "short arc" what means always guessing: Will it work this time or not ... :wink:
It works if the center point happens to be inside the visible bounding box of the arc (top / right / bottom / left orthogonal border).

Re: Wondering why it is so difficult to Autosnap to the center of a short arc ...

Posted: Tue Jun 09, 2020 8:20 am
by Husky
andrew wrote:
Tue Jun 09, 2020 8:03 am
It works if the center point happens to be inside the visible bounding box of the arc (top / right / bottom / left orthogonal border).
Bounding box? What do you mean?

Re: Wondering why it is so difficult to Autosnap to the center of a short arc ...

Posted: Tue Jun 09, 2020 8:32 am
by andrew
The bounding box is shown in red here:
Screenshot 2020-06-09 at 09.29.31.png
Screenshot 2020-06-09 at 09.29.31.png (34.71 KiB) Viewed 2272 times
As soon as the mouse cursor moves out of that box (+/- a margin), QCAD does not consider that entity for snapping.

"Easy", I hear you say, "simply extend that bounding box around the center - done!". Yes - that would lead to the behavior described above. I believe there's a bug report somewhere about that. The fix leads to the current behavior.

Re: Wondering why it is so difficult to Autosnap to the center of a short arc ...

Posted: Tue Jun 09, 2020 7:11 pm
by Husky
Thanks - understood. :wink: