QCAD.org Forum

Good Afternoon,

I'm often drawing an arc from a specific point, with a radius samples from elsewhere in my drawing.

For circles, I can use the 'Center, Radius' (CR) tool. I can click the centre, right click in the 'radius' field and select 'Insert measurement: point to Point'. (*Thanks Andrew!)

The Center, Point, Angles (AR) tool lets me pick the centre of an arc, but I cannot see how to sample a radius. I can enter a radius in the command line at the bottom, but I don't think I can use the same 'Insert measurement: point to Point' functionality as mentioned above.

The 2 Points and Radius tool lets me sample a distance for the radius, but I can only only select the end points of the arc, and not specify the centre.

If this isn't a feature I've overlooked, maybe it's something worth adding?

Regards
Derek

You can actually use the same procedure as outlined in

https://www.qcad.org/rsforum/viewtopic.php?f=31&t=11784

After the measurement, QCAD inserts the variable into the command line (e.g. d1). You can then confirm that value in the command line by pressing the enter key.

Thanks Andrew.

I must be doing something wrong.

I use AR;
Click for the centre;
Right-click in the command line, use Measurement Point-to-point;
On screen the circle still varies in size based on the cursor position. If I click into the command line again, and press Enter, my first attempt appeared to work correctly (d1), but subsequent attempts (d2, d3, d4, etc..) were giving me an arc with a radius much smaller than the distance I'd measured.

Any idea where I might be going wrong?

EDIT: I think I might have worked it out. The Return key and the Enter key appear to be giving me different behaviour. The Return key seems to be doing what I need. I don't understand what the Enter key is doing. Any further clarification or advice would be welcome.
(The procedure for this function isn't quite as smooth as right clicking in the radius/length field entry box and then taking a measurement—as with other tools—as this requires an additional two clicks in the command line and pressing Return. It would be nice if this could be streamlined, but at least I can get it to do what I was aiming for.
This is the button I'd like to see, though...

Regards,
Derek

Hi,

It seems that I can replicate your issues with variables in the latest version ...


To insert a measurement into the Command Line you must click right in the Command Line input field.
In other cases you can set the focus to the command Line input field with hitting the Space-Bar or with Ctrl+M.
See: List of all Keyboard Shortcuts

The procedure with a measured value would be:
- AR, the tool requires a center position.
- Indicated the center position or use the Command Line to specify a coordinate.

• The AR tool continues with asking for a radius.

- R-Click on Command Line > Insert Measurement > IP
- Indicate two points to take the measurement > The measurement tool inserts a variable 'd[n]'.

• This variable input is not auto-acknowledged because you can enter an expression based on such variables.
  'Insert Measurement' does nothing more than inserting a variable in the intended input field.
  If you move your cursor a little over the canvas the radius preview by circle still uses the cursor position.

- Set the focus on the Command Line with the Space-Bar.
- Acknowledge the Command Line input with Enter or Return ... Or complement the expression before acknowledging.

• The AR tool continues with asking the start and end orientations.
  The tool require a direction in regards with the Arc center.
  It accepts angle values, points by cursor or points by entering coordinates.
  Indicated points or coordinates must not be on the Arc.

- Indicate or specify the directions of the start and end of the Arc.

• The tool is reset for drawing the next Arc.
  This doesn't clear any defined variable.

Would be ...
Works perfectly as expected under QCAD 3.27.6
My first attempt under QCAD 3.32.3 with a measurement d1=10.000... resulted in an Arc with radius 2.56239811...
Drawing a next Arc in a second attempt with typing 'd1' in the Command Line followed by ENTER had the desired effect.
A third attempt after QQ > AR with a new measurement d2=10.000... also failed with yet another radius.

Restarted QCAD 3.32.3 and used the Return key to acknowledge instead but that acts just the same.

I can not replicate the same but faulty radius twice:
- While AR is still active.
- Restarting the AR tool.
- In any subsequent QCAD sessions.

There seems to be no repeatable logic behind the resulting radius.
Suspecting that 'Direct Distance Entry' is involved (See link) (I still owe a proper explanation about that)
Then the zoom state, GUI Viewport origin and the relative cursor position and/or angle when hitting ENTER may have an influence.
A recent change to allow entering relative coordinates without prefix may conflict.


HINT:
The advantage of variables is that they are in full Floating-Point notation while other displayed values are typically rounded.
Using measured values you can easily loose track what a certain variable stands for.
When you want to know the value of a variable you can use the Command Line as calculator.
Enter for example '=d1' or any other expression starting with the '=' sign.


Your proposed Arc tool would have a Radius option on the Option Toolbar.
The Command Line and the cursor would no longer be involved for defining a Radius.
The user interaction is then limited to 3 steps:
- Indicate or specify center position.
- Indicate or specify the directions of the start and end of the Arc.
Anytime before the third step you can alter the value in the Radius option by typing in a value or by inserting a variable.

Assigning a proper shortcut key sequence would be the hardest thing because AR would be the best suited analog of CR.
While the current AR is more the analog of CI and should be altered to AI ...
... There should also be a red dot on the Arc in the current AR tool icon for the second step of 4.

Regards,
CVH

Thank you CVH for very thoroughly testing this.

I was about to retest myself, because I still wasn't getting consistent behaviour. Your efforts have saved me typing up an almost identical message. Greatly appreciated.

Your summary of my proposed arc tool is exactly what I had in mind.

One tangential query I had about the variables (d1, d2, etc..) is how many are allowed? Does QCAD reach a point where it 'loops round' and starts again from d1? Is there a way to clear a cache of variables from a session? No doubt these questions are answered in the Help section, so I shall go looking there. I'm just thinking out loud.

Regards,
Derek

DerekGove wrote: ↑

Wed Aug 06, 2025 11:12 am

One tangential query I had about the variables (d1, d2, etc..) is how many are allowed?

As limited as integers are allowed ... Practically: About unlimited.

DerekGove wrote: ↑

Wed Aug 06, 2025 11:12 am

Is there a way to clear a cache of variables from a session?

By script code probably yes.
One could set the AutoVariableCounter custom key to zero.

The easiest way is to restart QCAD.

Regards,
CVH

CVH wrote: ↑

Wed Aug 06, 2025 3:20 pm

As limited as integers are allowed ... Practically: About unlimited.

Good to know, thanks.

CVH wrote: ↑

Wed Aug 06, 2025 3:20 pm

The easiest way is to restart QCAD.

That's what I've been doing. Seems to work fine.

Arbitrary radii can be replicated, the Math eludes me.
Please refer to the bug report FS#2688.

In the attached file in the report freeze the Layer 'First attempt' and try to replicate the Arcs on Layer 0 for example.
The position where you rest your cursor when hitting the Space-Bar followed by Enter has an influence.

Regards,
CVH

Derek,

DerekGove wrote: ↑

Tue Aug 05, 2025 3:27 pm

This is the button I'd like to see, though...

Proposed tool icon

Arc-Radius.png (2.52 KiB) Viewed 45083 times

ArcCPA.js (Draw Arc from Center, Point on arc and Angles)
Or the QCAD tool we know as AR (Arc Radius)
Or as command arcc (Arc Center)
Now includes a conditional 'Radius' option in the Option Toolbar.

See related commit 6f3992d.
That will take effect in a new stable release or in a newer Snapshot.

Instead of a dedicated tool (Analog for CR) it is piggy-backed on the existent AR (Rather the analog of CI).
Probably in order to not break with well-established habits.
As scripter I already get confused by: ArcCPA == AR == arcc ...


Derek, Andrew,
I already tested the new functionality with copies from Github.
- The radius option is conditionally but the value field is not greyed-out when not active. (Can be linked in the widget UI file)
- When active there is no preview of an Arc with that radius while defining the center position.
- When active and the center position is given:

• A full circular Arc is previewed.
  The left mouse button hint still reads "Point on Arc".
  The Command Line still reads "Point on Arc or radius".
  => The tool doesn't skips the "Point on Arc or radius" state.

- Only a next left click anywhere in the drawing area advances the tool state to indicate the Start angle.

• A limited Arc is previewed in CW or CWW direction with sweep = π/3.

- The tool then expects 2 additional coordinates for defining the angles just like before.

- A function is tied to the radius value field that advances the state 'Setting radius' to 'Setting angle 1' on Enter or Return.
Expected is that a numerical input is send to the first value field.
I typically don't terminate such an entry with Enter or Return and the tool state would not be advanced.

- One can edit the radius value field when active or non-active, the result of terminating that with Enter or Return varies with the state.
Again, terminating with Enter or Return is not required then.

- ...


Knowing that Andrew is not fond of tools with many or various additional options ...
... I think that this update to the AR tool needs more work.
Easier would be to split up the functionality as Derek proposed, analog to CI and CR.


Coming to the point of the topic: 'Arc tool than can use center and sampled radius'
- Inserting a measurement in the new radius option works well when set active.
- You need the extra left click anywhere to advance the tool state to indicate the Start angle.

But inserting a measurement in the Command Line still results in arbitrary radii (FS#2688)

Regards,
CVH

Follow up:

CVH wrote: ↑

Tue Sep 02, 2025 4:56 am

I think that this update to the AR tool needs more work.

The updates are throttled back.
The CE version of AR is turned back into its basic behavior: Draw Arc from Center, Point on arc and (2) Angles.
For Pro users there will be more options on the Options Toolbar.
Also see: QCAD Changelog Next Release after 3.32.3
Only part of an upcoming Snapshot or newer release of QCAD Pro.

Inserting a measurement in the Command Line should be resolved too.

Time to think further about extra options and methods for adding an Arc or bulging segment to a Polyline.

Regards,
CVH

Great to see this functionality included. Thanks to everyone involved.

After reading through CVH's message, I gather you would prefer an extra tool rather than too many options on the AR tool. I agree, but at least the functionality is now present.

Regards,
Derek