One dxf, two cads see different things -hiw to resolve?

[Buntehuhn]

Kindly requesting your help to use QCAD to resolve this issue.

Situation:
I want to lasercut parts for a model glider, from 2mm plywood, at the fablab nearby. The fablab accepts dxf-files as input, the fablab's laser-software is then used to create gcode.
The design is created in a specialized model plane 3D cad program, devwing, and exported as a dxf-file.
With QCAD I've opened the devcad-dxf and saved the parts (two ribs) to be cut from 2mm plywood in a separate dxf-file, for the fablab to check if it is suitable for the laser-software. They check that using CorelDraw, I think version 2007 or 2017; anyways, a very much not recent version, but definitely suitable for their compatabilty-check.
I run QCAD Professional 3.32.3.0 (3.32.3) on W10 Pro.

The issue
The file I supply, the attached Amokka Fin geodesie 5_1_Triplex2mm, contains two ribs, with the top one containing two round circles (that will become holes)
The fablab sees not those round circles, but sees each circle as two overlapping ellipses: see attached image What fablab sees_Amokka Finparts triplex2mm.

What I want the fablab to see, are the two round circles.

My question:
1. How can I make those ellipses in the file that I supply, visible in QCAD?

Kind regards,
Arjan

[Husky]

Hi Buntehuhn - welcome to the Qcad forum.

Your drawing was among other things edited with 3D. To be sure that everything is now 2D, I recommend first converting the drawing in Qcad to 2D.
Load the drawing and run ... "Flatten drawing to 2D": Menu ... Misc ... Modify ... Flatten Drawing to 2D.
... just to be on the safe side

Maybe this will already solve your problem - if not please feel free to ask again. Thank you.

[CVH]

Hi, and welcome to the QCAD forum.

IMHO there are no Z coordinates different from zero.
Flattening a drawing that is already 2D has no further effect.
It won't solve any of your issues.

The fablab sees not those round circles, but sees each circle as two overlapping ellipses:

The QCAD representation is perfect.
Model_Space shows 6 entities in total: 2 ribs outlines, 2 additional 'circles' and 2 text entities.

The top of 2 ribs is a Block Reference based on Block '*R20' what already includes 2 holes.
The leftmost hole has a duplicate: A Block Reference based on Block '*R21'.
The rightmost hole has also a duplicate: A Block Reference based on Block '*R22'.
When selecting entities that are superimposed try selecting with holding down the ALT key.

You should already fix Block names because starting with an asterisk is an invalid name (= Anonymous block).

• QCAD Pro .. Misc .. Block .. Fix Block Names

You could consider to remove the (almost) duplicates.
You could also consider to replace the circular shapes by real circles ...
... If you would be able to locate the correct center (See lower).
Eventually exploded to Polylines or exploded further into 2 Arcs by QCAD if that is required.
I would expect more holes to make the profiles even lighter.


In all 3 Blocks a circle is represented with a polyline with 2 semicircular segments.
'Arc like segments' or better 'Bulging segments' because one can convert them in both ways but never exact.
Bulging segments have end-points, Arcs have end-angles, an Arc has a center, a Polyline segment has none.
(Before anyone disagrees: QCAD explodes polylines internally for snapping to centers of Arc like segments for example)
For a perfect semicircle we expect them to have a bulge factor of 1.000...

Comes from bulge = tan(sweep/4) => tan(180/4) = 1.0

A fully circular 'Arc like segment' is not possible: tan(360/4) = tan(90) = Error (= infinite)
QCAD automatically prevents that on exploding a circle, on converting a circle to a polyline, ... , on saving and on loading.

I suspect that these polylines are not generated by QCAD:
- Would be a logical closed Polyline with 2 vertices, not as open with 3 vertices.
- The 2 vertices would be at 0 and 180 degrees.
- Both the bulge factors would be 1.000 exactly.

Why fablab displays them incorrect I can not tell and is outside the QCAD scope.
It has to do with the bulge factors and the orientation.
The third vertices typically has a bulge factor equal to the second.
So said 'circles' in Block '*R20' are drawn CW and in Block '*R21' and '*R22' they are drawn CCW.

Exploding the 'circular' polylines to Arcs reveals:
- Top Arcs in Block '*R20' have a sweep of -174.85847228
- Bottom Arcs in Block '*R20' have a sweep of -185.14143425
- Top and bottom arc of one so said circle don't have the exact same center, the radius is almost 2.5 but not exact and not equal.
- Both Arcs making up one (almost) circle don't connect tangentially.
Similar for Block '*R21' and Block '*R22' but then with positive sweeps.


The application that generated these polylines from circles did not a great job.

The same is true for the outlines of the ribs what probably were Splines at some point.
Stepping through the vertices of the top profile reveals connecting angles varying between 179.82747097 and 181.29127169 degrees.
Meaning that the transition from one Arc to the next is not smooth, your profiles have dents and kinks.
More pronounced at the front tip where small Arcs are replaced by short but straight segments.
The bottom profile then uses a small Arc at the tip: index 1 > about 185 degrees and that is a visual kink, index 16 > about 176.1 degrees.

QCAD explodes Spline to Polylines with tangentially connected Arc like segments.
Perhaps it is an idea to do all the conversion to polylines in QCAD.
Then you could avoid to explode Circles to Polylines, see: Application Preferences .. Modify .. Explode.
Remind that the stricter the exploding tolerance for Splines, the more Arc like segments are created.

Regards,
CVH

[CVH]

PS:
The QCAD book is not really the manual of QCAD.

The title is "An Introduction to Computer-Aided Design" and that at the hand of QCAD.
It does not cover all QCAD drawing methods.

It was issued in 2008.
QCAD is a living entity, updated and enhanced on regular base.

Best option that covers (almost) all QCAD methods is the Reference Manual: Online or offline included in QCAD under menu Help.

Regards,
CVH

[Buntehuhn]

Thank you for your quick replies!
I'll follow your advices and come back with updates of success or with new questions.

[CVH]

Thanks for this particular drawing file.
It is a good example of what I call a 'distant Block origin' where other applications may fail.
In essence: 'How it should be'.

Example:
Block '*R21' content is drawn around let's say (17.99790211, 362.20526181) what is the center of the bounding box.
Thus far from the drawing origin.
Inserted in Model_Space at (19.76712033, 16.2308315) and the content is also rendered around this point.
Seems logical but it isn't in all drawings we might encounter.

Instead the usual (0.0, 0.0) the Block origin is listed as (17.997902, 362.205262) ... Fairly a match with the above box.
If the Block origin was (0.0, 0.0) or not correctly included and reverted to (0.0, 0.0) ...
... Then the content would be displayed near (19.76712033+17.99790211, 16.2308315+362.20526181) or near (37.76502244, 378.43609331).

The rendered content would then be far form the insertion point.
An issue we see in many drawing files created by yet other applications.

Regards,
CVH

[Buntehuhn]

i've read the book and succesfully applied your advice in the previous posts.
However, one thing I can't accomplish: turn the red rib R-A-1a into a closed polyline.
I could do that to the white rib R-A-4a, by exploding its block and the command polylineautoclose | oe.
What do I have to do to close the polyline of red rib R-4-1a?

Amokka Fin geodesie 5_1_Triplex2mm_nodupls_lyr0_fonts_crcls_halfpolyclose.dxf

(110.13 KiB) Downloaded 470 times

[CVH]

Hi,

Please open a new topic for unrelated or new questions.
That keeps the forum a bit searchable.

The white rib in Block *R0 is already a Polyline.
As far as I can tell at glance that is not logically closed (Closed property) but it is geometrically closed (end = start).
OE - OJ converts between the two closed natures.

You don't have to explode an existent Block Reference first.
Now the white rib 'R-A-4a' in Model_Space is unrelated to the rib in Block *R0.

You can edit the content of Block *R0 and apply the conversion on the Block content.
Select the Block in the Block List followed by BE (Shortcut or pencil button)
Or simply double click on the Block name or even on the Block Reference.

Editing a Block by a double-click on a reference is by preference.
Block content can also be edited in-place, other things are then grayed.
See menu Edit .. Application Preferences .. Graphics View .. Behavior

The red shape in Model_Space are loose segments.
Double-click near one of them to select all connected segments. (Also by preference)
Called a contour but double-clicking selects all connected entities at endpoints.
The selection is not per definition a closed contour, more than 2 entities may connect in one point.

Then you can convert the selection into (a) Polyline(s) with 'Polyline from Selection' (OC) (HA) was wrong
If that is geometrically closed, the Polyline will automatically be constructed as logically closed.

If not resulted in logically closed or in more than one Polyline then there are gaps or more 'shapes' were selected.

Regards,
CVH

PS: *R0 is an invalid block name in QCAD what prevents inserting a Block Reference based on that: Use menu Misc .. Block .. Fix Block Names
PSS: I would avoid 'Double-click' = 'Edit block reference in-place' in print preview mode because the Options Toolbar doesn't seems to be complete.

[Buntehuhn]

Thank you for calling my attention to posting follow-up questions as separate posts.

I've managed to close all ribs (using command OC rather than HA), and will present the resulting file to the Fablab to have them check if it is laserable by their laser.
I'll report back in this thread.
Resulting new questions I'll post as new threads.

[CVH]

Sorry, my bad , (HA) was wrong, OC is indeed the correct shortcut.

The reason is that OC is a also a fine check for testing if a contour can be hatched.
When the resulting polyline is logically closed then HA won't throw an error with the perfect closed nature.
Otherwise there are gaps that may prevent hatching.

The associative mind jumped a notch in providing the shortcut .

Regards,
CVH

[Buntehuhn]

[Buntehuhn]

The fablab examined my latest dxf-files, resulting in that I should use a single line font (appearantly Arial ain't one) and another instance of him seeing something else using coreldraw, than me using qcad.

That instance is that I see the rib/object on the bottom as a red line, he sees it as a black line (see screenshot afbeelding.png).
The according dxf-file is Amokka Fin geodesie 5_1_Triplex2mm_nodupls_lyr0_fonts_crcls_polycloses_oneblock.dxf

What configuration item in qcad could explain that, and how should I correct it?

[CVH]

Hi,

Your second question tends to fall within the scope of the topic title:

I have no idea why CorelDRAW would see a black line instead of a red line.

"CorelDRAW" meaning the vector graphics editor is capable of importing DXF files, its native format is CDR.
It is also the name of the Corel graphics suite including various graphics-related programs.

In essence not really intended to be a CAD application, more geared to graphical illustrations.
Per example, no well established CAD application is listed under: Comparison of vector graphics editors

Shape R-A-1a lives on layer 0 and has a custom color attribute defined.
Entities living on layer 0 would typically inherit the layer attributes with 'By Layer'.
The color would then be white but this can be corrected to avoid white on white and black on black by an application preference in QCAD.
Perhaps you could place those entities on a layer that uses red ...

An online search on terms including "coreldraw dxf import limitations" may shed some light on many of the limitation.


TTF or OTF and so on are filled outline fonts.
The character glyph's are coded as outlines and an algorithm is used to fill the area between them.
One of the benefits is that these outlines as vectors can be scaled loss-less.
A game-changer replacing the historical pixel fonts.

Should not be an issue for a graphical oriented app, fills are supported widely.

Standard.cxf is for example a QCAD native font type that uses single stroke characters.
The vector art is rendered in QCAD with the configured Lineweight ... With a round pen.
For usage one a laser CNC that could be merely the laser spot size but then scorched, not cut.
Color coding like blue for engraving may apply.

Regards,
CVH

[Buntehuhn]

As the fablab uses Corel Draw to precheck dxf-files before they feed into the gcode-generating-software I'm bound to pass their test.
The fablab needs all objects to live on the same layer, so both that need to be cut and those that need to be scorched.
Lines to be cut must be in red, those to be scorched must be in blue.
That's why my objects are in layer 0, with custom colors red and blue, rather than 'color by layer'.
The font must be a single line font.

I've rechecked my file again, and found it to comply to the requirements above.
My file uses indeed the single line font 'standard' that lives in qcad.
The Fablab couldn't read it, and pointed me to the cambam single line fonts. I imported those - remarkably with a ttf-extension - and used them in th enext version of my dxf and sent it to fablab to find out which one they actually do see. I'll report back here.

Fablab couldn't explain why they see one object in white

Great suggestion to google on dxf importing limitations of/for Corel Draw; I could not find a useful suggestion

[CVH]

For the record, CamBam 'single line' TTF fonts are not single stroke.
No TTF can define such a thing.

The outline is coded in such a way that it looks ultra narrow, that there is nothing to fill.
In fact, it re-traces everything twice ...
... The laser would scorch everything double.

That fablab doesn't support standard.cxf ... A replacement for the AutoCAD standard.shx shape file I can understand.
Because it is a QCAD native font type intended for line-art just like shape files.

In that case explode (XP) the Text entity to single stroke line-art.
If you explode Text in an outline font like TTF, you get the outline(s) in line-art.
See the XP preferences for the outcome.

Remind that there is no easy way back.
As engraver I keep a copy of the text-art on a Locked/Hidden/Frozen layer and explode a copy to line-art.
So that I can easily re-edit the Text and replace the exploded art when required.


Re-reading and looking things up:
In the attached file "Amokka Fin geodesie 5_1_ ... _oneblock.dxf"
- The outline of Shape R-A-1a is indeed in red
- The circles in that shape use 'By Layer', text are in blue.

- The outline of R-A-4a also uses 'By Layer' ... White corrected to Black.
- The text in that shape is in blue.

Probably fixed with setting R-A-4a shape to red ...
The same would be true for the circles in the R-A-1a shape.
I can't explain the dotted red rectangle.

Arjan, I have sent you a personal private message.
See top right on the forum page.

Regards,
CVH