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HI,
i just want to remind that in AutoCAD Arcs are drawn in a counterclockwise direction by default. and by holding down the Ctrl key as you drag to draw in a clockwise direction. hope that help .
Regards,
All Arcs are counterclockwise. It doesn't matter in which direction or under which options it was drawn -- the data for it is always stored with the curve of it progressing counterclockwise from the stored start angle to the end angle.
Kent Cooper, AIA 
@BeKirra wrote:
....
Arcs are drawn in a counterclockwise direction in AutoCAD by default.
....
I drawn an arc starting at lower-right corner and ending at upper-left corner in 1st quadrant on "X-Y" plane.
... "3drotate" ... 180 degrees ... "Y" axis as rotation axis.
Now the arc is in 2nd quadrant.
When I use "pEdit" command to check the vertexes of the arc (actually it is converted to a polyline segment), I found the arc now is in clockwise direction.
....
Arc in 2nd quadrant before using "pEdit"
....
210
-0.0000000000000001
220
-0.0000000000000004
230
-1.0....
The group codes of extrusion direction "210", "220" and "230" are only shown in data of the 2nd quadrant arc. This may verify a clockwise arc.
I would word that first sentence quoted above differently -- the information about them is always stored as though they were drawn counterclockwise, but they can be drawn clockwise in several ways, at least in terms of the process of defining their geometry in a drawing.
The Polyline equivalent/conversion may "look" clockwise from your point of view [assuming you haven't changed that since you drew the initial Arc], but not from its own. Note the -1.0 Z coordinate [code 230] of the extrusion direction for the second Arc -- that's downward. Looking from "your" underneath [which is its own point of view because of that extrusion direction], it's still counterclockwise. The 210/220/230 codes are present only for the second one because when they are not present, the extrusion direction is 0,0,(+)1 -- see the DXF Reference for an Arc and note the "optional" under codes 210/220/230 and the description under 210 that 0,0,1 is assumed if not present.
Kent Cooper, AIA
@Anonymous wrote:
.... But group codes 210,220 and 230 are optional as per the DXF reference manual. It may be present or may not be present
::: KC -- And if they are not present, then the extrusion direction is 0,0,1 -- their absence doesn't leave anything indeterminate.
I got another method to find out whether it is clockwise or anti-clockwise. .... If it is a normal ARC that you have drawn then you need to calculate the value of the bulge using the formula.
::: KC -- If it is a normal Arc, its direction is counterclockwise, always -- there is no other possibility. If you want to know whether it's clockwise from the perspective of the World Coordinate System, as in your example after 3DRotating one, you don't need to calculate anything. Just look at its entity data --
(entget (car (entsel "\nSelect the Arc: ")))
will return a list that includes (210 0.0 0.0 -1.0) with the negative Z component. But from its own point of view, it's still counterclockwise.
....
Kent Cooper, AIA
@Anonymous
Once again: Any software can write and interpret (read) DXF data as it pleases.
(if you only read in your own data and you never share these files with others/other software, you can also give an object CIRCLE the look like a rectangle and the definition of a polyline)
That's your thing - but not useful.
But there is only one "correct" definition for the DXF (per version) and that comes from the company AutoDesk.
These should be used as a benchmark, everyone should do so, and thus the probability of success
is the highest in terms of trouble-free data exchange.
DXF is not documented to 100% but for these simple things it is more than enought.
"But one does draw the arc in clockwise direction then what will be the situation? "
You can draw an ARC (in a Coordinatesystem parallel to WCS / with the same direction)
in all kind you want. Left to right from roof to bottom, the DXF-definition is
to store the ARC-data anti/counter-clockwise.
"If I draw the arc in clockwise direction also group code did not appear in the DXF file."
Because you don't understand coordinatesystems / what the normal vector is (210,220,230) ? http://mathworld.wolfram.com/NormalVector.html
"Since I am developing a software to read the DXF file and display"
"the drawing I have to consider both clockwise and anti-clockwise."
For WRITING a DXF (blank ascii text, without an DXF-wrapper/programming interface) you have to consider both,
because you have to calculate the User-input data for the ARC to counterclockwise informations for writing to the DXF file.
If you write DXF ARC-Data in clockwise order, the ARC appears in other Software wrong, because all well DXF-import interfaces assume that the data is counterclockwise.
"One other thing, I am considering only 2D drawings and not 3D drawings."
It is possible, perfect and simple for you, BUT if you import a DXF which is not wrote by your program, you have to consider the 3.dimension
and so you have consider the normal 210,220,230 by import DXF-Datas too.
It is what it is.
Sebastian
@Anonymous wrote:
.... one does draw the arc in clockwise direction then what will be the situation?.....
The situation will be the same as if it were drawn in some other way -- that the entity data stored about that Arc will be a center point, a radius, starting and ending angles from the center to the ends, always with the "start" and "end" defined as proceeding in a counterclockwise direction, and an extrusion direction [and Layer, and other properties that all entity types share]. How the Arc was drawn is completely irrelevant after it has been drawn. And consider this: what if you drew a Circle, and then used Trim or Break to remove part of it? The resulting Arc wasn't drawn as an Arc, so how could one say in which "direction" it was drawn? But its entity data will be stored in the same way.
Kent Cooper, AIA
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