topic Re: Ellipse ARC Start Parameter from DXF ? in .NET Forum

Ellipse ARC Start Parameter from DXF ?

Anonymous — Fri, 09 May 2008 09:24:54 GMT

Hello to all DXF specialists,

how can I calculate the start angle and end angle of an ellipse arc, which has a start parameter and end parameter value in the dxf file ?

The documentation from Autodesk tells a formula which seems strange to me because it calculates a sum of two vectors and gives a number as result - which mathematically seems wrong to me and is not calculatable ...

The information that 0 to 2pi is a full ellipse is nice, but the values between 0 to pi/2 are not very concluant for an allipse which is rotated about 30 degrees.

The start parameter and the endparameter doesn't correspond to angles, but I need to calculate the angle to draw with .NET functions the ellipse arc from the dx file ...

Please help me to understand this formula ...

Anzbodz who has some code segment showing how to calculate these start angles and end angles (or sweep angle)

Best regards

Martin M. Mayr

Re: Ellipse ARC Start Parameter from DXF ?

Anonymous — Sun, 11 May 2008 23:14:14 GMT

This may help (It's from vba help files)
The start and end parameters of the ellipse are calculated based on the following equation:

P(Theta) = A * cos(Theta) + B * sin(Theta)

where A and B are the semimajor and -minor axes respectively.

For example, if your start angle is Theta1, the corresponding start parameter is P(Theta1) calculated based on the above formula. Similarly, if your end angle is Theta2, the corresponding end parameter is P(Theta2) calculated based on the above formula.

You can use either angle or parameter to create the ellipse.

Re: Ellipse ARC Start Parameter from DXF ?

Anonymous — Wed, 17 Sep 2008 06:48:40 GMT

i use that but not work for me
i try search at Autocad help I found this :
p(u) = c + a* cos(u) + b* sin(u)
the c parameter is the center
but not sure that work
any body can help me ?

Re: Ellipse ARC Start Parameter from DXF ?

Anonymous — Wed, 12 Jun 2019 08:38:11 GMT

Just calculate start angle from start parameter, which get from dxf file.

First, calculate the point at start parameter.

p(u) = a * sin(u) + b * cos(u).

Then contruct a line, the start point is center point, end point is p.

The start angle is the angle between line and major axis.

Re: Ellipse ARC Start Parameter from DXF ?

_gile — Wed, 12 Jun 2019 10:36:09 GMT

Hi,

You can use the following formula, where ratio is the ratio between minor axis and major axis (DXF group code 40) and assuming both param and angle are measured counterclockwise from the major axis vector (DXF group code 11).

double startAngle = Math.Atan2(Math.Sin(startParam) * ratio, Math.Cos(startParam));