Direction of arc by direction vector

[bsharp]

I need to determine the clockwise or counterclockwise direction of a circle or “arcâ€

[Bob Hansen]

I haven't worked with vectors, but this seems logical, just using the Start and End coordinates:
------------------------
If the Y value goes down (Ending Y Start X), it is clockwise (NE quadrant)
If the Y value goes down and the X value decreases, it is clockwise (SE)
If the Y value goes up and the X value decreases, it is clockwise (SW)
If the Y value goes up and the X value increases, it is clockwise (NW)

ELSE
All of the other combos (4 of them) must be counter clockwise
------------------------
The above references are not using screen coordinates, but are using the center of the circle as the X/Y references for increase/decrease.

[bsharp]

Ok I am trying to translate an APT file from one format to another. The file I am trying to read from gives me the coordinates of arcs like this.

A three arc circle going clockwise:
GOTO / 0.80353, -0.60825, 0.75000, 0.000000, 0.000000, 1.000000
INDIRV/ -0.86603, 0.50000, 0.00000
TLON,GOFWD/ (CIRCLE/ 0.86603, -0.50000, 0.75000,$
0.12500),ON,(LINE/ 0.86603, -0.50000, 0.75000,$
0.92853, -0.39175, 0.75000)
INDIRV/ 0.86603, -0.50000, 0.00000
TLON,GOFWD/ (CIRCLE/ 0.86603, -0.50000, 0.75000,$
0.12500),ON,(LINE/ 0.86603, -0.50000, 0.75000,$
0.97428, -0.56250, 0.75000)
INDIRV/ -0.50000, -0.86603, 0.00000
TLON,GOFWD/ (CIRCLE/ 0.86603, -0.50000, 0.75000,$
0.12500),ON,(LINE/ 0.86603, -0.50000, 0.75000,$
0.80353, -0.60825, 0.75000)
The same three arc circle going counterclockwise:
GOTO / 0.80353, -0.60825, 0.75000, 0.000000, 0.000000, 1.000000
INDIRV/ 0.86603, -0.50000, 0.00000
TLON,GOFWD/ (CIRCLE/ 0.86603, -0.50000, 0.75000,$
0.12500),ON,(LINE/ 0.86603, -0.50000, 0.75000,$
0.92853, -0.39175, 0.75000)
INDIRV/ -0.86603, 0.50000, 0.00000
TLON,GOFWD/ (CIRCLE/ 0.86603, -0.50000, 0.75000,$
0.12500),ON,(LINE/ 0.86603, -0.50000, 0.75000,$
0.75777, -0.43750, 0.75000)
INDIRV/ -0.50000, -0.86603, 0.00000
TLON,GOFWD/ (CIRCLE/ 0.86603, -0.50000, 0.75000,$
0.12500),ON,(LINE/ 0.86603, -0.50000, 0.75000,$
0.80353, -0.60825, 0.75000)

The “GOTOâ€

[bsharp]

OK I figured out what the "INDRV" values are. They are an incremental distance projected tangentially from the start of the arc. Now what I need to do is some how calculate when the arc center is left or right of the "INDRV" vector. This would give me the direction of the arc. I am trying to use the VBscript "Atn" function to do this but cannot really figure out how. Also in the MS command reference there is "ArcSin" and "ArcCos". If anybody could post an example of how to get an angle from three points it would definitely help me out.

[bsharp]

I have resolved my problem. I found an example at VB-Helper.com and converted it to MS. It could save someone a lot of time.
What it does is takes the cords of three points and converts it to an angle.
Using Vector mathematics.

Code: Select all

Let>Ax=%point1x%
Let>Ay=%point1y%
Let>Bx=%point2x%
Let>By=%point2y%
Let>Cx=%point3x%
Let>Cy=%point3y%


// Get the vectors' coordinates.
Let>BAx=%Ax%-%Bx%
Let>BAy=%Ay%-%By%
Let>BCx=%Cx%-%Bx%
Let>BCy=%Cy%-%By%

// DotProduct
// Calculate the dot product.
Let>DotProduct={((%BAx%*%BCx%)+(%BAy%*%BCy%))}

// CrossProduct
// Calculate the Z coordinate of the cross product.
Let>CrossProduct={((%BAx%*%BCy%)-(%BAy%*%BCx%))}


// Get the basic angle.
Let>adj=%DotProduct%
Let>opp=%CrossProduct%

VBEval>Abs(%adj%),adjab

	IF>{(%adjab%<0>angle=%PI%/2
    Else
	VBEval>Atn(%opp%/%adj%),opdad
	VBEval>Abs(%opdad%),opdadab
    Let>angle=%opdadab%
    EndIf

// See if we are in quadrant 2 or 3.
    IF>%adj%<0>angle=%PI%-%angle%
    EndIf

// See if we are in quadrant 3 or 4.
    IF>%opp%<0>angle=-%angle%
    EndIf

// Return the result.
    Let>angler=%angle%
    Let>angled={(%angler%/%PI%)*180}

[/code]