GraphicsPath.addArc adds line

General
1

Why does the following code add another line to the arc?

dim height as Double = 50
dim curve as Double = 50
dim offset as Double = 20
g.PenSize = 2
dim HalfPen as Double = g.PenSize/2
Const Pi = 3.14159

Dim p As New GraphicsPath
p.MoveToPoint(HalfPen, height + HalfPen)
p.AddLineToPoint(offset + HalfPen, height + HalfPen)
p.AddArc(offset + HalfPen, height + HalfPen - curve, curve, 0, Pi/2, False)
g.DrawPath(p, False)

The result should be a horizontal line and an arc. But I get an additional line:

Screenshot 2022-04-16 at 16.32.24
Screenshot 2022-04-16 at 16.32.241424×1080 58.7 KB

How can I remove the line in red in the screenshot above?

2

Try:

p.AddArc(offset + HalfPen, height + HalfPen - curve, curve, Pi/2, Pi/8, True)

Not sure about Pi/8

Edit: maybe is 3*Pi/2 instead of Pi/8

3

The code in the language references makes an arc that is identical to mine. So 0 as start angle and Pi/2 as final angle are okay:

Var arc As New GraphicsPath
arc.AddArc(50, 50, 20, 0, Pi / 2, False)
g.DrawPath(arc)
4

Explanation, this code:

p.MoveToPoint(HalfPen, height + HalfPen)

set the point here:
image
this code:

p.AddLineToPoint(offset + HalfPen, height + HalfPen)

draws a line to this point
image
this code

p.AddArc(offset + HalfPen, height + HalfPen - curve, curve, 0, Pi/2, False)

tells Xojo to Add an Arc starting here:
image
and ending here:
image

In other words, the line that you don’t want connects the end of the Line to the Start of the Arc.

5

Nope, that code starts an arc to the top right (0) and ends at the bottom left (pi/2).

It looks ok because there is no other reference or line connecting to it before you add that arc.

6

Here is the relevant part of the docs for AddArc:

x The x-coordinate of the center point of the arc.
y The y-coordinate of the center point of the arc.
radius The radius of the arc.
startAngle The angle (in radians) that determines the starting point of the arc, measured from the x-axis.
endAngle The angle (in radians) that determines the ending point of the arc, measured from the x-axis.
clockwise A Boolean value that specifies whether or not to draw the arc in the clockwise direction.

startAngle is 0, engAngle is Pi/2. So the arc is drawn upwards and not downwards. And I still don’t know how to get rid of the line.

7

Maybe the docs are not clear enough. You can do this test:

Var arc As New GraphicsPath
arc.AddArc(50, 50, 20, 0, Pi / 2, False)
arc.AddLineToPoint(50, 50)
g.DrawPath(arc)

Result:
image

8

I show you how, use this code:

Dim p As New GraphicsPath
p.MoveToPoint(HalfPen, height + HalfPen)
p.AddLineToPoint(offset + HalfPen, height + HalfPen)
p.AddArc(offset + HalfPen, height + HalfPen - curve, curve, Pi/2, 0, True)
g.DrawPath(p, False)

and get this:
image

Edit: I made a mistake it is 0 to create a 90 degree arc. 3*Pi/2 will create a 180 degree arc (my test cut the other 90 degree arc).

9

Thanks! I don’t get GraphicsPath, really.

10

Glad I was able to help.

It took me some time to understand that the graphics are upside/down, so the arc to create a circle is like this:
image
I edited the code to change 3*pi/2 to 0. You wanted to create an Arc from 90 to 0 but your code said to draw it from 0 to 90 that is why you got a ‘connecting’ line.

Also, the clockwise boolean is backward as we normally use because the graphics are flipped, so an arc from 90 to 0 that is counter-clockwise for us is clockwise for the graphic path.

I hope this information helps to make sense on how to use the arc.

11

This thing doesn’t make any sense at all. Yes, the image helps. That’s why I got so confused.