"Nik Coughlin" <nrkn.com@gmail.com> wrote in message
news:et4l6p$oam$1@aioe.org...
> Say I have drawn a number of lines on a form (forming a polygon), and I
> know the coordinates for each point in the polygon. Now, I want to draw a
> circle on the form, but I want to make sure that it doesn't overlap any of
> the lines.

Michael C wrote:
> "Nik Coughlin" <nrkn.com@gmail.com> wrote in message
> news:et4l6p$oam$1@aioe.org...
>> Say I have drawn a number of lines on a form (forming a polygon),
>> and I know the coordinates for each point in the polygon. Now, I
>> want to draw a circle on the form, but I want to make sure that it
>> doesn't overlap any of the lines.
>
> I can see 2 approaches. The first would be to use a region object for
> the circle and the polygon and check if they intersect (look up
> region in help). This won't me mathematically accurate but should be
> accurate to the pixel.

> The second would be to determine mathematically if the circle and line
> intersect. I'm not sure how to do the maths but you can use the 2
> equations for the circle: x2+y2=r2 (where the 2 means squared)
> for the line: y = ax + b
> substitue the second into the first and solve for x. If you get an
> invalid result (eg squareroot of a negative no) then they don't
> intersect. I'm a bit rusty on the old maths sorry.

> I should also mention that it's possible for the circle to be
> completely inside the polygon but intersect no lines so you should
> check for that too. That should be a bit easier if you divide the
> polygon into triangles and check if the centre of the circle is in
> any of those triangles.

"Nik Coughlin" <nrkn.com@gmail.com> wrote in message
news:et55aj$p1m$1@aioe.org...
> This won't tell me if the circle intersects one of the lines though will
> it, only if it intersects the polygon as a whole? Which is actually
> something I need to know as well, but I also need to know specifically if
> the circle intersects the lines making up the polygon.

> I'll give it a shot, thanks.

> I don't mind the circle being inside the polygon (in fact, I want it to
> be!), so long as it doesn't intersect any of the lines.
>
> What I specifically trying to do is allow the circle to be moved around
> within the polygon but not allow it to leave the interior of the polygon.
>
> The polygon may be of any level of complexity and to further complicate
> things, it may have holes in too.

Michael C wrote:
> "Nik Coughlin" <nrkn.com@gmail.com> wrote in message
> news:et55aj$p1m$1@aioe.org...
>> This won't tell me if the circle intersects one of the lines though
>> will it, only if it intersects the polygon as a whole? Which is
>> actually something I need to know as well, but I also need to know
>> specifically if the circle intersects the lines making up the
>> polygon.
>
> That's true but you will be able to tell if the circle outside the
> polygon by getting the regions that don't interset.

>> I'll give it a shot, thanks.
>

> if h, i, j or k are unreal numbers (roots of negative) then they don't
> intersect.
>
>> I don't mind the circle being inside the polygon (in fact, I want it
>> to be!), so long as it doesn't intersect any of the lines.
>>
>> What I specifically trying to do is allow the circle to be moved
>> around within the polygon but not allow it to leave the interior of
>> the polygon. The polygon may be of any level of complexity and to further
>> complicate things, it may have holes in too.
>
>
> That shouldn't be too much of an issue, just treat the holes as
> seperate polygons and do the opposite with them. The other problem
> you will have is calculating the position the circle needs to be to
> just touch the edge. For example if the circle is right at the centre
> of the screen and inside the polygon and then it is moved suddenly to
> the very left of the screen and outside the polygon, leaving it at
> the centre will look poor, you'll need to determine the edge of the
> polygon and move it there. Does that make sense?

"Nik Coughlin" <nrkn.com@gmail.com> wrote in message
news:et5958$o4$1@aioe.org...
> Thankfully this won't be a problem as I'll only be moving the circle one
> pixel at a time.
>
> Thank you very much for your assistance!

"Nik Coughlin" <nrkn.com@gmail.com> wrote in message
news:et4l6p$oam$1@aioe.org...
> Say I have drawn a number of lines on a form (forming a polygon), and I
> know the coordinates for each point in the polygon. Now, I want to draw a
> circle on the form, but I want to make sure that it doesn't overlap any of
> the lines.
>
> The approach I am thinking of goes something like this:
>
> The center of my circle is at this point on the form. Are any of the
> pixels that make up any of the lines at a distance from the center of
> circle that is less than or equal to the radius of the circle? If so then
> the circle overlaps a line.
>
> Now, can GDI+ help me with this? I understand the problem but I'm having
> trouble phrasing a Google search that finds me anything meaningful. What
> keywords can I use to search for information, or what classes should I be
> looking at?
>
> Thanks heaps :)
>