1. mathmatical question..
Hi everyone!
Can anyone give me nice answer over the problem below;
--- here goes problem,
Assumes the polygon with arbitrary numbers of points is drawn on the
pixel-graphics screen, for example;
polygon(RED,1,{{100,200},{200,200},......,}
Problem is:
How can I know whether the mouse pointer is INSIDE the polygon or not,
at the time the mouse button is pressed?
Bye!
from Lee, WooSeob
2. Re: mathmatical question..
> Assumes the polygon with arbitrary numbers of points is drawn on the
> pixel-graphics screen, for example;
>
> polygon(RED,1,{{100,200},{200,200},......,}
>
> Problem is:
> How can I know whether the mouse pointer is INSIDE the polygon or not,
> at the time the mouse button is pressed?
>
> Bye!
> from Lee, WooSeob
It's not even a mathematical problem...it's more like a question of "Is the
mouse pointer over an image?"
There's two ways of attacking this. One way is to use get_pixel() using the
returned mouse x and y co-ordinates to obtain the pixel colour of where the
mouse pointer is at. In the case of your polygon above, you know you are
over the polygon if the colour RED is returned. Admittedly. it's not good
for a polygon with mulitple or shared colours, but it works if the polygon
is a complex shape.
Another way is to make a sequence representing a map of what space the
polygon occupies, and then search it using find() with the x and y mouse
pointer co-ordinates as the search argument. If you get a hit, you know you
are over the shape. This is much more preferable if the icon you are trying
to make clickable is multi-coloured.
Hope this helps
David Gay
http://www.digital-liquid.com/abgte
"A Beginner's Guide To Euphoria"
3. Re: mathmatical question..
Jiri's Reply has SOME merit mathmatically.
However it is only 50% accurate.
EXAMPLE
________
| _ |
| / | |
|/ | |
. |____|
/|\
|
[point]
If you take this polygon.
You click to where his check comes exactly
across the point. This would cause an odd count
and still be inside the polygon.
The easiest way is David Gay's suggestion.
Next!!! to do it mathmatically you would
have break the polygon into triangles.
Then you could use Jiri's idea accurately.
You would check each triangle as a seperate
polygon. If it is within or on any of the
triangles thin it is within or on the polygon.
Triangled view
________
| /| /|
| / | / |
|/ | / |
|/___|
--Lucius Lamar Hilley III
-- E-mail at luciuslhilleyiii at juno.com
-- I support transferring of files less than 60K.
-- I can Decode both UU and Base64 format.
4. Re: mathmatical question..
Hello again!
Just had another thought.
Do as Jiri said go from click point in one
direction until you hit a polygon line or
reach the screen edge.
Now go from the click point in the opposite
direction until a polygon line or reach the
screen edge.
If first hit line & second hit line then
You are in the polygon.
else
You are not.
end if
--Lucius Lamar Hilley III
-- E-mail at luciuslhilleyiii at juno.com
-- I support transferring of files less than 60K.
-- I can Decode both UU and Base64 format.
5. Re: mathmatical question..
- Posted by Michael Bolin <michaeltom at GEOCITIES.COM>
May 29, 1997
-
Last edited May 30, 1997
> Jiri's Reply has SOME merit mathmatically.
> However it is only 50% accurate.
>
> EXAMPLE
>
> ________
> | _ |
> | / | |
> |/ | |
> . |____|
> /|\
> |
> [point]
>
> If you take this polygon.
> You click to where his check comes exactly
> across the point. This would cause an odd count
> and still be inside the polygon.
No, in the case of ANY closed polygon, drawing a line from a point
and counting the number of intersections will ALWAYS work.
When counting the number of intersections mathematically, lines that
lie straight across another do not count as intersection points.
It's just a simple matter of topology. This system must ALWAYS work.
Michael Bolin
6. Re: mathmatical question..
- Posted by Michael Bolin <michaeltom at GEOCITIES.COM>
May 30, 1997
-
Last edited May 31, 1997
> Hi everyone!
>
> Can anyone give me nice answer over the problem below;
>
> --- here goes problem,
>
> Assumes the polygon with arbitrary numbers of points is drawn on the
> pixel-graphics screen, for example;
>
> polygon(RED,1,{{100,200},{200,200},......,}
>
> Problem is:
> How can I know whether the mouse pointer is INSIDE the polygon or not,
> at the time the mouse button is pressed?
Here's a function that will do this. It computes the number of line
intersections mathematically, and will work for any polygon.
Regards,
Michael Bolin
---------------------------------------------------
function inside(sequence point,sequence points)
atom x1,y1,x2,y2,x,y,min,max
integer intersections
intersections=0
x=point[1]
y=point[2]
x2=points[length(points)][1]
y2=points[length(points)][2]
for t=1 to length(points) do
point=points[t]
x1=point[1]
y1=point[2]
if y1!=y2 then
min=y1
max=y2
if y2<y1 then
min=y2
max=y1
else
min=y1
max=y2
end if
if y>min and y<=max then
if x<=x1+(x2-x1)/(y2-y1)*(y-y1) then
intersections=intersections+1
end if
end if
end if
x2=x1
y2=y1
end for
return remainder(intersections,2)
end function
------------------------------------
