Is There a Ready Routine for Intersection Calculation in Polygons?
lazarus / 39-Graphics

----------------------------------------------------------------------
loaded · 2021-01-03 18:02

Hi Everyone.

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  procedure
TForm1.FormPaint(Sender: TObject);var  poly:array[0..3] of TPoint; 
cline:array[0..1] of TPoint;begin  poly[0]:=Point(100,100); 
poly[1]:=Point(ClientWidth-100,100); 
poly[2]:=Point(ClientWidth-100,ClientHeight-100); 
poly[3]:=Point(100,ClientHeight-100);  canvas.brush.color:=clred; 
canvas.brush.Style:=bsCross;  canvas.Polygon(poly); 
cline[0]:=Point(round(ClientWidth/2)+10,10); 
cline[1]:=Point(round(ClientWidth/2)-10,ClientHeight-10); 
canvas.pen.Width:=3;  canvas.Polyline(cline);end; 
With the code above
There is an example polygon that I have drawn (it consists of 4 points
in the drawing, but it can actually consist of 3-50 points). And I
want to cut this polygon according to a certain line. So actually I
want to get their intersections as fields. In the future, I will work
to calculate the combination or differences of intertwined areas.
Before dealing with geometry formulas, I wanted to consult you.
What is the best method for this job?

----------------------------------------------------------------------
wp · 2021-01-03 21:33

This is pretty complicated stuff for the general case... There is a
Delphi library "Clipper" by Angus Johnson from which you maybe get an
idea what to do. It is for Graphics32, but I think the algorithm
should be general enough so that you can extend it to your purpose:
https://sourceforge.net/projects/polyclipping/.

I am attaching the demo project as well as Angus' clipper unit,
converted to Lazarus. Please note that the demo required Graphics32 to
be installed -- you can get it via Online-Package-Manager as
pl_Graphics32.

P.S.
Maybe similar stuff is in BGRABitmap, I don't know.

----------------------------------------------------------------------
circular · 2021-01-03 21:35

Not yet in BGRABitmap. But that would be cool

----------------------------------------------------------------------
Seenkao · 2021-01-03 22:40

Разбейте полигон на треугольники.

Eng: Divide the polygon into triangles.

----------------------------------------------------------------------
loaded · 2021-01-04 11:24

wp, circular, seenkao, thank you individually.
wp, I'm trying to run the demo you posted. I hope it works.
circular, I hope you add these features to a BGRAbitmap, and we pray
for you.
seenkao, my path will definitely cross with the triangulation process
one day. This is an issue that I am aware of but always avoid.
If I can't find an easier way, I'll gradually learn to triangulate.

----------------------------------------------------------------------
marcov · 2021-01-04 11:29

Angus library has a bit weird license.

----------------------------------------------------------------------
BeanzMaster · 2021-01-22 07:05

Hi load look at the function TBZ2DPolygonTool.IntersectWithLine(aLine
: TBZ2DLineTool; out IntersectPoints : TBZArrayOfFloatPoints) :
Boolean; (line 423) here https://github.com/jdelauney/BZScene/blob/mas
ter/Source/Utils/BZGeoTools.pas perhaps it can help you a bit.

Cheers