GLU Tesselation problem

[JernejL]

I've created this very simple tesselator of convex polyons into concave ones using GLU, but it crashes if i remove the last showmessage line.. any ideas why?!?!?!?!?!?!?!?!

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, Buttons, opengl;

type

DVector = packed record
x, y, z: double;
end;

  TForm1 = class(TForm)
    BitBtn1: TBitBtn;
    output: TMemo;
    procedure BitBtn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

const
  GLU_TESS_VERTEX_DATA = 100107; 

implementation

{$R *.DFM}

procedure TForm1.BitBtn1Click(Sender: TObject);
var
tess: GLUtesselator;

   procedure tessError(errno : GLEnum); stdcall;
   begin
   showmessage('error, ERROR!');
   end;

   procedure ElementBegin (mode: GLenum); stdcall;
   begin
   unit1.Form1.output.lines.add('BEGIN ' + inttostr(mode));
   end;

   procedure ElementEnd; stdcall;
   begin
   unit1.Form1.output.lines.add('END');
   end;

  procedure ElementPrimitiveV2(vertexData : Pointer; polygonData : Pointer); stdcall;
   var
     pv: DVector;
   begin

   move(vertexData^, pv, sizeof(dvector));

   unit1.Form1.
   output.lines.add(
     format('%f %f %f', [
     pv.x,
     pv.y,
     pv.z])
   );

  end;
{
   procedure ElementPrimitive(vertexData : Pointer); stdcall;
   begin

   end;
}
var
poly: array of DVector;
i: integer;
begin
tess:= gluNewTess;

gluTessCallback(tess, GLU_TESS_BEGIN, @ElementBegin);
//gluTessCallback(tess, GLU_TESS_VERTEX, @ElementPrimitive); // directly to opengl...
gluTessCallback(tess, GLU_TESS_VERTEX_DATA, @ElementPrimitiveV2);
gluTessCallback(tess, GLU_TESS_END, @ElementEnd);
gluTessCallback(tess, GLU_TESS_ERROR, @tessError);

gluTessBeginPolygon(tess);
gluTessBeginContour(tess);

setlength(poly, 4);
poly[0].x:= 0;
poly[0].y:= 0;
poly[0].z:= 0;

poly[1].x:= 1;
poly[1].y:= 0;
poly[1].z:= 0;

poly[2].x:= 1;
poly[2].y:= 1;
poly[2].z:= 0;

poly[3].x:= 0;
poly[3].y:= 1;
poly[3].z:= 0;


for i:= 0 to high(poly) do begin
gluTessVertex(tess, @poly[i].x, @poly[i].x);
end;

gluTessEndContour(tess);
gluTessEndPolygon(tess);

gluDeleteTess(tess);

application.processmessages;

showmessage(output.lines.gettext); // UNCOMMENT ME = CRASH.

end;

end.

[cronodragon]

In my programs I have found those misterious errors which are solved switching 2 lines of code :? Sometimes the compiler does weird things.

But it could also be memory corruption in your code that doesn't crash until that line is removed... try debugging with MemProof, I found Delphi's debugger is not enough to catch every problem. Also Fast Memory Manager could help you better if it's a matter of memory corruption.

[JernejL]

i have no idea what it is, i remove that and the program disappears crashing in totally silence, if i run with delphi's debugger it tells me weird AV error message on addresses way beyond application's range..

[tpascal]

It happens to me one or two times; I think this is related to a bad pointer access or bad stack memory access.

In your case i think the problem is you are defining everithing in a local stack procedure ( TForm1.BitBtn1Click ), probably including the showmessage procedure makes the compiler build the stack size in a way that hide the problem.

I have not idea how much the variable Tess grow in size but most likely it is passing the stack size; try to move the variable tess: GLUtesselator as global, up after the Form1: TForm1;


good luck.

[JernejL]

It happens to me one or two times; I think this is related to a bad pointer access or bad stack memory access.

In your case i think the problem is you are defining everithing in a local stack procedure ( TForm1.BitBtn1Click ), probably including the showmessage procedure makes the compiler build the stack size in a way that hide the problem.

I have not idea how much the variable Tess grow in size but most likely it is passing the stack size; try to move the variable tess: GLUtesselator as global, up after the Form1: TForm1;


good luck.

i tried making all procedures global, same effect, also tess does not grow at all.. it is just a identifier / pointer, variable poly however grows, to 96 bytes only.. and i allocated much more data in local procedures.

[Setharian]

maybe because you're using nested procedures as callbacks? I think it's disallowed, you bypass the compiler by putting "@" in front of the function...probably the stack gets corrupted and you're only lucky it gets executed ok with that last line present.....if it's not the case, then only disassembly could give you a hint on what's wrong....

[JSoftware]

my gluTessBeginPolygon looks like this:
procedure gluTessBeginPolygon( tess: GLUtesselator; gon_data: Pointer ); stdcall;

[JernejL]

my gluTessBeginPolygon looks like this:
procedure gluTessBeginPolygon( tess: GLUtesselator; gon_data: Pointer ); stdcall;

you use that definition if you use GLU_TESS_BEGIN_DATA, GLU_TESS_BEGIN is different.

[JSoftware]

my gluTessBeginPolygon looks like this:
procedure gluTessBeginPolygon( tess: GLUtesselator; gon_data: Pointer ); stdcall;

you use that definition if you use GLU_TESS_BEGIN_DATA, GLU_TESS_BEGIN is different.

Well that doesn't make much sense to me. All I know is that your code compiles for me using the abovementioned calling convention passing nil to gon_data and it works fine.

Would stdcall make your call pass too few parameters on the stack?

[JernejL]

I don't know what you did, please post the code, it crashes for me if i use your function definition (AND remove the last showmessage line).

edit: are you by any chance using different glu.dll?