Re: still a tessellation problem with OpenGL

[André Bleau <andre_bleau at hotmail dot com>]

<Denis dot Roegel at loria dot fr> wrote:

> Hi,

Hi Denis,

> using an advice by Andre Bleau a year ago, I got the two
> tessellation examples (tess.c and tesswind.c) from OpenGL working.
> However, the example below, slightly different from
> tess.c doesn't work. I get a segmentation fault, but I have no idea
> how to solve the problem.

Well, reading the manual about how to properly call "glu" functions would be 
a step in the right direction; see below.

> I would appreciate any help as this is
> a reduced case of a larger program which works well on linux,
> and that I'd love to port on cygwin. I suspect this problem is related with 
> tessellation, but I am not sure.

Your problem is indeed related to tessellation, but this time it is not 
Cygwin-specific. I'll give you some help anyway.

> Thanks in advance,
>
> Denis
>
> $ gcc -g -o mytess mytess.c -lopengl32 -lglu32 -lglut32

First thing: the proper order for linking is -lglut32 -lglu32 -lopengl32
, as documented in /usr/share/doc/opengl-1.1.0/README.txt
Order is important; you were just lucky this time.

> $mytess.exe
> Ok here
> Segmentation fault (core dumped)
>
> ...
-> ------------------------------------------------------------------------
> // adapted from tess.c
>
> #include <GL/glut.h>
> #include <stdlib.h>
> #include <stdio.h>
>
> #ifndef CALLBACK #define CALLBACK __attribute__ ((__stdcall__))
> #endif
>
> ...
>
> void CALLBACK vertexCallback(GLvoid *vertex)
> {
>    const GLdouble *pointer;
>
>    pointer = (GLdouble *) vertex;
>    glColor3dv(pointer+3);
>    glVertex3dv(vertex);
> }
>
> ...
>
> void tess_properties(GLUtesselator *tobj) {
>    gluTessProperty (tobj, GLU_TESS_WINDING_RULE,
>    GLU_TESS_WINDING_POSITIVE);  gluTessCallback(tobj,
>    GLU_TESS_VERTEX,(_GLUfuncptr)glVertex3dv); gluTessCallback(tobj,
>    GLU_TESS_BEGIN,(_GLUfuncptr)beginCallback); gluTessCallback(tobj,
>    GLU_TESS_END,(_GLUfuncptr)endCallback);
>    gluTessCallback(tobj, GLU_TESS_ERROR,(_GLUfuncptr)errorCallback);
>    gluTessCallback(tobj, GLU_TESS_COMBINE, (_GLUfuncptr)combineCallback);
>    gluTessCallback(tobj,
>    GLU_TESS_VERTEX_DATA,(_GLUfuncptr)&vertexCallback);
> }
>
> ...


Here's the problem: vertexCallback is your  GLU_TESS_VERTEX_DATA callback 
function. Proper GLU_TESS_VERTEX_DATA callback functions need to have the 
following prototype:

void CALLBACK vertexData (void * vertex_data, void * polygon_data);

Maybe your version of linux and/or gcc on linux is more tolerant to missing 
arguments. Or maybe you screwed-up the stack there too, only just not enough 
for crashing your program.

You may find other useful info about gluTessCallback in the man pages for 
glu. There are available at many places on the web. Example:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/opengl/glufnc01_9bu3.asp

I've modified a single line in your test program:

void CALLBACK vertexCallback(GLvoid *vertex)

was changed to:

void CALLBACK vertexCallback(GLvoid *vertex, GLvoid *polygon)

Running it opens an all-black window. Probably not what you would like, but 
certainly no crash. As this is not some problem specific to cygwin's support 
of glut, glu or open GL, please use some open GL forum, such as:

http://www.opengl.org/discussion_boards/cgi_directory/ultimatebb.cgi?ubb=forum;f=2

if you need further help with your program.

Regards,

André Bleau, Cygwin's OpenGL package maintainer.

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