Very Computer

Hi,

I have seem to have some problem with my triangles.
I have two triangles, A & B

- A : in front of the B (on the z-axis).

When I use GL_SMOOTH, and I end up seeing the color of the back triangle
overlapping the front triangle.

I'm very sure that they are placed correctly, as I rotate the two
triangles,
I can see that A is infront of B.

What is wrong? Is it some depth problem? Did I miss some gl keywords?

Thanks,
Rach

Did you read the friendly manual?

Mine says:

glEnable(GL_POLYGON_SMOOTH);

... For correct anti-aliased polygons, an alpha buffer
is needed and the polygons must de sorted front to back.

--
<\___/>
/ O O \
\_____/  FTB.

Is z-buffering enabled?

Quote:> Hi,

> I have seem to have some problem with my triangles.
> I have two triangles, A & B

> - A : in front of the B (on the z-axis).

> When I use GL_SMOOTH, and I end up seeing the color of the back triangle
> overlapping the front triangle.

> I'm very sure that they are placed correctly, as I rotate the two
> triangles,
> I can see that A is infront of B.

> What is wrong? Is it some depth problem? Did I miss some gl keywords?

> Thanks,
> Rach

Thanks fungus and Folker,

Yes, I have the following:

  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
  glEnable(GL_DEPTH_TEST);

and also    

  glDepthRange(0.0, 0.5);

I'm not quite sure if I need glDepthRange, but one of the examples
placed it in.
I have inserted more triangles into my program. Some did not overlap,
some did :-(

Is it the problem with the depth range?

Thanks,
Rach

Quote:> Thanks fungus and Folker,

> Yes, I have the following:

>   glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
>   glEnable(GL_DEPTH_TEST);

> and also

>   glDepthRange(0.0, 0.5);

> I'm not quite sure if I need glDepthRange, but one of the examples
> placed it in.
> I have inserted more triangles into my program. Some did not
overlap,
> some did :-(

> Is it the problem with the depth range?

I'd suggest setting the depth-range from 0.0 to 1.0. Though this
probably won't solve your problem. Check if the z-coordinates of the
triangles are correct ... stupid hint, yes .. but you never know :)

--
jb

(replace y with x to reply)

Having read

http://www.opengl.org/developers/faqs/technical/depthbuffer.htm#0070

I have set:

gluPerspective( 80, 1.0, 1, 100.0) to gluPerspective( 80, 1.0, 10,
100.0)
to reduce the near and far, but still I see cuts over front and back
triangles.

I think the buffer is too small(?). I have around 10,000 triangles ;-(
The suggestion given in that website is to use the multipass technique!!
Oh dear!

Any pointers on where I can find examples or codes to implement
multipass?

Thanks,
Rach

Quote:> Having read

> http://www.opengl.org/developers/faqs/technical/depthbuffer.htm#0070

> I have set:

> gluPerspective( 80, 1.0, 1, 100.0) to gluPerspective( 80, 1.0, 10,
> 100.0)
> to reduce the near and far, but still I see cuts over front and back
> triangles.

> I think the buffer is too small(?). I have around 10,000 triangles
;-(
> The suggestion given in that website is to use the multipass
technique!!
> Oh dear!

> Any pointers on where I can find examples or codes to implement
> multipass?

I don't think you have an "astronomically large scene". After all, 10K
triangles is not thaaat much. What did you set the glDepthFunc to?
Also, it might help uploading a screenshot to some website and then
posting the link here.

--
jb

(replace y with x to reply)

After a tough time printing screen, I have attached the
pictures of my program. Please visit it at:

http://www.dcs.warwick.ac.uk/~csvhl/triangles.html

I do not understand why the polygons seems to cut (overlap).
The triangles are displayed correctly.

thanks in advance,
Rachel

Hi,

glDepthFunc?? I miss that in my code. Could that be the problem?
What do I need to set it to? Is it just:

glDepthFunc (GL_LEQUAL);                    

Thanks,
Rach

Hiya

Judging by the images it appears that the depth buffer isn't working yet,
try adding

glClearDepth(1.0f);

somewhere in your initialisation section.
At the beginning of your painting routine you should have a line along the
lines of:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

This will clear the screen & reset the depth buffer to the value that you
specified in glClearDepth().

hope this helps

cu|K

from the images it appears that your program fails to

After a tough time printing screen, I have attached the
pictures of my program. Please visit it at:

http://www.dcs.warwick.ac.uk/~csvhl/triangles.html

I do not understand why the polygons seems to cut (overlap).
The triangles are displayed correctly.

thanks in advance,
Rachel

Hi,

I've added glClearDepth, but still the same image. I have the following
in my code:

main()
{
  glutInit(&argc,argv);
  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
  init();
  showTriangles();

Quote:}

void init(void)
{
   glClearColor(0.0, 0.0, 0.0, 0.0);
   glEnable(GL_DEPTH_TEST);
   glShadeModel(GL_POLYGON_SMOOTH);
   glDepthRange(0, 1);
   glClearDepth(1.0f);        //added in, but still no effect :-(

Quote:}

showTriangles()
{
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);  //yes, I have a
glClear
  drawTriangles();
  drawMoreTriangles();
  glutSwapBuffers();

Quote:}

drawTriangles()
{
  glPushMatrix();
    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
    glBegin(GL_POLYGON);
      //draw triangle
    glEnd();
  glPopMatrix();

Quote:}

drawMoreTriangles()
{
  //same as drawTriangles..

Quote:}

Do I need glFrontFace? or etc? I'm just drawing triangles though.

Thanks,
rach

Quote:> glDepthFunc?? I miss that in my code. Could that be the problem?
> What do I need to set it to? Is it just:

> glDepthFunc (GL_LEQUAL);

Default value is GL_LESS, which should be fine...

Hi,

There is something very wrong with my z-buffer. Every triangles that
I create in the drawMoreTriangles subroutine seems to be on top of
drawTriangles, just because I call drawMoreTriangles later in the
program!

Why?!! What*up?

Rach

looks all correct...

Quote:> > Hi,

> > There is something very wrong with my z-buffer. Every triangles
that
> > I create in the drawMoreTriangles subroutine seems to be on top of
> > drawTriangles, just because I call drawMoreTriangles later in the
> > program!

> > Why?!! What*up?
> looks all correct...

Well, I don't see a call to glDepthFunc ... Maybe set that to GL_LESS.

--
jb

(replace y with x to reply)