pyopengl-users

[PyOpenGL-Users] Need help about Picking

[Marco A. <mar...@ex...>]

Hi, I followed a lot of tutorials about picking and I decided to use the
Selection Buffer.

 

Now, the code that I wrote works as attended but I am in trouble to discover
"the nearest object". 

 

In fact, I wrote in my code(as I successfully learned at
http://pyopengl.sourceforge.net/documentation/opengl_diffs.html) :

.

hits = glRenderMode(GL_RENDER)

for record in hits:

min_depth, max_depth, names = record

.

 

and I tried to obtain the lower value of "min_depth" (= "the nearest
object") but the result is no correct: the reason is that all my returned
values of min_depth are identical. 

 

Probably it is perhaps caused by a truncated internal representation - I am
not sure, I am a novice in python too -, or by a bad pyopengl configuration.

 

For example, at page
http://www.dei.isep.ipp.pt/~matos/cg/docs/manual/gluPerspective.3G.html I
read:

 

"roughly log2r bits of depth buffer precision are lost. Because r approaches
infinity as zNear approaches 0, zNear must never be set to 0."

 

and I used "gluPerspective(45, 1.0*width/height, 0.05, 100.0)" with the
value "0.05" near to 0.  I tried to change "0.05" in "1.0" but I obtain the
bad effect that my model disappear soon when I zoom into ).

 

Can you suggest me the correct way to calculate the lower value of
"min_depth"? Or can you suggest me a alternative method to simulate a pick?
I'd like to avoid color based method.

 

Thank you for your help.

 

Marco Avellino

 

 

 

 

 

 

 

Re: [PyOpenGL-Users] Need help about Picking

[Ian M. <geo...@gm...>]

On Mon, May 24, 2010 at 7:48 AM, Marco Avellino <
mar...@ex...> wrote:

>  Hi, I followed a lot of tutorials about picking and I decided to use the
> Selection Buffer.
>
I've never used it, but it looks interesting.

> hits = glRenderMode(GL_RENDER)
>
> for record in hits:
>
> *min_depth*, max_depth, names = record
>
In context would be more helpful.  For example, did you use
glRenderMode(GL_SELECT) and do drawing stuff before this?  Could we see your
actual source file?

> and I tried to obtain the lower value of “min_depth“ (= “the nearest
> object”) but the result is no correct: the reason is that all my returned
> values of *min_depth* are identical.
>
...telling me that nothing is being written to this buffer.  Which could
mean many things--it's not configured correctly, you're not drawing
anything, you don't have a graphics card, etc.

Could we see the actual code?

Thanks,
Ian

Re: [PyOpenGL-Users] Need help about Picking

[Ian M. <geo...@gm...>]

Hi,

Unfortunately, I can't immediately detect any problems with this code.
-The name stack is, by default, only 64 large.  There's no advantage to
having a 1024-length selection buffer.
-What happens if you use "print hits"?  Is it all zeroes?

Ian

[PyOpenGL-Users] R: Need help about Picking

[Marco A. <mar...@ex...>]

Hi, 

when I use "print hits" I read the value 0.50000000016 for both min_depth,
max_depth. 
 
For example, when I call "print min_depth, max_depth, names" l'output is:
0.50000000016 0.50000000016 [object_X's name]
0.50000000016 0.50000000016 [object_Y's name]
.
 
If I use GLdouble(min_depth or max_depth), I read 0.5000000001641532
 
I read always 0.50000000016 except when I draw objects very near. At that
moment I read values like 0.49., 0.48, .. etc. and min_depth != max_depth

It seems that Selection works only in range [0.2,0.5] where 0.2 is forced by
gluPerspective(45, 1.0*width/height, 0.2, 2.0)

 

If you want I can send you my code but I will do it only if you are agree.

 

PS.

Sorry if I write you no at sourceforge.net. It seems that I am not able to
create a new account

 

 

 

  _____  

Da: Ian Mallett [mailto:geo...@gm...] 
Inviato: domenica 30 maggio 2010 17.31
A: Marco Avellino
Cc: pyopengl-users
Oggetto: Re: [PyOpenGL-Users] Need help about Picking

 

Hi,

Unfortunately, I can't immediately detect any problems with this code.  
-The name stack is, by default, only 64 large.  There's no advantage to
having a 1024-length selection buffer.
-What happens if you use "print hits"?  Is it all zeroes?

Ian

Re: [PyOpenGL-Users] Need help about Picking

[Ian M. <geo...@gm...>]

On Mon, May 31, 2010 at 9:10 AM, Marco Avellino <
mar...@ex...> wrote:

>  Hi,
>
> when I use "print hits” I read the value 0.50000000016 for both min_depth, max_depth.
>
>
>
> For example, when I call “print min_depth, max_depth, names” l’output is:
>
> 0.50000000016 0.50000000016 [object_X’s name]
>
> 0.50000000016 0.50000000016 [object_Y’s name]
>
> …
>
>
>
> If I use GLdouble(min_depth or max_depth), I read 0.5000000001641532
>
>
>
> I read always 0.50000000016 except when I draw objects very near. At that moment I read values like 0.49…, 0.48, …. etc. and min_depth != max_depth
>
> It seems that Selection works only in range [0.2,0.5] where 0.2 is forced
> by gluPerspective(45, 1.0*width/height, *0.2*, 2.0)
>
>
>
> If you want I can send you my code but I will do it only if you are agree.
>
Yes, that might be helpful.  I'll fuss with it, and see what I can do.
Ian

>
>
> PS.
>
> Sorry if I write you no at sourceforge.net. It seems that I am not able to
> create a new account
>

Re: [PyOpenGL-Users] Need help about Picking

[Ian M. <geo...@gm...>]

On Thu, Jun 3, 2010 at 2:31 AM, Marco Avellino <mar...@ex...
> wrote:

>  *Challenge:*
>
> Obtain reliable values of min_depth in everywhere I see models in the
> scene. Actually I obtain reliable values of min_depth only in a great level
> of zoom (= Page Down).
>
Seems picking is working . . .

Also, the depth seems to be working fine too.  Do however remember that it's
returning the depth value in z-buffer units.  These measure the part
difference of the fragment from the near plane to the far one.  For example,
in your code, the near and far planes are at 0.1 and 2.0, respectively.  If
a fragment is 1.05 OpenGL units away from the camera, then in Z, your
fragment is 0.5, because it is halfway between the near and far planes.
However, in practice, the Z-buffer is mapped to be non-linear, so the
conversion is more difficult.

Do note that for selecting, you can just use the values returned from the
selection buffer "as-is".  Conversion is not necessary for a
comparison--only for getting actual OpenGL coordinates.

If you still need to convert to OpenGL units, you can do the following:


mpos = #mouse position--ensure that it is measured in
       #pixels from the *lower right* of the screen!

returned_z_value = #whatever value you want to convert

viewport = glGetIntegerv(GL_VIEWPORT)
modelview = glGetDoublev(GL_MODELVIEW_MATRIX)
projection = glGetDoublev(GL_PROJECTION_MATRIX)

#pos is the actual position of the object in OpenGL coordinates
pos =
gluUnProject(mpos[0],mpos[1],returned_z_value,modelview,projection,viewport)


#tells how far away, in OpenGL units, the object is
distance = length(pos-camerapos)


Other than this possible pitfall, I don't see any problem.

Ian

[PyOpenGL-Users] R: Need help about Picking

[Marco A. <mar...@ex...>]

Ok, I’ll try to follow your advice

 

If it works, I will send a new mail to confirm it.

 

Thank you for your precious help

 

Marco

 

  _____  

Da: Ian Mallett [mailto:geo...@gm...] 
Inviato: venerdì 4 giugno 2010 19.33
A: Marco Avellino
Cc: pyopengl-users
Oggetto: Re: [PyOpenGL-Users] Need help about Picking

 

On Thu, Jun 3, 2010 at 2:31 AM, Marco Avellino
<mar...@ex...> wrote:

Challenge: 

Obtain reliable values of min_depth in everywhere I see models in the scene.
Actually I obtain reliable values of min_depth only in a great level of zoom
(= Page Down).

Seems picking is working . . .

Also, the depth seems to be working fine too.  Do however remember that it's
returning the depth value in z-buffer units.  These measure the part
difference of the fragment from the near plane to the far one.  For example,
in your code, the near and far planes are at 0.1 and 2.0, respectively.  If
a fragment is 1.05 OpenGL units away from the camera, then in Z, your
fragment is 0.5, because it is halfway between the near and far planes.
However, in practice, the Z-buffer is mapped to be non-linear, so the
conversion is more difficult.  

Do note that for selecting, you can just use the values returned from the
selection buffer "as-is".  Conversion is not necessary for a
comparison--only for getting actual OpenGL coordinates.

If you still need to convert to OpenGL units, you can do the following:


mpos = #mouse position--ensure that it is measured in
       #pixels from the *lower right* of the screen!

returned_z_value = #whatever value you want to convert

viewport = glGetIntegerv(GL_VIEWPORT)
modelview = glGetDoublev(GL_MODELVIEW_MATRIX)
projection = glGetDoublev(GL_PROJECTION_MATRIX)

#pos is the actual position of the object in OpenGL coordinates
pos =
gluUnProject(mpos[0],mpos[1],returned_z_value,modelview,projection,viewport)


#tells how far away, in OpenGL units, the object is
distance = length(pos-camerapos)


Other than this possible pitfall, I don't see any problem.

Ian

[PyOpenGL-Users] R: Need help about Picking

[Marco A. <mar...@ex...>]

Hi Ian,

 

I followed your advice ( I convert now in OpenGL units) and it works.

 

Thank you for your precious help !!!

 

Marco

  _____  

Da: Marco Avellino [mailto:mar...@ex...] 
Inviato: lunedì 7 giugno 2010 13.52
A: 'Ian Mallett'
Cc: 'pyopengl-users'
Oggetto: R: [PyOpenGL-Users] Need help about Picking

 

Ok, I’ll try to follow your advice

 

If it works, I will send a new mail to confirm it.

 

Thank you for your precious help

 

Marco

 

  _____  

Da: Ian Mallett [mailto:geo...@gm...] 
Inviato: venerdì 4 giugno 2010 19.33
A: Marco Avellino
Cc: pyopengl-users
Oggetto: Re: [PyOpenGL-Users] Need help about Picking

 

On Thu, Jun 3, 2010 at 2:31 AM, Marco Avellino
<mar...@ex...> wrote:

Challenge: 

Obtain reliable values of min_depth in everywhere I see models in the scene.
Actually I obtain reliable values of min_depth only in a great level of zoom
(= Page Down).

Seems picking is working . . .

Also, the depth seems to be working fine too.  Do however remember that it's
returning the depth value in z-buffer units.  These measure the part
difference of the fragment from the near plane to the far one.  For example,
in your code, the near and far planes are at 0.1 and 2.0, respectively.  If
a fragment is 1.05 OpenGL units away from the camera, then in Z, your
fragment is 0.5, because it is halfway between the near and far planes.
However, in practice, the Z-buffer is mapped to be non-linear, so the
conversion is more difficult.  

Do note that for selecting, you can just use the values returned from the
selection buffer "as-is".  Conversion is not necessary for a
comparison--only for getting actual OpenGL coordinates.

If you still need to convert to OpenGL units, you can do the following:


mpos = #mouse position--ensure that it is measured in
       #pixels from the *lower right* of the screen!

returned_z_value = #whatever value you want to convert

viewport = glGetIntegerv(GL_VIEWPORT)
modelview = glGetDoublev(GL_MODELVIEW_MATRIX)
projection = glGetDoublev(GL_PROJECTION_MATRIX)

#pos is the actual position of the object in OpenGL coordinates
pos =
gluUnProject(mpos[0],mpos[1],returned_z_value,modelview,projection,viewport)


#tells how far away, in OpenGL units, the object is
distance = length(pos-camerapos)


Other than this possible pitfall, I don't see any problem.

Ian

Re: [PyOpenGL-Users] Need help about Picking

[Ian M. <geo...@gm...>]

Awesome!  Glad I could help.