Hidden Line Removal

aheymann · Post by **aheymann** » Sun Apr 06, 2008 4:42 pm

Hallo

I would like to display a scene, but with hidden lines removed. I have read various articles about enabling Z buffers and/or stencil buffers. Here is a 'definition' of hidden line removal :

http://en.wikipedia.org/wiki/Hidden_line_removal

Can anybody please provide me with some ideas?

Thanks

Anton

hybrid · Post by **hybrid** » Sun Apr 06, 2008 10:10 pm

Simply enable the wireframe mode, which will give you that effect automatically.

Bob Finnie · Post by **Bob Finnie** » Thu Apr 30, 2009 10:08 am

Does this hide the lines that shouldnt be visible though? I thought the wireframe mode showed all lines...

vitek · Post by **vitek** » Thu Apr 30, 2009 9:54 pm

Why not take 5 seconds to modify one of the example programs and see for yourself?

Travis

Bob Finnie · Post by **Bob Finnie** » Tue May 05, 2009 11:51 am

I did and it dosen't.

smartarse

hybrid · Post by **hybrid** » Tue May 05, 2009 1:13 pm

A simply approach for the fixed pipeline: Use an invisible version of the mesh, scaled by 0.999 in all dimensions, and turn of the color write.This should fill the zbuffer in order to avoid painting the wires at positions invisible from the current viewpoint. Pretty much the same as for the outline rendering, just that the inner mesh is invisible this time.
Not rendering the full tris of the wireframe is not possible without much manual rendering, though, and lots of edge detection and stuff AFAIK.

Bob Finnie · Post by **Bob Finnie** » Tue May 05, 2009 4:25 pm

Hi Hybrid,
This may be something I will implement as I too need 'retro style' wireframe graphics..
Just a couple of questions though:

1/ You mention 'color write', what is color write and how do I turn it off?
2/ When you say invisible mesh for the inner part - presumably you dont mean:

Code: Select all

 scenenode->setVisible(false);

as this would not render anything.. could you clarify please?

Many Thanks,
Bob

Mel · Post by **Mel** » Tue May 05, 2009 7:39 pm

Scaling would not be the proper operation to get the hidden-line effect, but to shift the vertices along the normals a bit inside the object, and then, proceed exactly as Hybrid has suggested.

Or a 2 passes effect, where the first renders only the Zbuffer of the scene, and, without clearing the Zbuffer, the next renders only the wireframe lines.

hybrid · Post by **hybrid** » Tue May 05, 2009 8:10 pm

Yes, I meant the latter: First render the mesh into the zbuffer, then render the wireframe.
ColorMask can be set in SMaterial in SVN/trunk (i.e. Irrlicht 1.6).

Bob Finnie · Post by **Bob Finnie** » Tue May 12, 2009 9:44 am

Thanks Hybrid. I guess I'll wait for the official 1.6 release then to try it.

Just out of curiosity, Why hasn't Irrlicht got a basic Hidden line removal algortihm in? Is it harder to implement than I imagine?

In any case I think it would be a great addition to a great renderer.

hybrid · Post by **hybrid** » Wed May 13, 2009 10:54 pm

I guess it's of not much use for usual games. And it's also not a typical material property. So it's up the some user to prepare such an effect, could be added to irrExt then maybe.

cassini · Post by **cassini** » Thu May 14, 2009 1:56 am

Actually it is not necessary to do any scaling at all, two passes do the trick very well.
First past a zbuffer write with alpha test set to never write to the color buffer,
The second pass set to write if buffer is equal or less.
If you do a line thickness in OpenGl the resulting image looks just like Maya, or Xsi wire frame render.

hybrid · Post by **hybrid** » Thu May 14, 2009 7:46 am

Hmm, guess I should try now

hybrid · Post by **hybrid** » Thu May 14, 2009 9:31 am

Ok, just as cassini suggested. Works right away, just render the mesh two times. First with ColorMask set to ECP_NONE, then with wireframe enabled, ZBuffer set to ECF_LESSEQUAL (to avoid missing lines where the mesh is intersecting the wireframe) and Thickness set tom something larger than 1.f for better recognizable effect (the latter only works under OpenGL). Looks terrific