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Vertex array & wireframe problem

Started by Halconero Mar 23, 2007 at 8:46 PM 4 replies 3.6k views

Original Post

Mar 23, 2007 at 8:46 PM

Hi everyone, I'm new to vertex arrays and I'm trying to do a 3D mesh using them. I get to draw the mesh with filled quads. But when I switch to wireframe mode (glPolygonMode(GL_FRONT, GL_LINE) nothing gets drawn.. Is it obligatory to specify an edge flag / color arrays along with the vertex array or these are taken from the default / current values? I tried it also but nothing happened. Besides, I draw quads using glBegin and they draw perfectly in any of the two modes. Please help! Thank you. [smile] [Edited by - Halconero on March 23, 2007 9:20:21 PM]

polymorphed

Mar 24, 2007 at 2:02 AM

Try using glPolygonMode(GL_FRONT_AND_BACK, GL_LINE) instead, it's possible the vertices of your quads are in the "wrong" order, which leads to them being culled.

Halconero

Mar 24, 2007 at 8:23 PM

Thank you

Well it seems something strange happened here because I tried changing the orientation of the quads by changing the indices passed to glDrawElements but still one moment I have a filled quad and then I remove the comment to the line:

glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

and the quad disappears!

By the way I changed to glDrawArrays to render my quads and it worked fine.

MARS_999

Mar 24, 2007 at 8:57 PM

Sounds like you have your indices setup wrong. Your order is incorrect. With glDrawElements you need to have the order and amount setup correctly for it to work right, vs. with glDrawArrays() you just tell it the number of indices to render.

Halconero

Mar 24, 2007 at 11:58 PM

Yeah that's true, with glDrawArrays you have to provide the array with ordered vertices instead of providing ordered indices to glDrawElements, but I'm almost sure that they were correct, as a proof of this the quads drew right in fill mode. I think is a local issue (My PC!).

Halconero

Mar 25, 2007 at 9:50 PM

I found a similar case:

http://209.85.165.104/search?q=cache:-qSpjYACzvUJ:www10.dcccafe.com/NewsPortal-0.37/article.php%3Fid%3D152931%26group%3Dcomp.graphics.api.opengl+glDrawElements+not+working+GL_LINE&hl=es&ct=clnk&cd=1≷=cl

If you can't see the link here goes:

Quote:

Re: glPolygonMode( GL_FRONT_AND_BACK, GL_LINE ) not working with vertex arrays on laptop
Newsgroups Answer

Subject: Re: glPolygonMode( GL_FRONT_AND_BACK, GL_LINE ) not working with vertex arrays on laptop
From: nospam@nospam.net (Carl Bateman)
Newsgroups: comp.graphics.api.opengl
Organization: Aioe.org NNTP Server
Date: Nov 21 2006 11:39:06

"jbwest" wrote in message
news:leadnUDioLiAK8LYnZ2dnUVZ_tudnZ2d@comcast.com...

Do you have backface/frontface culling on ?

jbw

Short answer, I don''t know, I''m a noob. However, I''ve tried variations of
glEnable/glDisable (GL_CULL_FACE)
but to no avail.

However,
glDrawElements(GL_LINE_STRIP, 4, GL_UNSIGNED_INT, cube_indexes);
and
glDrawElements(GL_LINES, 4, GL_UNSIGNED_INT, cube_indexes);
do give me lines.

I program during my daily commute, but since OpenGL doesn''t seem to properly
implemented I have a problem. It slows dev work, but nothing catastrophic.
I''m asking about this more out of curiosity than anything else, I was
wondering whether this is a known issue and what the cause might be.

Sorry for delay in replying I have limited access to news servers I can post
from.

CB

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