View more

View more

View more

### Image of the Day Submit

IOTD | Top Screenshots

### The latest, straight to your Inbox.

Subscribe to GameDev.net Direct to receive the latest updates and exclusive content.

# glDrawElements only drawing one of two triangles...

Old topic!

Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.

2 replies to this topic

### #1Chris867  Members

Posted 29 October 2012 - 02:10 PM

I'm trying to draw an image into a window using OpenGL, but I can't seem to get it to draw both the triangles for the quad. It only draws one of them. I'm using two VBOs and a VAO with glDrawElements to draw. The code for the scene and the GL renderer are in the links below as well as a link to an image of the rendered single triangle.

scene2d.cpp: http://pastebin.com/L5umEBhf
ecglrenderer.cpp: http://pastebin.com/QjheWNXj
image: http://minus.com/mbklHNfIXZvkWr

What's the problem here? How would I go about debugging this issue?

Thanks,
Chris

### #2karwosts  Members

Posted 29 October 2012 - 02:56 PM

Can I ask why these vertices are arranged so oddly?

m_verticies[0].x = -1.0f; m_verticies[1].y = 1.0f; m_verticies[1].z = 0.0f;
m_verticies[0].u = 0.0f; m_verticies[1].v = 0.0f;

m_verticies[1].x = -1.0f; m_verticies[0].y = -1.0f; m_verticies[0].z = 0.0f;
m_verticies[1].u = 0.0f; m_verticies[0].v = 1.0f;

m_verticies[2].x = 1.0f; m_verticies[3].y = -1.0f; m_verticies[3].z = 0.0f;
m_verticies[2].u = 1.0f; m_verticies[3].v = 1.0f;

m_verticies[3].x = 1.0f; m_verticies[2].y = 1.0f; m_verticies[2].z = 0.0f;
m_verticies[3].u = 1.0f; m_verticies[2].v = 0.0f;

Note that if you draw out the triangles (0,1,2) and (3,2,1) it doesn't make a quad, but rather a weird shape that only covers 75% of a quad (the second triangle should be (3,2,0))

Also your two triangles as they are are wound in opposing directions. (0,1,2) is clockwise, and (3,2,1) is counterclockwise. Do you have any face culling enabled?
My Projects:
Portfolio Map for Android - Free Visual Portfolio Tracker
Electron Flux for Android - Free Puzzle/Logic Game

### #3Chris867  Members

Posted 29 October 2012 - 03:00 PM

Yes. I have backface culling enabled. If the verticies aren't correct that's probably the reason why one of the triangles isn't showing up. I'll get them set up correctly and try again.

Edit:
The indices order was the problem. Switching it to (2,1,0,2,0,3) makes it work.

Edited by Chris867, 29 October 2012 - 03:04 PM.

Old topic!

Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.