Draw a triangle and his borders too??

Vincent_Dumont · April 11, 2006, 5:34pm

Hello, I am new to OpenGL and I’d like to draw triangles but also to draw their borders.

The standard way draws solid/plain triangles.
glPolygonMode(GL_FRONT_AND_BACK,GL_LINE) draws triangles’ borders.

I would like both, is that possible? Thank you!

system · April 11, 2006, 6:54pm

you mean draw a triangle in both wire frame and solid? what’s stopping you?

Vincent_Dumont · April 11, 2006, 7:28pm

Well I don’t know how

if I do for example

glBegin(GL_TRIANGLES);				
		glVertex3f( 0.0f, 1.0f, 0.0f);		
		glVertex3f(-1.0f,-1.0f, 0.0f);		
		glVertex3f( 1.0f,-1.0f, 0.0f);		
glEnd();

This will do solid triangles

And

glPolygonMode(GL_FRONT_AND_BACK,GL_LINE)
glBegin(GL_TRIANGLES);				
		glVertex3f( 0.0f, 1.0f, 0.0f);		
		glVertex3f(-1.0f,-1.0f, 0.0f);		
		glVertex3f( 1.0f,-1.0f, 0.0f);		
glEnd();

This will do wire frame

But how can I do wireframe and solid at the same time? Do I have to draw them separatly?

system · April 11, 2006, 7:52pm

it would help if you could detail what effect you’re after. if this is some sort of selection outline for a solid in an editor, i’ve never really come up with anything satisfactory. i’ve played with stippled lines, texmapped lines, plain old lines, wireframe solid lines, green eggs and lines… nothing quite suits me. i like the texmapped lines the best, i think, for selection anyway.

system · April 11, 2006, 8:13pm

You should be able to do both, just draw with one mode then with the other.

// Filled
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL)
glBegin(GL_TRIANGLES);
glVertex…
glEnd();

// Wireframe
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE)
glBegin(GL_TRIANGLES);
glVertex…
glEnd();

Vincent_Dumont · April 11, 2006, 8:13pm

I will post a screenshot of what I’m looking for tomorrow morning (don’t have the models here)

Vincent_Dumont · April 11, 2006, 8:22pm

I tried this and it’s nearly what I’m looking for. My problem is that the lines lack width and they look stippled

I use glPolygonMode(GL_FRONT_AND_BACK, GL_FILL) for the solid ones, and GL_LINE for the wire frame.

system · April 11, 2006, 8:30pm

yeah, that’s z-fighting, and it’s considered by many the worst thing imaginable. you could try glPolygonOffset to slide the solid part “into” screen a bit, but this isn’t perfect either.

if you don’t care about the lines being visible through the object you could of course disable the depth test or you could try playing with the stencil buffer to mask the parts that are backfacing…

system · April 11, 2006, 10:13pm

it’s been a while, but i just did a quick test with a cube and it worked pretty well. you can go 2 routes: push the solid geometry in, or pull the lines out.

push solids in:
glPolygonOffset(1,1);
glEnable(GL_POLYGON_OFFSET_FILL);
draw solids…
glDisable(GL_POLYGON_OFFSET_FILL);
draw lines normally

pull lines out:
draw solids normally…
glPolygonOffset(-1,-1);
glEnable(GL_POLYGON_OFFSET_LINE);
draw lines…
glDisable(GL_POLYGON_OFFSET_LINE);

maybe the 2nd approach is better, but who really knows for sure…

Vincent_Dumont · April 12, 2006, 7:22am

This improved the drawing a lot, but still there are some small glitches with it.

See the following:

Relic · April 12, 2006, 8:42am

This is an indication for depth buffer imprecision.

Make sure your pixelformat has 24 bits of depth.
glGetIntegerv(GL_DEPTH_BITS, &numDepthBits).
If not, change your pixelformat selection code. 24 bits of depth is the max. what today’s hardware offers mainly.

Then check your viewing frustum zNear and zFar settings and make the ratio zFar/zNear as small as possible without clipping your geometry, that is push zNear out and draw zFar in to increase the depth buffer precision.

Vincent_Dumont · April 12, 2006, 9:35am

Hi Relic, thank you for the answer.

glGetIntegerv(GL_DEPTH_BITS, &numDepthBits) returned 16 bits

How can I set the pixel format to 24? (Sorry I’ve only been doing OpenGL for 3 days now)

I am using wxWidgets (don’t know if that means something for the pixel format)

Also, for the zNear and zFar settings, are you talking about something like this —>

This is what I have:

    GLint viewport[4]; 
    glGetIntegerv(GL_VIEWPORT, viewport); 

    const float aspectRatio = (float)(viewport[2]) / (float)(viewport[3]); 
    gluPerspective(45, aspectRatio, 0.1, 200);

system · April 12, 2006, 10:56am

u try nehe tuts? they helped me!

Vincent_Dumont · April 12, 2006, 11:53am

Yeha but they set it related with Windows API

system · April 12, 2006, 12:04pm

be careful with the value you use with polygonoffset. the factor and units need to be in proportion to your depth range. the last image suggests to me that your values are out of whack. try something smaller… experiment.

i dunno about the wxWidget stuff. don’t they have a forum or tutorials or documents? the windows setup stuff is tedious but very straightforward, and you need only do it once.

Vincent_Dumont · April 12, 2006, 12:25pm

Setting the PolygonOffset to -0,485 is way better, but not perfect. I still need to figure out how to set the pixel format to 24 bits.

Sorry I am quite new with OpenGL.

What do you mean by “depth range”? Are you talking about this: gluPerspective(45, aspectRatio, 0.1, 200); ???
I’ve found nothing in wxWidget’s doc but I’ll ask on forums
How can I enlarge the width of the GL_LINES?

system · April 12, 2006, 12:32pm

http://pyopengl.sourceforge.net/documentation/manual/gluPerspective.3G.html

and

http://www.rush3d.com/reference/opengl-redbook-1.1/

and give that wiki a go… looks interesting

system · April 12, 2006, 1:02pm

uh, you’re welcome, i guess. gees, this is a tough crowd!

Vincent_Dumont · April 12, 2006, 1:19pm

Thank you very much indeed I’ll read some more.

songho · April 12, 2006, 1:45pm

Originally posted by Vincent Dumont:
- What do you mean by “depth range”? Are you talking about this: gluPerspective(45, aspectRatio, 0.1, 200); ???

Relic meant that increase the near value AND decrease the far value, so the depth range gets narrower and the depth precision is increased as a result.
==song==