Wireframes in OpenGL

Question

glPolygoneMode(GL_FRONT_AND_BACK, GL_FILL) is ON, and how can I get wireframe at this situation? Is there any way to get wireframe rendering without switch Polygone Mode to GL_LINE?

Answer 1

glPolygonMode( GL_FRONT_AND_BACK, GL_LINE );

Fill fills it... you want lines.

EDIT: Remember, you can always put it back to fill when you're done, like this

glPolygonMode( GL_FRONT_AND_BACK, GL_LINE );
 // add polygons here
glPolygonMode( GL_FRONT_AND_BACK, GL_FILL );

Answer 2

From your question I assume you have some huge middleware, and have only access to shaders, am I right ?

In this case, create in your vertex shader a varying called, for instance, vertexID :

varying float vertexID; # in GLSL <= 3, or
out float vertexID;     # in GLSL 4

and assign it the built-in gl_VertexID :

vertexID = (float)gl_VertexID;

This way, you'll get access to gl_VertexID from the fragment shader, but interpolated ! This is great because it will be, for instance, 2 at one vertex, 3 at the neighbour, and 2.5 inbetween.

So you just have to check if vertexID is close to 2 (resp. 3) : if so, you're close to an edge. That should be good enough for visualisation & debug.

in float vertexID; # or
varying float vertexID;

// blabla

if (fract(vertexID) < 0.1){
    outcolor = vec4(1,0,0,0); # in glsl 4
}

Note that you'll get only 2 edges of each triangle this way. And maybe some will be ver thin ( if one vertexID is 2 and the other 1000, the 0.1 zone is little ).

Answer 3

Your question is not clear, but I guess that you want to render both the filled polygon AND the wireframe. Turns out that's not so trivial. A classic way to solve the problem is:

glPolygonMode( ... GL_FILL);
draw();
glPolygonOffset( param1, param2 );
glPolygonMode( ... GL_LINE );
draw();

but this is not without problems, such as figuring out the values to pass to glPolygonOffset, etc.