How do you render primitives as wireframes in OpenGL

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From http   cone3d gamedev net cgi-bin index pl page tutorials ogladv tut5     Turn on wireframe mode glPolygonMode GL FRONT  GL LINE   glPolygonMode GL BACK  GL LINE       Draw the box DrawBox        Turn off wireframe mode glPolygonMode GL FRONT  GL FILL   glPolygonMode GL BACK  GL FILL

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If you are using the fixed pipeline  OpenGL  lt  3 3  or the compatibility profile you can use    Turn on wireframe mode glPolygonMode GL FRONT AND BACK  GL LINE      Draw the scene with polygons as lines  wireframe  renderScene       Turn off wireframe mode glPolygonMode GL FRONT AND BACK  GL FILL     In this case you can change the line width by calling glLineWidth  Otherwise you need to change the polygon mode inside your draw method  glDrawElements  glDrawArrays  etc  and you may end up with some rough results because your vertex data is for triangles and you are outputting lines  For best results consider using a Geometry shader or creating new data for the wireframe

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glPolygonMode  GL FRONT AND BACK  GL LINE      to switch on   glPolygonMode  GL FRONT AND BACK  GL FILL      to go back to normal   Note that things like texture-mapping and lighting will still be applied to the wireframe lines if they re enabled  which can look weird

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use this function  void glPolygonMode  GLenum face   GLenum mode    face   Specifies the polygons that mode applies to  can be GL FRONT for front side of the polygone and GL BACK for his back and GL FRONT AND BACK for both   mode   Three modes are defined and can be specified in mode    GL POINT  Polygon vertices that are marked as the start of a boundary edge are drawn as points    GL LINE   Boundary edges of the polygon are drawn as line segments   your target   GL FILL   The interior of the polygon is filled   P S   glPolygonMode controls the interpretation of polygons for rasterization in the graphics pipeline    for more information look at the OpenGL reference pages in khronos group   https   www khronos org registry OpenGL-Refpages gl4 html glPolygonMode xhtml

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If it s OpenGL ES 2 0 you re dealing with  you can choose one of draw mode constants from  GL LINE STRIP  GL LINE LOOP   GL LINES  to draw lines   GL POINTS  if you need to draw only vertices   or  GL TRIANGLE STRIP  GL TRIANGLE FAN  and GL TRIANGLES to draw filled triangles  as first argument to your   glDrawElements GLenum mode  GLsizei count  GLenum type  const GLvoid   indices    or  glDrawArrays GLenum mode  GLint first  GLsizei count  calls

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Assuming a forward-compatible context in OpenGL 3 and up  you can either use glPolygonMode as mentioned before  but note that lines with thickness more than 1px are now deprecated  So while you can draw triangles as wire-frame  they need to be very thin  In OpenGL ES  you can use GL LINES with the same limitation   In OpenGL it is possible to use geometry shaders to take incoming triangles  disassemble them and send them for rasterization as quads  pairs of triangles really  emulating thick lines  Pretty simple  really  except that geometry shaders are notorious for poor performance scaling   What you can do instead  and what will also work in OpenGL ES is to employ fragment shader  Think of applying a texture of wire-frame triangle to the triangle  Except that no texture is needed  it can be generated procedurally  But enough talk  let s code  Fragment shader     in vec3 v barycentric     barycentric coordinate inside the triangle uniform float f thickness     thickness of the rendered lines  void main         float f closest edge   min v barycentric x          min v barycentric y  v barycentric z       see to which edge this pixel is the closest     float f width   fwidth f closest edge      calculate derivative  divide f thickness by this to have the line width constant in screen-space      float f alpha   smoothstep f thickness  f thickness   f width  f closest edge      calculate alpha     gl FragColor   vec4 vec3  0   f alpha       And vertex shader   in vec4 v pos     position of the vertices in vec3 v bc     barycentric coordinate inside the triangle  out vec3 v barycentric     barycentric coordinate inside the triangle  uniform mat4 t mvp     modeview-projection matrix  void main         gl Position   t mvp   v pos      v barycentric   v bc     just pass it on     Here  the barycentric coordinates are simply  1  0  0    0  1  0  and  0  0  1  for the three triangle vertices  the order does not really matter  which makes packing into triangle strips potentially easier    The obvious disadvantage of this approach is that it will eat some texture coordinates and you need to modify your vertex array  Could be solved with a very simple geometry shader but I d still suspect it will be slower than just feeding the GPU with more data

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The easiest way is to draw the primitives as GL LINE STRIP   glBegin GL LINE STRIP      Draw vertices here    glEnd

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You can use glut libraries like this     for a sphere   glutWireSphere radius 20 20    for a Cylinder    GLUquadric  quadratic   gluNewQuadric    gluQuadricDrawStyle quadratic GLU LINE   gluCylinder quadratic 1 1 1 12 1    for a Cube   glutWireCube 1 5

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In Modern OpenGL OpenGL 3 2 and higher   you could use a Geometry Shader for this      version 330  layout  triangles  in  layout  line strip   for lines  use  points  for points    max vertices 3  out   in vec2 texcoords pass      Texcoords from Vertex Shader in vec3 normals pass      Normals from Vertex Shader  out vec3 normals    Normals for Fragment Shader out vec2 texcoords    Texcoords for Fragment Shader  void main void        int i      for  i   0  i  lt  gl in length    i                  texcoords texcoords pass i     Pass through         normals normals pass i     Pass through         gl Position   gl in i  gl Position    Pass through         EmitVertex              EndPrimitive        Notices      for points  change layout  line strip  max vertices 3  out  to layout  points  max vertices 3  out  Read more about Geometry Shaders

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A good and simple way of drawing anti-aliased lines on a non anti-aliased render target is to draw rectangles of 4 pixel width with an 1x4 texture  with alpha channel  values of  0  1  1  0    and use linear filtering with mip-mapping off  This will make the lines 2 pixels thick  but you can change the texture for different thicknesses  This is faster and easier than barymetric calculations

[opengl] How do you render primitives as wireframes in OpenGL?

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