How do I set the opacity of a vertex in OpenGL?

The following snippet draws a gray square.

glColor3b(50, 50, 50);

glBegin(GL_QUADS);
glVertex3f(-1.0, +1.0, 0.0); // top left
glVertex3f(-1.0, -1.0, 0.0); // bottom left
glVertex3f(+1.0, -1.0, 0.0); // bottom right
glVertex3f(+1.0, +1.0, 0.0); // top right
glEnd();

In my application, behind this single square exists a colored cube.

What function should I use to make square (and only this square) opaque?

From stackoverflow

• glColor4f(float r,float g, float b, flaot alpha);
  (in your case maybe clColor4b)
  also make sure, that blending is enabled.
  (you have to reset the color to non-alpha afterwads, which might involve a glGet* to save the old vertexcolor)

• Use glColor4 instead of glColor3. For example:

  glBlendFunc(GL_SRC_ALPHA,GL_ONE);
  glColor4f(1.0f,1.0f,1.0f,0.5f);
  

  nbolton : Thanks. Hmm, doesn't seem to be working for me, what could I be doing wrong?

• You can set colors per vertex

  glBegin(GL_QUADS);
  glColor4f(1.0, 0.0, 0.0, 0.5); // red, 50% alpha
  glVertex3f(-1.0, +1.0, 0.0); // top left
  // Make sure to set the color back since the color state persists
  glVertex3f(-1.0, -1.0, 0.0); // bottom left
  glVertex3f(+1.0, -1.0, 0.0); // bottom right
  glVertex3f(+1.0, +1.0, 0.0); // top right
  glEnd();
  

• In the init function, use these two lines:

  glEnable(GL_BLEND);
  glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
  

  And in your render function, ensure that glColor4f is used instead of glColor3f, and set the 4th argument to the level of opacity required.

  glColor4f(1.0, 1.0, 1.0, 0.5);
  
  glBegin(GL_QUADS);
  glVertex3f(-1.0, +1.0, 0.0); // top left
  glVertex3f(-1.0, -1.0, 0.0); // bottom left
  glVertex3f(+1.0, -1.0, 0.0); // bottom right
  glVertex3f(+1.0, +1.0, 0.0); // top right
  glEnd();